Introduction Prerequisites
Requirements
Components Used
Conventions BGP Case Studies 1
How Does BGP Work?
eBGP and iBGP
Enabling BGP Routing
Forming BGP Neighbors
BGP and Loopback Interfaces
eBGP Multihop
eBGP Multihop (Load Balancing)
Route Maps
match and set Configuration Commands
Network Command
Redistribution
Static Routes and Redistribution
iBGP
The BGP Decision Algorithm BGP Case Studies 2
AS_PATH Attribute
Origin Attribute
BGP Next Hop Attribute
BGP Backdoor
Synchronization
Weight Attribute
Local Preference Attribute
Metric Attribute
Community Attribute BGP Case Studies 3
BGP Filtering
AS Regular Expression
BGP Neighbors and Route Maps BGP Case Studies 4
CIDR and Aggregate Addresses
BGP Confederation
Route Reflectors
Route Flap Dampening
How BGP Selects a Path BGP Case Studies 5
Practical Design Example Related Information
8540MSR
Router Platforms Sample Designs
Design 1: Simple, but to be avoided...
Design 2: More complex, but safer and more efficient... Guidelines
Guideline #1
Guideline #2
Guideline #3
Guideline #4
Guideline #5
Guideline #6
Guideline #7
Guideline #8
Guideline #9
Guideline #10
Guideline #11 Related Information
Introduction
This document contains five BGP case studies.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
The information presented in this document was created from devices in a specific lab environment. All of the
devices used in this document started with a cleared (default) configuration. If you are working in a live
network, ensure that you understand the potential impact of any command before using it.
Conventions
For more information on document conventions, refer to Cisco Technical Tips Conventions.
BGP Case Studies 1
The Border Gateway Protocol (BGP), defined in RFC 1771 , allows you to create loop-free interdomain
routing between autonomous systems (AS). An AS is a set of routers under a single technical administration.
Routers in an AS can use multiple interior gateway protocols to exchange routing information inside the AS
and an exterior gateway protocol to route packets outside the AS.
How Does BGP Work?
BGP uses TCP as its transport protocol (port 179). Two BGP routers form a TCP connection between one
another (peer routers) and exchange messages to open and confirm the connection parameters.
BGP routers exchange network reachability information. This information is mainly an indication of the full
paths (BGP AS numbers) that a route should take in order to reach the destination network. This information
helps in constructing a graph of ASs that are loop-free and where routing policies can be applied in order to
enforce some restrictions on the routing behavior.
Any two routers that have formed a TCP connection in order to exchange BGP routing information are called
peers, or neighbors. BGP peers initially exchange their full BGP routing tables. After this exchange,
incremental updates are sent as the routing table changes. BGP keeps a version number of the BGP table,
which should be the same for all of its BGP peers. The version number changes whenever BGP updates the
table due to routing information changes. Keepalive packets are sent to ensure that the connection is alive
between the BGP peers and notification packets are sent in response to errors or special conditions.
eBGP and iBGP
If an AS has multiple BGP speakers, it could be used as a transit service for other ASs. As you see below,
It is necessary to ensure reachability for networks within an AS before sending the information to external
ASs. This is done by a combination of internal BGP (iBGP) peering between routers inside an AS and by
redistributing BGP information to Internal Gateway Protocols (IGPs) running in the AS.
As far as this paper is concerned, when BGP is running between routers belonging to two different ASs, we
call this exterior BGP (eBGP). When BGP is running between routers in the same AS, we call this iBGP.
Enabling BGP Routing
Use these steps to enable and configure BGP.
Let us assume you want to have two routers, RTA and RTB, talk BGP. In the first example RTA and RTB are
in different ASs and in the second example both routers belong to the same AS.
We start by defining the router process and the AS number to which the routers belong. Use this command to
enable BGP on a router:
router bgp autonomous-system
RTA#
router bgp 100
RTB#
router bgp 200
The above statements indicate that RTA is running BGP and it belongs to AS100 and RTB is running BGP
and it belongs to AS200.
The next step in the configuration process is to define BGP neighbors, which indicates the routers that are
trying to talk BGP.
Forming BGP Neighbors
Two BGP routers become neighbors once they establish a TCP connection between each other. The TCP
connection is essential in order for the two peer routers to start exchanging routing updates.
Once the TCP connection is up, the routers send open messages in order to exchange values such as the AS
number, the BGP version they're running, the BGP router ID and the keepalive hold time. After these values
are confirmed and accepted the neighbor connection is established. Any state other than "established" is an
indication that the two routers didn't become neighbors, and BGP updates won't be exchanged.
Use this neighbor command to establish a TCP connection:
The remote-as number is the AS number of the router we're trying to connect to using BGP. The ip-address
is the next hop directly-connected address for eBGP and any IP address on the other router for iBGP.
It's essential that the two IP addresses used in the neighbor command of the peer routers be able to reach one
another. One sure way to verify reachability is an extended ping between the two IP addresses. The extended
ping forces the pinging router to use as source the IP address specified in the neighbor command rather than
the IP address of the interface the packet is going out from.
It is important to reset the neighbor connection in case any BGP configuration changes are made in order for
the new parameters to take effect.
clear ip bgp address (where address is the neighbor address)
clear ip bgp * (clear all neighbor connections)
By default, BGP sessions begin using BGP version 4 and negotiating downward to earlier versions if
necessary. To prevent negotiations and force the BGP version used to communicate with a neighbor, perform
the following task in router configuration mode:
neighbor {ip address|peer-group-name}version value
In the above example RTA and RTB are running eBGP. RTB and RTC are running iBGP. The difference
between eBGP and iBGP is manifested by having the remote-as number pointing to either an external or an
internal AS. Also, the eBGP peers are directly connected while the iBGP peers are not. iBGP routers don't
have to be directly connected, as long as there is some IGP running that allows the two neighbors to reach one
another.
The following is an example of the information that the show ip bgp neighbors command displays. Pay
special attention to the BGP state, since anything other than state "established" indicates the peers aren't up.
You should also note the BGP version is 4, the remote router ID (highest IP address on the router or the
highest loopback interface in case it exists) and the table version (this is the state of the table, any time new
information comes in, the table increases the version and a version that keeps incrementing indicates that
some route is flapping causing routes to continuously be updated).
# show ip bgp neighbors
BGP neighbor is 129.213.1.1, remote AS 200, external link
BGP version 4, remote router ID 175.220.12.1
BGP state = Established, table version = 3, up for 0:10:59
Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 2828 messages, 0 notifications, 0 in queue
Sent 2826 messages, 0 notifications, 0 in queue
Connections established 11; dropped 10
BGP and Loopback Interfaces
Using a loopback interface to define neighbors is common with iBGP, but not with eBGP. Normally the
loopback interface is used to make sure the IP address of the neighbor stays up and is independent of
hardware functioning properly. In the case of eBGP, peer routers are frequently directly connected and
loopback does not apply.
If you use the IP address of a loopback interface in the neighbor command, you need some extra
configuration on the neighbor router. The neighbor router needs to tell BGP it's using a loopback interface
rather than a physical interface to initiate the BGP neighbor TCP connection. The command used to indicate a
loopback interface is:
neighbor ip-address update-source interface
The following example illustrates the use of this command.
In the above example, RTA and RTB are running iBGP inside AS 100. RTB is using in its neighbor
command the loopback interface of RTA (150.212.1.1); in this case RTA has to force BGP to use the
loopback IP address as the source in the TCP neighbor connection. RTA does this by adding the
update-source int loopback configuration (neighbor 190.225.11.1 update-source loopback 1) and this
statement forces BGP to use the IP address of its loopback interface when talking to neighbor 190.225.11.1.
Note that RTA has used the physical interface IP address (190.225.11.1) of RTB as a neighbor, which is why
RTB does not need any special configuration. Refer to Sample Configuration for iBGP and eBGP With or
Without a Loopback Address for a complete network scenario sample configuration.
eBGP Multihop
In some cases, a Cisco router can run eBGP with a third party router that doesn't allow the two external peers
to be directly connected. To achieve this, you can use eBGP multihop, which allows the neighbor connection
to be established between two non-directly-connected external peers. The multihop is used only for eBGP
and not for iBGP. The following example illustrates of eBGP multihop.
RTA is indicating an external neighbor that isn't directly connected. RTA needs to indicate that it's using
ebgp-multihop. On the other hand, RTB is indicating a neighbor that is directly connected (129.213.1.2),
which is why it doesn't need the ebgp-multihop command. You should also configure an IGP or static
routing to allow the non-connected neighbors to reach each other.
The following example shows how to achieve load balancing with BGP in a particular case where we have
eBGP over parallel lines.
eBGP Multihop (Load Balancing)
RTA#
int loopback 0
ip address 150.10.1.1 255.255.255.0
router bgp 100
neighbor 160.10.1.1 remote-as 200
neighbor 160.10.1.1 ebgp-multihop
neighbor 160.10.1.1 update-source loopback 0
network 150.10.0.0
ip route 160.10.0.0 255.255.0.0 1.1.1.2
ip route 160.10.0.0 255.255.0.0 2.2.2.2
RTB#
int loopback 0
ip address 160.10.1.1 255.255.255.0
router bgp 200
neighbor 150.10.1.1 remote-as 100
neighbor 150.10.1.1 update-source loopback 0
neighbor 150.10.1.1 ebgp-multihop
network 160.10.0.0
ip route 150.10.0.0 255.255.0.0 1.1.1.1
ip route 150.10.0.0 255.255.0.0 2.2.2.1
The above example illustrates the use of loopback interfaces, update-source and ebgp-multihop. This is a
workaround in order to achieve load balancing between two eBGP speakers over parallel serial lines. In
normal situations, BGP picks one of the lines to send packets on, and load balancing wouldn't happen. By
introducing loopback interfaces, the next hop for eBGP is the loopback interface. We use static routes (we
could also use an IGP) to introduce two equal cost paths to reach the destination. RTA has two choices to
reach next hop 160.10.1.1: one via 1.1.1.2 and the other one via 2.2.2.2, and the same for RTB.
Route Maps
Route maps are used heavily with BGP. In the BGP context, the route map is a method used to control and
modify routing information. This is done by defining conditions for redistributing routes from one routing
protocol to another or controlling routing information when injected in and out of BGP. The format of the
route map follows:
The map tag is just a name you give to the route map. Multiple instances of the same route map (same
name-tag) can be defined. The sequence number is just an indication of the position a new route map is to
have in the list of route maps already configured with the same name.
For example, if there are two instances of the route map defined, called MYMAP, the first instance will have a
sequence-number of 10, and the second will have a sequence number of 20.
route-map MYMAP permit 10 (first set of conditions goes here.)
route-map MYMAP permit 20 (second set of conditions goes here.)
When applying route map MYMAP to incoming or outgoing routes, the first set of conditions will be applied
via instance 10. If the first set of conditions is not met then we proceed to a higher instance of the route map.
match and set Configuration Commands
Each route map will consist of a list of match and set configuration. The match will specify a match criteria
and set specifies a set action if the criteria enforced by the match command are met.
For example, you could define a route map that checks outgoing updates and if there is a match for IP address
1.1.1.1 then the metric for that update will be set to 5. The above can be illustrated by the following
commands:
match ip address 1.1.1.1
set metric 5
Now, if the match criteria are met and we have a permit then the routes will be redistributed or controlled as
specified by the set action and we break out of the list.
If the match criteria are met and we have a deny then the route will not be redistributed or controlled and we
break out of the list.
If the match criteria are not met and we have a permit or deny then the next instance of the route map
(instance 20 for example) will be checked, and so on until we either break out or finish all the instances of the
route map. If we finish the list without a match then the route we are looking at will not be accepted nor
forwarded.
In Cisco IOSŪ software releases earlier than 11.2, when you use route maps for filtering BGP updates (as we
will see later), rather redistributing between protocols, you cannot filter on the inbound when using a match
command on the IP address. Filtering on the outbound is acceptable. This restriction is lifted in Cisco IOS
Software Release 11.2 and later.
The related commands for match are:
match as-path
match community
match clns
match interface
match ip address
match ip next-hop
match ip route-source
match metric
match route-type
match tag
The related commands for set are:
set as-path
set clns
set automatic-tag
set community
set interface
set default interface
set ip default next-hop
set level
set local-preference
set metric
set metric-type
set next-hop
set origin
set tag
set weight
Let us look at some route map examples:
Example 1:
Assume RTA and RTB are running RIP; RTA and RTC are running BGP. RTA is getting updates via BGP
and redistributing them to RIP. If RTA wants to redistribute to RTB routes about 170.10.0.0 with a metric of 2
and all other routes with a metric of 5 then we might use the following configuration:
router bgp 100
neighbor 2.2.2.3 remote-as 300
network 150.10.0.0
route-map SETMETRIC permit 10
match ip-address 1
set metric 2
route-map SETMETRIC permit 20
set metric 5
access-list 1 permit 170.10.0.0 0.0.255.255
In the above example if a route matches the IP address 170.10.0.0 it will have a metric of 2 and then we break
out of the route map list. If there is no match then we go down the route map list which says, set everything
else to metric 5. It is always very important to ask the question, what will happen to routes that do not match
any of the match statements, because they will be dropped by default.
Example 2:
Suppose in the above example we did not want AS100 to accept updates about 170.10.0.0. Since route maps
cannot be applied on the inbound when matching based on an IP address, we have to use an outbound route
map on RTC:
Now that you feel more comfortable with how to start BGP and how to define a neighbor, let us look at how
to start exchanging network information.
There are multiple ways to send network information using BGP. The following sections will go through
these methods one-by-one.
Network Command
The format of the network command follows:
network network-number [mask network-mask]
The network command controls what networks are originated by this box. This is a different concept from
what you are used to configuring with IGRP and RIP. With this command we are not trying to run BGP on a
certain interface, rather we are trying to indicate to BGP what networks it should originate from this box. The
mask portion is used because BGP version 4 (BGP4) can handle subnetting and supernetting. A maximum of
200 entries of the network command are accepted.
The network command will work if the network you are trying to advertise is known to the router, whether
connected, static or learned dynamically.
The above example indicates that router A, will generate a network entry for 192.213.0.0/16. The /16
indicates that we are using a supernet of the class C address and we are advertizing the first two octets (the
first 16 bits).
Note that we need the static route to get the router to generate 192.213.0.0 because the static route will put a
matching entry in the routing table.
Redistribution
The network command is one way to advertise your networks via BGP. Another way is to redistribute your
IGP (IGRP, OSPF, RIP, EIGRP, and so on) into BGP. This sounds scary because now you are dumping all of
your internal routes into BGP, some of these routes might have been learned via BGP and you do not need to
send them out again. Careful filtering should be applied to make sure you are sending to the internet only
routes that you want to advertise and not everything you have. Let us look at the example below.
RTA is announcing 129.213.1.0 and RTC is announcing 175.220.0.0. Look at RTC's configuration:
If you use a network command you will have:
RTC#
router eigrp 10
network 175.220.0.0
redistribute bgp 200
default-metric 1000 100 250 100 1500
router bgp 200
neighbor 1.1.1.1 remote-as 300
network 175.220.0.0 mask 255.255.0.0
!--- This will limit the networks
!--- originated by your AS to 175.220.0.0
This will cause 129.213.1.0 to be originated by your AS. This is misleading because you are not the source of
129.213.1.0 but AS100 is. So you would have to use filters to prevent that network from being sourced out by
your AS. The correct configuration would be:
The access-list command is used to control what networks are to be originated from AS200.
Redistribution of OSPF into BGP is slightly different than for other IGPs. The simple configuration of
redistribute ospf 1 under router bgp does not work. Specific keywords such as internal, external, and
nssa-external are required to redistribute respective routes. Refer to Understanding Redistribution of OSPF
Routes into BGP for more details.
Static Routes and Redistribution
You could always use static routes to originate a network or a subnet. The only difference is that BGP will
consider these routes as having an origin of incomplete (unknown). In the above example the same could have
been accomplished by doing:
The null 0 interface means disregard the packet. So if I get the packet and there is a more specific match than
175.220.0.0 (which exists of course) the router will send it to the specific match otherwise it will disregard it.
This is a nice way to advertise a supernet.
We have discussed how we can use different methods to originate routes out of our autonomous system.
Please remember that these routes are generated in addition to other BGP routes that BGP has learned via
neighbors (internal or external). BGP passes on information that it learns from one peer to other peers. The
difference is that routes generated by the network command, or redistribution or static, will indicate your AS
as the origin for these networks.
Injecting BGP into IGP is always done by redistribution.
RTC#
router bgp 300
neighbor 150.10.20.1 remote-as 100
neighbor 160.10.20.1 remote-as 200
network 170.10.00
Note that you do not need network 150.10.0.0 or network 160.10.0.0 in RTC unless you want RTC to also
generate these networks on top of passing them on as they come in from AS100 and AS200. Again the
difference is that the network command will add an extra advertisement for these same networks indicating
that AS300 is also an origin for these routes.
An important point to remember is that BGP will not accept updates that have originated from its own AS.
This is to insure a loop free interdomain topology.
For example, assume AS200 above had a direct BGP connection into AS100. RTA will generate a route
150.10.0.0 and will send it to AS300 then RTC will pass this route to AS200 with the origin kept as AS100,
RTB will pass 150.10.0.0 to AS100 with origin still AS100. RTA will notice that the update has originated
from its own AS and will ignore it.
iBGP
iBGP is used if an AS wants to act as a transit system to other ASs. You might ask, why can't we do the same
thing by learning via eBGP redistributing into IGP and then redistributing again into another AS? We can, but
iBGP offers more flexibility and more efficient ways to exchange information within an AS; for example
iBGP provides us with ways to control what is the best exit point out of the AS by using local preference (will
be discussed later).
Note: An important point to remember, is that when a BGP speaker receives an update from other BGP
speakers in its own AS (IBGP), the receiving BGP speaker will not redistribute that information to other BGP
speakers in its own AS. The receiving BGP speaker will redistribute that information to other BGP speakers
outside of its AS. That is why it is important to sustain a full mesh between the IBGP speakers within an AS.
In the above diagram, RTA and RTB are running iBGP and RTA and RTD are running iBGP also. The BGP
updates coming from RTB to RTA will be sent to RTE (outside of the AS) but not to RTD (inside of the AS).
This is why an iBGP peering should be made between RTB and RTD in order not to break the flow of the
updates.
The BGP Decision Algorithm
After BGP receives updates about different destinations from different autonomous systems, the protocol will
have to decide which paths to choose in order to reach a specific destination. BGP will choose only a single
path to reach a specific destination.
The decision process is based on different attributes, such as next hop, administrative weights, local
preference, the route origin, path length, origin code, metric and so on.
BGP will always propagate the best path to its neighbors. Refer to BGP Best Path Selection Algorithm for
more information.
The following sections will try to explain these attributes and show how they are used.
BGP Case Studies 2
AS_PATH Attribute
Whenever a route update passes through an AS, the AS number is prepended to that update. The AS_PATH
attribute is actually the list of AS numbers that a route has traversed in order to reach a destination. An
AS_SET is an ordered mathematical set {} of all the ASs that have been traversed. An example of AS_SET is
given later.
In the above example, network 190.10.0.0 is advertised by RTB in AS200, when that route traverses AS300
and RTC will append its own AS number to it. So when 190.10.0.0 reaches RTA it will have two AS numbers
attached to it: first 200 then 300. So as far as RTA is concerned the path to reach 190.10.0.0 is (300,200).
The same applies for 170.10.0.0 and 180.10.0.0. RTB will have to take path (300,100), such as traverse
AS300 and then AS100 in order to reach 170.10.0.0. RTC will have to traverse path (200) in order to reach
190.10.0.0 and path (100) in order to reach 170.10.0.0.
Origin Attribute
The origin is a mandatory attribute that defines the origin of the path information. The origin attribute can
assume three values:
IGP: Network Layer Reachability Information (NLRI) is interior to the originating AS. This normally happens
when we use the bgp network command or when IGP is redistributed into BGP, then the origin of the path
info will be IGP. This is indicated with an "i" in the BGP table.
EGP: NLRI is learned via EGP (Exterior Gateway Protocol). This is indicated with an "e" in the BGP table.
INCOMPLETE: NLRI is unknown or learned via some other means. This usually occurs when we redistribute
a static route into BGP and the origin of the route will be incomplete. This is indicated with an "?" in the BGP
table.
RTA will reach 170.10.0.0 via: 300 i (which means the next AS path is 300 and the origin of the route is IGP).
RTA will also reach 190.10.50.0 via: i (which means, the entry is in the same AS and the origin is IGP). RTE
will reach 150.10.0.0 via: 100 i (the next AS is 100 and the origin is IGP). RTE will also reach 190.10.0.0 via:
100 ? (the next AS is 100 and the origin is incomplete "?", coming from a static route).
BGP Next Hop Attribute
The BGP next hop attribute is the next hop IP address that is going to be used to reach a certain destination.
For eBGP, the next hop is always the IP address of the neighbor specified in the neighbor command. In the
above example, RTC will advertise 170.10.0.0 to RTA with a next hop of 170.10.20.2 and RTA will advertise
150.10.0.0 to RTC with a next hop of 170.10.20.1. For IBGP, the protocol states that the next hop advertised
by EBGP should be carried into IBGP. Because of that rule, RTA will advertise 170.10.0.0 to its IBGP peer
RTB with a next hop of 170.10.20.2. So according to RTB, the next hop to reach 170.10.0.0 is 170.10.20.2
and not 150.10.30.1.
You should make sure that RTB can reach 170.10.20.2 via IGP, otherwise RTB will drop packets destined to
170.10.0.0 because the next hop address would be inaccessible. For example, if RTB is running IGRP you
could also run igrp on RTA network 170.10.0.0. You would want to make IGRP passive on the link to RTC
so BGP is only exchanged.
*RTC will advertise 170.10.0.0 to RTA with a next hope = 170.10.20.2
*RTA will advertise 170.10.0.0 to RTB with a next hope = 170.10.20.2 (The eBGP next hop is carried in
iBGP.)
Special care should be taken when dealing with multiaccess and NBMA networks as described in the
following sections.
BGP Next Hop (Multiaccess Networks)
The following example shows how the next hop will behave on a multiaccess network such as Ethernet.
Assume that RTC and RTD in AS300 are running OSPF. RTC is running BGP with RTA. RTC can reach
network 180.20.0.0 via 170.10.20.3. When RTC sends a BGP update to RTA regarding 180.20.0.0 it will use
as next hop 170.10.20.3 and not its own IP address (170.10.20.2). This is because the network between RTA,
RTC and RTD is a multiaccess network and it makes more sense for RTA to use RTD as a next hop to reach
180.20.0.0 rather than making an extra hop via RTC.
*RTC will advertise 180.20.0.0 to RTA with a next hop 170.10.20.3.
If the common media to RTA, RTC and RTD was not multiaccess, but NBMA (Non Broadcast Media
Access) then further complications will occur.
BGP Next Hop (NBMA)
If the common media as you see in the shaded area above is a frame relay or any NBMA cloud then the exact
behavior will occur as if we were connected via Ethernet. RTC will advertise 180.20.0.0 to RTA with a next
hop of 170.10.20.3.
The problem is that RTA does not have a direct PVC to RTD, and cannot reach the next hop. In this case
routing will fail.
In order to remedy this situation a command called next-hop-self is created.
The next-hop-self Command
Because of certain situations with the next hop as we saw in the previous example, a command called
next-hop-self is created. The syntax is:
RTC advertises 180.20.0.0 with a next hop = 170.10.20.2
BGP Backdoor
Consider the above diagram, RTA and RTC are running eBGP, and RTB and RTC are running eBGP. RTA
and RTB are running some kind of IGP (RIP, IGRP, and so on). By definition, eBGP updates have a distance
of 20 which is lower than the IGP distances. Default distance is 120 for RIP, 100 for IGRP, 90 for EIGRP,
and 110 for OSPF.
RTA will receive updates about 160.10.0.0 via two routing protocols: eBGP with a distance of 20 and IGP
with a distance higher than 20.
By default, BGP has the following distances, but that could be changed by the distance command:
RTA will pick eBGP via RTC because of the lower distance.
If we want RTA to learn about 160.10.0.0 via RTB (IGP), then we have two options:
Change eBGP's external distance or IGP's distance, which is not recommended.
Use BGP backdoor.
BGP backdoor makes the IGP route the preferred route.
Use the following network address backdoor command.
The configured network is the network that we would like to reach via IGP. For BGP this network will be
treated as a locally assigned network except it will not be advertised in BGP updates.
Network 160.10.0.0 is treated as a local entry, but is not advertised as a normal network entry.
RTA learns 160.10.0.0 from RTB via EIGRP with distance 90, and also learns it from RTC via eBGP with
distance 20. Normally eBGP is preferred, but because of the backdoor command EIGRP is preferred.
Synchronization
Before we discuss synchronization let us look at the following scenario. RTC in AS300 is sending updates
about 170.10.0.0. RTA and RTB are running iBGP, so RTB will get the update and will be able to reach
170.10.0.0 via next hop 2.2.2.1 (remember that the next hop is carried via iBGP). In order to reach the next
hop, RTB will have to send the traffic to RTE.
Assume that RTA has not redistributed network 170.10.0.0 into IGP, so at this point RTE has no idea that
170.10.0.0 even exists.
If RTB starts advertising to AS400 that he can reach 170.10.0.0 then traffic coming from RTD to RTB with
destination 170.10.0.0 will flow in and get dropped at RTE.
Synchronization states: If your autonomous system is passing traffic from another AS to a third AS, BGP
should not advertise a route before all routers in your AS have learned about the route via IGP. BGP will wait
until IGP has propagated the route within the AS and then will advertise it to external peers. This is called
synchronization.
In the above example, RTB will wait to hear about 170.10.0.0 via IGP before it starts sending the update to
RTD. We can fool RTB into thinking that IGP has propagated the information by adding a static route in RTB
pointing to 170.10.0.0. Care should be taken to make sure that other routers can reach 170.10.0.0 otherwise
we will have a problem reaching that network.
Disabling Synchronization
In some cases you do not need synchronization. If you will not be passing traffic from a different autonomous
system through your AS, or if all routers in your AS will be running BGP, you can disable synchronization.
Disabling this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more
quickly.
Disabling synchronization is not automatic, if you have all your routers in the AS running BGP and you are
not running any IGP, the router has no way of knowing that, and your router will be waiting forever for an
IGP update about a certain route before sending it to external peers. You have to disable synchronization
manually in this case for routing to work correctly:
router bgp 100
no synchronization
(Make sure you do a clear ip bgp address to reset the session.)
The weight attribute is a Cisco defined attribute. The weight is used for a best path selection process. The
weight is assigned locally to the router. It is a value that only makes sense to the specific router and which is
not propagated or carried through any of the route updates. A weight can be a number from 0 to 65535. Paths
that the router originates have a weight of 32768 by default and other paths have a weight of zero.
Routes with a higher weight are preferred when multiple routes exist to the same destination. Let us study the
above example. RTA has learned about network 175.10.0.0 from AS4 and will propagate the update to RTC.
RTB has also learned about network 175.10.0.0 from AS4 and will propagate it to RTC. RTC has now two
ways for reaching 175.10.0.0 and has to decide which way to go. If on RTC we can set the weight of the
updates coming from RTA to be higher than the weight of updates coming from RTB, then we will force RTC
to use RTA as a next hop to reach 175.10.0.0. This is achieved by using multiple methods:
The same outcome as above can be achieved by using route maps.
RTC#
router bgp 300
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 route-map setweightin in
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 route-map setweightin in
...
ip as-path access-list 5 permit ^100$
...
route-map setweightin permit 10
match as-path 5
set weight 200
!-- Anything that applies to access-list 5, such as packets from AS100, have weight 200
route-map setweightin permit 20
set weight 100
!-- Anything else would have weight 100
Local Preference Attribute
Local preference is an indication to the AS about which path is preferred to exit the AS in order to reach a
certain network. A path with a higher local preference is more preferred. The default value for local
preference is 100.
Unlike the weight attribute which is only relevant to the local router, local preference is an attribute that is
exchanged among routers in the same AS.
Local preference is set via the bgp default local-preference value command or with route maps as will be
demonstrated in the following example:
The bgp default local-preference command will set the local preference on the updates out of the router
going to peers in the same AS. In the above diagram, AS256 is receiving updates about 170.10.0.0 from two
different sides of the organization. Local preference will help us determine which way to exit AS256 in order
to reach that network. Let us assume that RTD is the preferred exit point. The following configuration will set
the local preference for updates coming from AS300 to 200 and those coming from AS100 to 150.
In the above configuration RTC will set the local preference of all updates to 150. The same RTD will set the
local preference of all updates to 200. Since local preference is exchanged within AS256, both RTC and RTD
will realize that network 170.10.0.0 has a higher local preference when coming from AS300 rather than when
coming from AS100. All traffic in AS256 addressed to that network will be sent to RTD as an exit point.
More flexibility is provided by using route maps. In the above example, all updates received by RTD will be
tagged with local preference 200 when they reach RTD. This means that updates coming from AS34 will also
be tagged with the local preference of 200. This might not be needed. This is why we can use route maps to
specify what specific updates need to be tagged with a specific local preference as shown below:
route-map setlocalin permit 10
match as-path 7
set local-preference 200
route-map setlocalin permit 20
set local-preference 150
With this configuration, any update coming from AS300 will be set with a local preference of 200. Any other
updates such as those coming from AS34 will be set with a value of 150.
Metric Attribute
The metric attribute which is also called MULTI_EXIT_DISCRIMINATOR, MED (BGP4) or INTER_AS
(BGP3) is a hint to external neighbors about the preferred path into an AS. This is a dynamic way to influence
another AS on which way to choose in order to reach a certain route given that we have multiple entry points
into that AS. A lower value of a metric is more preferred.
Unlike local preference, metric is exchanged between ASs. A metric is carried into an AS but does not leave
the AS. When an update enters the AS with a certain metric, that metric is used for decision making inside the
AS. When the same update is passed on to a third AS, that metric will be set back to 0 as shown in the above
diagram. The Metric default value is 0.
Unless otherwise specified, a router will compare metrics for paths from neighbors in the same AS. In order
for the router to compare metrics from neighbors coming from different ASs the special configuration
command bgp always-compare-med should be configured on the router.
Note: There are two BGP configuration commands that can influence the Multi Exit Discriminator
(MED)-based path selection, the bgp deterministic-med and the bgp always-compare-med commands.
Enabling the bgp deterministic-med command ensures the comparison of the MED variable when choosing
routes advertised by different peers in the same autonomous system (AS). Enabling the bgp
always-compare-med command ensures the comparison of the MED for paths from neighbors in different
autonomous systems. The bgp always-compare-med command is useful when multiple service providers or
enterprises agree on a uniform policy for setting MED. Refer to How the bgp deterministic-med Command
Differs from the bgp always-compare-med Command to understand how BGP path selection is influenced by
these commands.
In the above diagram, AS100 is getting information about network 180.10.0.0 via three different routers:
RTC, RTD and RTB. RTC and RTD are in AS300 and RTB is in AS400.
Assume that we have set the metric coming from RTC to 120, the metric coming from RTD to 200 and the
metric coming from RTB to 50. Given that by default a router compares metrics coming from neighbors in the
same AS, RTA can only compare the metric coming from RTC to the metric coming from RTD and will pick
RTC as the best next hop because 120 is less than 200. When RTA gets an update from RTB with metric 50,
he can not compare it to 120 because RTC and RTB are in different ASs (RTA has to choose based on some
other attributes).
In order to force RTA to compare the metrics we have to add bgp always-compare-med to RTA. This is
illustrated in the configs below:
With the above configs, RTA will pick RTC as next hop, considering all other attributes are the same. In order
to have RTB included in the metric comparison, we have to configure RTA as follows:
!--- Causes RTB to send out 180.10.0.0 with a metric of 50
Community Attribute
The community attribute is a transitive, optional attribute in the range 0 to 4,294,967,200. The community
attribute is a way to group destinations in a certain community and apply routing decisions (accept, prefer,
redistribute, etc.) according to those communities.
We can use route maps to set the community attributes. The route map set command has the following syntax:
set community community-number [additive]
A few predefined well known communities (community-number) are:
no-export (Do not advertise to EBGP peers; keep this route within an AS)
no-advertise (Do not advertise this route to any peer, internal or external)
internet (Advertise this route to the internet community, any router belongs to it)
local-AS (Use in confederation scenarios to prevent sending packets outside the local AS).
An example of route maps where community is set is:
route-map communitymap
match ip address 1
set community no-advertise
or
route-map setcommunity
match as-path 1
set community 200 additive
If the additive keyword is not set, 200 replaces any old community that already exits; if we use the keyword
additive then the 200 is added to the community. Even if we set the community attribute, this attribute is not
sent to neighbors by default. In order to send the attribute to our neighbor we have to use the following:
In Cisco IOS Software release 12.0 and later, you can configure communities in three different formats:
decimal, hexadecimal, and AA:NN. By default, IOS uses the older decimal format. To configure and display
in AA:NN, where the first part is the AS number and the second part is a 2-byte number, use the ip
bgp-community new-format global configuration command.
Here's an example.
Without the ip bgp-community new-format command in global configuration, the show ip bgp 6.0.0.0
command below displays the community attribute value in decimal format (6553620).
Router# show ip bgp 6.0.0.0
BGP routing table entry for 6.0.0.0/8, version 7
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
1
10.10.10.1 from 10.10.10.1 (200.200.200.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: 6553620
Let us configure the ip bgp-community new-format command globally on this router.
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# ip bgp-community new-format
Router(config)# exit
With the ip bgp-community new-format global configuration command, the community value is displayed
in AA:NN format (100:20) in the output of the show ip bgp 6.0.0.0 command below.
Router# show ip bgp 6.0.0.0
BGP routing table entry for 6.0.0.0/8, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
1
10.10.10.1 from 10.10.10.1 (200.200.200.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: 100:20
BGP Case Studies 3
BGP Filtering
Sending and receiving BGP updates can be controlled by using a number of different filtering methods. BGP
updates can be filtered based on route information, on path information or on communities. All methods will
achieve the same results, choosing one over the other depends on the specific network configuration.
Route Filtering
In order to restrict the routing information that the router learns or advertises, you can filter BGP based on
routing updates to or from a particular neighbor. In order to achieve this, an access-list is defined and applied
to the updates to or from a neighbor. Use the following command in the router configuration mode:
In the following example, RTB is originating network 160.10.0.0 and sending it to RTC. If RTC wanted to
stop those updates from propagating to AS100, we would have to apply an access-list to filter those updates
and apply it when talking to RTA:
!-- Filter out all routing updates about 160.10.x.x
Using access lists is a bit tricky when you are dealing with supernets that might cause some conflicts.
Assume in the above example that RTB has different subnets of 160.10.X.X and our goal is to filter updates
and advertise only 160.0.0.0/8 (this notation means that we are using 8 bits of subnet mask starting from the
far left of the IP address; this is equivalent to 160.0.0.0 255.0.0.0).
The command access-list 1 permit 160.0.0.0 0.255.255.255 permits 160.0.0.0/8,160.0.0.0/9 and so on. In
order to restrict the update to only 160.0.0.0/8 we have to use an extended access list of the following format:
access-list 101 permit ip 160.0.0.0 0.255.255.255 255.0.0.0 0.0.0.0. This list permits 160.0.0.0/8 only.
Refer to How to Block One or More Networks From a BGP Peer for sample configurations on how to filter
networks from BGP peers using distribute-list with standard and extended ACLs, as well as prefix list
filtering.
Another type of filtering is path filtering, which is described in the next section.
Path Filtering
You can specify an access list on both incoming and outgoing updates based on the BGP autonomous system
paths information. In the above figure we can block updates about 160.10.0.0 from going to AS100 by
defining an access list on RTC that prevents any updates that have originated from AS200 from being sent to
AS100. To do this use the following statements.
ip as-path access-list access-list-number {permit|deny} as-regular-expression
ip as-path access-list 1 deny ^200$
ip as-path access-list 1 permit .*
In the above example, access-list 1 states: deny any updates with path information that start with 200 (^) and
end with 200 ($). The ^200$ is called a regular expression, with ^ meaning "starts with" and $ meaning "ends
with". Since RTB sends updates about 160.10.0.0 with path information starting with 200 and ending with
200, this update matches the access list and will be denied.
The .* is another regular expression with the period meaning "any character" and the * meaning "the
repetition of that character". So .* is actually any path information, which is needed to permit all other updates
to be sent.
What would happen if instead of using ^200$ we have used ^200? If you have an AS400 (see figure above),
updates originated by AS400 will have path information of the form (200, 400) with 200 being first and 400
being last. Those updates will match the access list ^200 because they start with 200 and will be prevented
from being sent to RTA which is not the required behavior.
A good way to check whether we have implemented the correct regular expression is to use the show ip bgp
regexp regular expression command. This shows all the paths that have matched the configured regular
expression.
The next section explains what is involved in creating a regular expression.
AS Regular Expression
A regular expression is a pattern to match against an input string. By building a regular expression we specify
a string that input must match. In case of BGP we are specifying a string consisting of path information that
an input should match.
In the previous example we specified the string ^200$ and wanted path information coming inside updates to
match it in order to perform a decision.
The regular expression is composed of the following:
Range
A range is a sequence of characters contained within left and right square brackets. For example: [abcd]
Atom
An atom is a single character, such as the following:
. (Matches any single character)
^ (Matches the beginning of the input string)
$ (Matches the end of the input string)
\ (Matches the character)
- (Matches a comma (,), left brace ({), right brace (}), the beginning of the input string, the end of the input
string, or a space.
Piece
A piece is an atom followed by one of the following symbols:
* (Matches 0 or more sequences of the atom)
+ (Matches 1 or more sequences of the atom)
? (Matches the atom or the null string)
Branch
A branch is a 0 or more concatenated pieces.
Examples of regular expressions follow:
a* (Any occurrence of the letter "a", including none)
a+ ( At least one occurrence of the letter "a" should be present)
ab?a (This matches "aa" or "aba")
_100_ (Via AS100)
_100$ (Origin AS100)
^100 .* (Coming from AS100)
^$ (Originated from this AS)
Refer to Using Regular Expressions in BGP for sample configurations on regular expression filtering.
BGP Community Filtering
We have already seen route filtering and as-path filtering. Another method is community filtering.
Community has been discussed previously and here are few examples of how we can use it.
We would like RTB above to set the community attribute to the BGP routes it is advertising such that RTC
would not propagate these routes to its external peers. The no-export community attribute is used:
RTB#
router bgp 200
network 160.10.0.0
neighbor 3.3.3.1 remote-as 300
neighbor 3.3.3.1 send-community
neighbor 3.3.3.1 route-map setcommunity out
route-map setcommunity
match ip address 1
set community no-export
access-list 1 permit 0.0.0.0 255.255.255.255
Note that we have used the route-map setcommunity command in order to set the community to no-export.
Note also that we had to use the neighbor send-community command in order to send this attribute to RTC.
When RTC gets the updates with the attribute NO_EXPORT, it will not propagate them to its external peer
RTA.
In the example below, RTB has set the community attribute to 100 200 additive. The value 100 200 will be
added to any existing community value before being sent to RTC.
route-map setcommunity
match ip address 2
set community 100 200 additive
access-list 2 permit 0.0.0.0 255.255.255.255
A community list is a group of communities that we use in a match clause of a route map which allows us to
do filtering or setting attributes based on different lists of community numbers.
ip community-list community-list-number {permit|deny} community-number
For example we can define the following route map, match-on-community:
route-map match-on-community
match community 10
!-- 10 is the community-list number
set weight 20
ip community-list 10 permit 200 300
!-- 200 300 is the community number
We can use the above in order to filter or set certain parameters like weight and metric based on the
community value in certain updates. In example two above, RTB was sending updates to RTC with a
community of 100 200. If RTC wants to set the weight based on those values we could do the following:
route-map check-community permit 10
match community 1
set weight 20
route-map check-community permit 20
match community 2 exact
set weight 10
route-map check-community permit 30
match community 3
ip community-list 1 permit 100
ip community-list 2 permit 200
ip community-list 3 permit internet
In the above example, any route that has 100 in its community attribute will match list 1 and will have the
weight set to 20. Any route that has only 200 as community will match list 2 and will have weight 20. The
keyword exact states that community should consist of 200 only and nothing else. The last community list is
here to make sure that other updates are not dropped. Remember that anything that does not match, will be
dropped by default. The keyword internet means all routes because all routes are members of the internet
community.
Refer to Using BGP Community Values to Control Routing Policy in an Upstream Provider Network for
more information.
BGP Neighbors and Route Maps
The neighbor command can be used in conjunction with route maps to perform either filtering or parameter
setting on incoming and outgoing updates.
Route maps associated with the neighbor statement have no affect on incoming updates when matching based
on the IP address:
neighbor ip-address route-map route-map-name
Assume in the above diagram we want RTC to learn from AS200 about networks that are local to AS200 and
nothing else. Also, we want to set the weight on the accepted routes to 20. We can achieve this with a
combination of neighbor and as-path access lists.
Any updates that originate from AS200 have a path information that starts with 200 and ends with 200 and
will be permitted. Any other updates will be dropped.
Assume that we want the following:
Updates originating from AS200 to be accepted with weight 20.
route-map stamp permit 10
match as-path 1
set weight 20
route-map stamp permit 20
match as-path 2
set weight 10
ip as-path access-list 1 permit ^200$
ip as-path access-list 2 permit ^200 600 .*
The above statement will set a weight of 20 for updates that are local to AS200, and will set a weight
of 10 for updates that are behind AS400 and will drop updates coming from AS400.
Use of set as-path prepend Command
In some situations we are forced to manipulate the path information in order to manipulate the BGP decision
process. The command that is used with a route map is:
set as-path prepend ...
Suppose in the above diagram that RTC is advertising its own network 170.10.0.0 to two different ASs:
AS100 and AS200. When the information is propagated to AS600, the routers in AS600 will have network
reachability information about 150.10.0.0 via two different routes, the first route is via AS100 with PATH
(100, 300) and the second one is via AS400 with PATH (400, 200,300). Assuming that all other attributes are
the same AS600 will pick the shortest path and will choose the route via AS100.
AS300 will be getting all its traffic via AS100. If we want to influence this decision from the AS300 end we
can make the PATH through AS100 look like it is longer than the PATH going through AS400. We can do
this by prepending autonomous system numbers to the existing path info advertised to AS100. A common
practice is to repeat our own AS number using the following:
Because of the above configuration, AS600 will receive updates about 170.10.0.0 via AS100 with a PATH
information of: (100, 300, 300, 300) which is longer than (400, 200, 300) received from AS100.
BGP Peer Groups
A BGP peer group, is a group of BGP neighbors with the same update policies. Update policies are usually set
by route maps, distribute-lists and filter-lists, etc. Instead of defining the same policies for each separate
neighbor, we define a peer group name and we assign these policies to the peer group.
Members of the peer group inherit all of the configuration options of the peer group. Members can also be
configured to override these options if these options do not affect outbound updates; you can only override
options set on the inbound.
To define a peer group use the following:
neighbor peer-group-name peer-group
In the following example we will see how peer groups are applied to internal and external BGP neighbors.
In the above configuration, we have defined a peer group named internalmap and we have defined some
policies for that group, such as a route map SETMETRIC to set the metric to 5 and two different filter lists 1
and 2. We have applied the peer group to all internal neighbors RTE, RTF and RTG. We have defined a
separate filter-list 3 for neighbor RTE, and this will override filter-list 2 inside the peer group. Note that we
could only override options that affect inbound updates.
Now, let us look at how we can use peer groups with external neighbors. In the same diagram we will
configure RTC with a peer-group externalmap and we will apply it to external neighbors.
Note that in the above configs we have defined the remote-as statements outside of the peer group because
we have to define different external ASs. Also we did an override for the inbound updates of neighbor 1.1.1.2
by assigning filter-list 3.
For more information on peer groups, refer to BGP Peer Groups.
Note: In Cisco IOS 12.0(24)S and later, Cisco introduced BGP Dynamic Update Peer Groups feature. This
feature introduces a new algorithm that dynamically calculates and optimizes update groups of neighbors that
share the same outbound policies and can share the same update messages. In previous versions of Cisco IOS
software, BGP update messages were grouped together based on peer group configurations. This method of
grouping updates limited outbound policies and specific session configurations. The BGP Dynamic Update
Peer Group feature separates update group replication from peer group configuration, which improves
convergence time and flexibility of neighbor configuration. Refer to BGP Dynamic Update Peer-Groups for
more details.
BGP Case Studies 4
CIDR and Aggregate Addresses
One of the main enhancements of BGP4 over BGP3 is Classless Interdomain Routing (CIDR). CIDR or
supernetting is a new way of looking at IP addresses. There is no notion of classes anymore (class A, B or C).
For example, network 192.213.0.0 which used to be an illegal class C network is now a legal supernet
represented by 192.213.0.0/16 where the 16 is the number of bits in the subnet mask counting from the far left
of the IP address. This is similar to 192.213.0.0 255.255.0.0.
Aggregates are used to minimize the size of routing tables. Aggregation is the process of combining the
characteristics of several different routes in such a way that a single route can be advertised. In the example
below, RTB is generating network 160.10.0.0. We will configure RTC to propagate a supernet of that route
160.0.0.0 to RTA.
RTC will propagate the aggregate address 160.0.0.0 to RTA.
Aggregate Commands
There is a wide range of aggregate commands. It is important to understand how each one works in order to
have the desired aggregation behavior.
The first command is the one used in the previous example:
aggregate-address address mask
This will advertise the prefix route, and all of the more specific routes. The command aggregate-address
160.0.0.0 will propagate an additional network 160.0.0.0 but will not prevent 160.10.0.0 from being also
propagated to RTA. The outcome of this is that both networks 160.0.0.0 and 160.10.0.0 have been propagated
to RTA. This is what we mean by advertising the prefix and the more specific route.
Please note that you can not aggregate an address if you do not have a more specific route of that address in
the BGP routing table.
For example, RTB can not generate an aggregate for 160.0.0.0 if it does not have a more specific entry of
160.0.0.0 in its BGP table. The more specific route could have been injected into the BGP table via incoming
updates from other ASs, from redistributing an IGP or static into BGP or via the network command (network
160.10.0.0).
In case we would like RTC to propagate network 160.0.0.0 only and NOT the more specific route then we
would have to use the following:
aggregate-address address mask summary-only
This will a advertise the prefix only; all the more specific routes are suppressed.
The command aggregate 160.0.0.0 255.0.0.0 summary-only will propagate network 160.0.0.0 and will
suppress the more specific route 160.10.0.0.
Please note that if we are aggregating a network that is injected into our BGP via the network statement (ex:
network 160.10.0.0 on RTB) then the network entry is always injected into BGP updates even though we are
using "the aggregate summary-only" command. The upcoming CIDR example discusses this situation.
aggregate-address address mask as-set
This advertises the prefix and the more specific routes but it includes as-set information in the path
information of the routing updates.
aggregate 129.0.0.0 255.0.0.0 as-set
This command will be discussed in an example by itself in the following sections.
In case we would like to suppress more specific routes when doing the aggregation we can define a route map
and apply it to the aggregates. This will allow us to be selective about which more specific routes to suppress.
This advertises the prefix and the more specific routes but it suppresses advertisement according to a
route-map. In the previous diagram, if we would like to aggregate 160.0.0.0 and suppress the more specific
route 160.20.0.0 and allow 160.10.0.0 to be propagated, we can use the following route map:
This allows us to set the attributes (such as metric) when aggregates are sent out. The following route map
when applied to the aggregate attribute-map command will set the origin of the aggregates to IGP.
For more information, refer to Understanding Route Aggregation in BGP.
CIDR Example 1
Request: Allow RTB to advertise the prefix 160.0.0.0 and suppress all the more specific routes. The problem
here is that network 160.10.0.0 is local to AS200, meaning AS200 is the originator of 160.10.0.0. You cannot
have RTB generate a prefix for 160.0.0.0 without generating an entry for 160.10.0.0 even if you use the
aggregate summary-only command because RTB is the originator of 160.10.0.0. There are two solutions to
this problem.
The first solution is to use a static route and redistribute it into BGP. The outcome is that RTB will advertise
the aggregate with an origin of incomplete (?).
!-- This generates an update for 160.0.0.0
!-- with the origin path as *incomplete*
ip route 160.0.0.0 255.0.0.0 null0
In the second solution, in addition to the static route we add an entry for the network command. This has the
same effect except that the origin of the update will be set to IGP.
AS-SETS are used in aggregation to reduce the size of the path information by listing the AS number only
once, regardless of how many times it may have appeared in multiple paths that were aggregated. The as-set
aggregate command is used in situations were aggregation of information causes loss of information regarding
the path attribute. In the following example RTC is getting updates about 160.20.0.0 from RTA and updates
about 160.10.0.0 from RTB. Suppose RTC wants to aggregate network 160.0.0.0/8 and send it to RTD. RTD
would not know what the origin of that route is. By adding the aggregate as-set statement we force RTC to
generate path information in the form of a set {}. All the path information is included in that set irrespective
of which path came first.
RTC does not have an as-set statement. RTC will send an update 160.0.0.0/8 to RTD with path information
(300) as if the route has originated from AS300.
!-- This causes RTC to send RTD updates about 160.0.0.0/8 with no indication
!-- that 160.0.0.0 is actually coming from two different autonomous
!-- systems, this may create loops if RT4 has an entry back into AS100
.
Case 2:
!-- Causes RTC to send RTD updates about 160.0.0.0/8 with an
!-- indication that 160.0.0.0 belongs to a set {100 200}.
The next two subjects, confederation and route reflectors, are designed for ISPs who would like to further
control the explosion of iBGP peering inside their autonomous systems.
BGP Confederation
BGP confederation is implemented in order to reduce the iBGP mesh inside an AS. The trick is to divide an
AS into multiple ASs and assign the whole group to a single confederation. Each AS by itself will have iBGP
fully meshed and has connections to other AS's inside the confederation. Even though these ASs will have
EBGP peers to ASs within the confederation, they exchange routing as if they were using iBGP; next hop,
metric and local preference information are preserved. To the outside world, the confederation (the group of
ASs) will look like a single AS.
To configure a BGP confederation use the following:
bgp confederation identifier autonomous-system
The confederation identifier will be the AS number of the confederation group. The group of ASs will look to
the outside world as one AS with the AS number being the confederation identifier.
Peering within the confederation between multiple ASs is done via the following command:
Let us assume that you have an autonomous system 500 consisting of nine BGP speakers (other non BGP
speakers exist also, but we are only interested in the BGP speakers that have EBGP connections to other ASs).
If you want to make a full iBGP mesh inside AS500 then you would need nine peer connections for each
router, 8 iBGP peers and one EBGP peer to external ASs.
By using confederation we can divide AS500 into multiple ASs: AS50, AS60 and AS70. We give the AS a
confederation identifier of 500. The outside world will see only one AS500. For each AS50, AS60 and AS70
we define a full mesh of iBGP peers and we define the list of confederation peers using the bgp confederation
peers command.
Let us look at a sample configuration of routers RTC, RTD and RTA. Note that RTA has no knowledge of
ASs 50, 60 or 70. RTA has only knowledge of AS500.
Another solution for the explosion of iBGP peering within an autonomous system is Route Reflectors (RR).
As demonstrated in the Internal BGP section, a BGP speaker will not advertise a route learned via another
iBGP speaker to a third iBGP speaker. By relaxing this restriction a bit and by providing additional control,
we can allow a router to advertise (reflect) iBGP learned routes to other iBGP speakers. This will reduce the
number of iBGP peers within an AS.
In normal cases, a full iBGP mesh should be maintained between RTA, RTB and RTC within AS100. By
utilizing the route reflector concept, RTC could be elected as a RR and have a partial iBGP peering with RTA
and RTB. Peering between RTA and RTB is not needed because RTC will be a route reflector for the updates
coming from RTA and RTB.
neighbor route-reflector-client
The router with the above command would be the RR and the neighbors pointed at would be the clients of that
RR. In our example, RTC would be configured with the neighbor route-reflector-client command pointing
at RTA and RTB's IP addresses. The combination of the RR and its clients is called a cluster. RTA, RTB and
RTC above would form a cluster with a single RR within AS100.
Other iBGP peers of the RR that are not clients are called non-clients.
An autonomous system can have more than one route reflector; a RR would treat other RRs just like any other
iBGP speaker. Other RRs could belong to the same cluster (client group) or to other clusters. In a simple
configuration, the AS could be divided into multiple clusters, each RR will be configured with other RRs as
non-client peers in a fully meshed topology. Clients should not peer with iBGP speakers outside their cluster.
Consider the above diagram. RTA, RTB and RTC form a single cluster with RTC being the RR. According to
RTC, RTA and RTB are clients and anything else is a non-client. Remember that clients of an RR are pointed
at using the neighbor route-reflector-client command. The same RTD is the RR for its clients RTE and
RTF; RTG is a RR in a third cluster. Note that RTD, RTC and RTG are fully meshed but routers within a
cluster are not. When a route is received by a RR, it will do the following depending on the peer type:
Route from a non-client peer: reflect to all the clients within the cluster.
Route from a client peer: reflect to all the non-client peers and also to the client peers.
Route from an EBGP peer: send the update to all client and non-client peers.
The following is the relative BGP configuration of routers RTC, RTD and RTB:
As the iBGP learned routes are reflected, it is possible to have the routing information loop. The Route
Reflector scheme has a few methods to avoid this:
Originator-id: This is an optional, non transitive BGP attribute that is four bytes long and is created
by a RR. This attribute will carry the router-id (RID) of the originator of the route in the local AS.
Thus, due to poor configuration, if the routing information comes back to the originator, it will be
ignored.
Cluster-list: This will be discussed in the next section.
Multiple RRs within a Cluster
Usually, a cluster of clients will have a single RR. In this case, the cluster will be identified by the router ID of
the RR. In order to increase redundancy and avoid single points of failure, a cluster might have more than one
RR. All RRs in the same cluster need to be configured with a 4 byte cluster-id so that a RR can recognize
updates from RRs in the same cluster.
A cluster list is a sequence of cluster IDs that the route has passed. When a RR reflects a route from its clients
to non-clients outside of the cluster, it will append the local cluster ID to the cluster list. If this update has an
empty cluster list the RR will create one. Using this attribute, a RR can identify if the routing information is
looped back to the same cluster due to poor configuration. If the local cluster ID is found in the cluster list, the
advertisement will be ignored.
In the above diagram, RTD, RTE, RTF and RTH belong to one cluster with both RTD and RTH being RRs
for the same cluster. Note the redundancy in that RTH has a fully meshed peering with all the RRs. In case
RTD goes down, RTH will take its place. The following are the configuration of RTH, RTD, RTF and RTC:
Note that we did not need the cluster command for RTC because only one RR exists in that cluster.
An important thing to note, is that peer groups were not used in the above configuration. If the clients inside a
cluster do not have direct iBGP peers among one another and they exchange updates through the RR, peer
goups should not be used. If peer groups were to be configured, then a potential withdrawal to the source of a
route on the RR would be sent to all clients inside the cluster and could cause problems.
The router subcommand bgp client-to-client reflection is enabled by default on the RR. If BGP
client-to-client reflection were turned off on the RR and redundant BGP peering was made between the
clients, then using peer groups would be alright.
RR and Conventional BGP Speakers
It is normal in an AS to have BGP speakers that do not understand the concept of route reflectors. We will call
these routers conventional BGP speakers. The route reflector scheme will allow such conventional BGP
speakers to coexist. These routers could be either members of a client group or a non-client group. This
would allow easy and gradual migration from the current iBGP model to the route reflector model. One could
start creating clusters by configuring a single router as RR and making other RRs and their clients normal
iBGP peers. Then more clusters could be created gradually.
In the above diagram, RTD, RTE and RTF have the concept of route reflection. RTC, RTA and RTB are what
we call conventional routers and cannot be configured as RRs. Normal iBGP mesh could be done between
these routers and RTD. Later on, when we are ready to upgrade, RTC could be made a RR with clients RTA
and RTB. Clients do not have to understand the route reflection scheme; it is only the RRs that would have to
be upgraded.
The following is the configuration of RTD and RTC:
router bgp 100
neighbor 4.4.4.4 remote-as 100
neighbor 2.2.2.2 remote-as 100
neighbor 1.1.1.1 remote-as 100
neighbor 14.14.14.14 remote-as 400
When we are ready to upgrade RTC and make it a RR, we would remove the iBGP full mesh and have RTA
and RTB become clients of RTC.
Avoiding Looping of Routing Information
We have mentioned so far two attributes that are used to prevent potential information looping: originator-id
and cluster-list.
Another means of controlling loops is to put more restrictions on the set clause of out-bound route-maps. The
set clause for out-bound route-maps does not affect routes reflected to iBGP peers.
More restrictions are also put on nexthop-self, which is a per neighbor configuration option. When used on
RRs the nexthop-self will only affect the next hop of EBGP learned routes because the next hop of reflected
routes should not be changed.
Route Flap Dampening
Route dampening (introduced in Cisco IOS version 11.0) is a mechanism to minimize the instability caused
by route flapping and oscillation over the network. To accomplish this, criteria are defined to identify poorly
behaved routes. A route which is flapping gets a penalty for each flap (1000). As soon as the cumulative
penalty reaches a predefined "suppress-limit", the advertisement of the route will be suppressed. The penalty
will be exponentially decayed based on a preconfigured "half-time". Once the penalty decreases below a
predefined "reuse-limit", the route advertisement will be un-suppressed.
Routes, external to an AS, learned via iBGP will not be dampened. This is to avoid the iBGP peers having
higher penalty for routes external to the AS.
The penalty will be decayed at a granularity of 5 seconds and the routes will be un-suppressed at a granularity
of 10 seconds. The dampening information is kept until the penalty becomes less than half of "reuse-limit" ,
at that point the information is purged from the router.
Initially, dampening will be off by default. This might change if there is a need to have this feature enabled by
default. The following are the commands used to control route dampening:
bgp dampening (will turn on dampening) ·
no bgp dampening (will turn off dampening) ·
bgp dampening half-life-time (will change the half-life-time) ·
A command that sets all parameters at the same time is:
RTB is configured for route dampening with default parameters. Assuming the EBGP link to RTD is stable,
RTB's BGP table would look like this:
RTB#show ip bgp
BGP table version is 24, local router ID is 203.250.15.2 Status codes: s
suppressed, d damped, h history, * valid, > best, i - internal Origin
codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 192.208.10.0 192.208.10.5 0 0 300 i
*> 203.250.15.0 0.0.0.0 0 32768 i
In order to simulate a route flap, use clear ip bgp 192.208.10.6 on RTD. RTB's BGP table will look like this:
RTB# show ip bgp
BGP table version is 24, local router ID is 203.250.15.2 Status codes: s
suppressed, d damped, h history, * valid, > best, i - internal Origin
codes: i - IGP, e - EGP, ? - incomplete
;Network Next Hop Metric LocPrf Weight Path
h 192.208.10.0 192.208.10.5 0 0 300 i
*> 203.250.15.0 0.0.0.0 0 32768
The BGP entry for 192.208.10.0 has been put in a "history" state. Which means that we do not have a best
path to the route but information about the route flapping still exists.
RTB# show ip bgp 192.208.10.0
BGP routing table entry for 192.208.10.0 255.255.255.0, version 25
Paths: (1 available, no best path)
300 (history entry)
192.208.10.5 from 192.208.10.5 (192.208.10.174)
Origin IGP, metric 0, external
Dampinfo: penalty 910, flapped 1 times in 0:02:03
The route has been given a penalty for flapping but the penalty is still below the "suppress limit" (default is
2000). The route is not yet suppressed. If the route flaps few more times we will see the following:
RTB# show ip bgp
BGP table version is 32, local router ID is 203.250.15.2 Status codes:
s suppressed, d damped, h history, * valid, > best, i - internal Origin codes:
i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*d 192.208.10.0 192.208.10.5 0 0 300 i
*> 203.250.15.0 0.0.0.0 0 32768 i
RTB# show ip bgp 192.208.10.0
BGP routing table entry for 192.208.10.0 255.255.255.0, version 32
Paths: (1 available, no best path)
300, (suppressed due to dampening)
192.208.10.5 from 192.208.10.5 (192.208.10.174)
Origin IGP, metric 0, valid, external
Dampinfo: penalty 2615, flapped 3 times in 0:05:18 , reuse in 0:27:00
The route has been dampened (suppressed). The route will be reused when the penalty reaches the "reuse
value", in our case 750 (default).The dampening information will be purged when the penalty becomes less
than half of the reuse-limit, in our case (750/2=375). The following are the commands used to show and clear
flap statistics information:
show ip bgp flap-statistics (displays flap statistics for all the paths) ·
show ip bgp-flap-statistics regexp regexp (displays flap statistics for all paths that match the
regexp)
show ip bgp flap-statistics filter-list list (displays flap statistics for all paths that pass the filter) ·
show ip bgp flap-statistics A.B.C.D m.m.m.m (displays flap statistics for a single entry) ·
show ip bgp flap-statistics A.B.C.D m.m.m.m longer-prefixes (displays flap statistics for more
specific entries)
show ip bgp neighbor [dampened-routes] | [flap-statistics] (displays flap statistics for all paths
from a neighbor)
clear ip bgp flap-statistics (clears flap statistics for all routes) ·
clear ip bgp flap-statistics regexp regexp (clears flap statistics for all the paths that match the
regexp)
clear ip bgp flap-statistics filter-list list (clears flap statistics for all the paths that pass the filter) ·
clear ip bgp flap-statistics A.B.C.D m.m.m.m (clears flap statistics for a single entry) ·
clear ip bgp A.B.C.D flap-statistics (clears flap statistics for all paths from a neighbor)
How BGP Selects a Path
Now that we are familiar with the BGP attributes and terminology, refer to BGP Best Path Selection
Algorithm.
The following section contains a design example that shows the configuration and routing tables as they
actually appear on Cisco routers.
BGP Case Studies 5
Practical Design Example
We'll build the above configuration step by step and see what can go wrong along the way. Whenever you
have an AS that is connected to two ISPs via eBGP, it's always good to run iBGP within your AS in order to
have a better control of your routes. In this example we run iBGP inside AS100 between RTA and RTB, and
we run OSPF as an IGP. Assuming that we're connected to two ISPs, AS200 and AS300, the following is the
first run of the configurations for all the routers. These aren't the final configurations.
RTA#
hostname RTA
ip subnet-zero
interface Loopback0
ip address 203.250.13.41 255.255.255.0
RTE#
hostname RTE
ip subnet-zero
interface Loopback0
ip address 200.200.10.1 255.255.255.0
interface Serial0
ip address 195.211.10.2 255.255.255.252
interface Serial1
clockrate 1000000
router bgp 400
network 200.200.10.0
neighbor 128.213.63.5 remote-as 200
RTG#
hostname RTG
ip subnet-zero
interface Loopback0
ip address 195.211.10.174 255.255.255.192
interface Serial0
ip address 192.208.10.1 255.255.255.252
interface Serial1
ip address 195.211.10.1 255.255.255.252
router bgp 500
network 195.211.10.0
neighbor 192.208.10.2 remote-as 300
neighbor 195.211.10.2 remote-as 400
It's always better to use the network command or redistribute static entries into BGP to advertise networks,
rather than redistributing IGP into BGP. This is why, throughout this example I use the network command to
inject networks into BGP.
Let us start with the s1 interface on RTB shutdown, as if the link between RTB and RTD doesn't exist. The
following is RTB's BGP table.
RTB# show ip bgp BGP
table version is 4, local router ID is 203.250.15.2 Status
codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*i128.213.0.0 128.213.63.2 0 100 0 200 i
300 i
*i195.211.10.0 128.213.63.2 100 0 200 400 500 i
*i200.200.10.0 128.213.63.2 100 0 200 400 i
*>i203.250.13.0 203.250.13.41 0 100 0 i
*>i203.250.14.0 203.250.13.41 0 100 0 i
*>203.250.15.0 0.0.0.0 0 32768 i
Let me go over the basic notations of the above table. The "i" at the beginning means that the entry was
learned via an iBGP peer. The "i" at the end indicates the origin of the path information to be IGP. The path
info is intuitive. For example, network 128.213.0.0 is learned via path 200 with a next hop of 128.213.63.2.
Note that any locally generated entry, such as 203.250.15.0, has a next hop 0.0.0.0.
The > symbol indicates that BGP has chosen the best route based on the list of decision steps that I have gone
through earlier in this document under "How BGP selects a Path". BGP picks one best path to reach a
destination, installs it in the IP routing table and advertises it to other BGP peers. Notice the next hop
attribute. RTB knows about 128.213.0.0 via a next hop of 128.213.63.2, which is the eBGP next hop carried
into iBGP.
Let us look at the IP routing table:
RTB# show ip route
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
default
Gateway of last resort is not set
203.250.13.0 255.255.255.255 is subnetted, 1 subnets
O 203.250.13.41 [110/75] via 203.250.15.1, 02:50:45, Serial0
203.250.15.0 255.255.255.252 is subnetted, 1 subnets
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 02:50:46, Serial0
It doesn't look like any of the BGP entries has made it to the routing table. There are two problems here,
which we'll examine in turn.
The first problem is the next hop for these entries, 128.213.63.2, is unreachable. This is true because we don't
have a way to reach that next hop via our IGP (OSPF). RTB hasn't learned about 128.213.63.0 via OSPF. We
can run OSPF on RTA's s0 interface and make it passive, and this way RTB would know how to reach the
next hop 128.213.63.2. Doing this, RTA's configuration would be as shown below. We could also change the
next hop by using the bgp nexthopself command between RTA and RTB.
hostname RTA
ip subnet-zero
interface Loopback0
ip address 203.250.13.41 255.255.255.0
interface Ethernet0
ip address 203.250.14.1 255.255.255.0
interface Serial0
ip address 128.213.63.1 255.255.255.252
passive-interface Serial0
network 203.250.0.0 0.0.255.255 area 0
network 128.213.0.0 0.0.255.255 area 0
router bgp 100
network 203.250.0.0 mask 255.255.0.0
neighbor 128.213.63.2 remote-as 200
neighbor 203.250.15.2 remote-as 100
neighbor 203.250.15.2 update-source Loopback0
The new BGP table on RTB now looks like this:
RTB# show ip bgp
BGP table version is 10, local router ID is 203.250.15.2
i - internal Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i128.213.0.0 128.213.63.2 0 100 0 200 i
*>i192.208.10.0 128.213.63.2 100 0 200 400 500
300 i
*>i195.211.10.0 128.213.63.2 100 0 200 400 500 i
*>i200.200.10.0 128.213.63.2 100 0 200 400 i
*>i203.250.13.0 203.250.13.41 0 100 0 i
*>i203.250.14.0 203.250.13.41 0 100 0 i
*> 203.250.15.0 0.0.0.0 0 32768 i
Note that all the entries have >, which means that BGP can reach the next hop. Let us look at the routing
table:
RTB# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * -
candidate default
Gateway of last resort is not set
203.250.13.0 255.255.255.255 is subnetted, 1 subnets
O 203.250.13.41 [110/75] via 203.250.15.1, 00:04:46, Serial0
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 00:04:46, Serial0
128.213.0.0 255.255.255.252 is subnetted, 1 subnets
O 128.213.63.0 [110/138] via 203.250.15.1, 00:04:47, Serial0
The second problem is that we still don't see the BGP entries in the routing table; the only difference is that
128.213.63.0 is now reachable via OSPF. This is a synchronization issue: BGP isn't putting these entries in the
routing table and won't send them in BGP updates because it's not synchronized with the IGP. Note that RTF
has no notion of networks 192.208.10.0 and 195.211.10.0 because we haven't redistributed BGP into OSPF
yet.
In this scenario, if we turn synchronization off, the entries appear in the routing table, but connectivity is still
be broken.
If you turn off synchronization on RTB this is what will happen:
RTB# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * -
candidate default
Gateway of last resort is not set
B 200.200.10.0 [200/0] via 128.213.63.2, 00:01:07
B 192.208.10.0 [200/0] via 128.213.63.2, 00:01:07
203.250.13.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.13.41 255.255.255.255
[110/75] via 203.250.15.1, 00:12:37, Serial0
B 203.250.13.0 255.255.255.0 [200/0] via 203.250.13.41, 00:01:08
203.250.15.0 255.255.255.252 is subnetted, 1 subnets
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 00:12:37, Serial0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
B 128.213.0.0 255.255.0.0 [200/0] via 128.213.63.2, 00:01:08
[110/138] via 203.250.15.1, 00:12:37, Serial0
The routing table looks fine, but there's no way we can reach those networks because RTF in the middle
doesn't know how to reach them:
RTF# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * -
candidate default
Gateway of last resort is not set
203.250.13.0 255.255.255.255 is subnetted, 1 subnets
O 203.250.13.41 [110/11] via 203.250.14.1, 00:14:15, Ethernet0
203.250.15.0 255.255.255.252 is subnetted, 1 subnets
C 203.250.15.0 is directly connected, Serial1
C 203.250.14.0 is directly connected, Ethernet0
128.213.0.0 255.255.255.252 is subnetted, 1 subnets
O 128.213.63.0 [110/74] via 203.250.14.1, 00:14:15, Ethernet0
So, turning off synchronization in this situation didn't help, but we'll need it for other issues later on. Let us
redistribute BGP into OSPF on RTA, with a metric of 2000.
RTA#
hostname RTA
ip subnet-zero
interface Loopback0
ip address 203.250.13.41 255.255.255.0
interface Ethernet0
ip address 203.250.14.1 255.255.255.0
interface Serial0
ip address 128.213.63.1 255.255.255.252
router ospf 10
redistribute bgp 100 metric 2000 subnets
