Open Shortest Path First v3

Agenda

• Starting from Open Shortest Path First version 2 (OSPFv2)
• Link State Advertisement (LSA) Details
• OSPFv3 Revisited Concepts
• Configuring OSPFv3
• Troubleshooting Commands

OSPFv2

• April 1998 was the most recent revision (RFC 2328)
• OSPF uses a 2-level hierarchical model
• SPF calculation is performed independently for each area
• Typically faster convergence than DVRPs
• Relatively low, steady state bandwidth requirements

Introduction to OSPFv2

• Uses metrics—path cost
• Typically faster convergence than DVRPs
• Support for CIDR, VLSM, authentication, multipath, and IP unnumbered
• Relatively low, steady state bandwidth requirements

Cost = Metric

• Cost applied on all router link paths
• 16-bit positive number 1–65,535
• The lower the more desirable
• Relevant going out an interface only
• Route decisions made on total cost of path

Areas

• OSPF uses a 2 level hierarchical model
• Areas defined with 32 bit number Defined in IP address format Can also be defined using single decimal value (ie. Area 0.0.0.0, or Area 0)
• 0.0.0.0 reserved for the backbone area
• All areas must connect to area 0.0.0.0

OSPF Areas—Example

Different Types of Routers

• Internal routers (inside an area)
• Backbone routers (inside area 0)
• Area Border Routers (ABR) An ABR sits between two or more areas Must touch backbone area (area 0)
• Autonomous System Boundary Routers (ASBR) Redistribution makes a router an ASBR

Location of Different Routers

OSPF Algorithm

• Network changes generates LSAs
• All routers exchange LSAs to build and maintain a consistent database
• The protocol remains relatively quiet during steady-state conditions Periodic refresh of LSAs every 30 minutes Otherwise, updates only sent when there are changes

Routing Protocol Packets

• They share a common protocol header
• Routing protocol packets are sent with type of service (TOS) of 0
• Five types of OSPF routing protocol packets
  Hello
  Database description
  Link-state request
  Link-state update
  Link-state acknowledgment

OSPF Hello Packets

• Multicast 224.0.0.5 on all router interfaces
• Unicast on virtual links
• Hello interval 10 sec. LAN, 30 sec. NBMA
• Used to form adjacencies between routers
• Describes the optional capabilities

Different Types of LSAs

• Router link (LSA type 1)
• Network link (LSA type 2)
• Network summary (LSA type 3)
• ASBR (LSA type 4)
• External (LSA type 5)
• NSSA external (LSA type 7)

OSPFv3 overview

• OSPF for IPv6
• Based on OSPFv2, with enhancements
• Distributes IPv6 prefixes
• Runs directly over IPv6
• Ships-in-the-night with OSPFv2

OSPFv3 / OSPFv2 Similarities

• Basic packet types
  Hello, DBD, LSR, LSU, LSA
• Mechanisms for neighbor discovery and adjacency formation
• Interface types
  P2P, P2MP, Broadcast, NBMA, Virtual
• LSA flooding and aging
• Nearly identical LSA types

What IPv6 Attributes Affect OSPF?

• 128 bit addresses
• Link-Local address
• Multiple addresses per interface
• Authentication

OSPFv3 / OSPFv2 Differences

• OSPFv3 runs over a link, rather than a subnet
• Multiple instances per link
• OSPFv2 topology not IPv6-specific Router ID Link ID
• Standard authentication mechanisms
• Uses link-local addresses
• Generalized flooding scope

Removal of Address Semantic

• IPv6 address is not present in OSPFv3 packets Exception: LSA payload
• Router-LSA and Network-LSA expressing topology
• Router ID, area ID, LSA link state ID remain a 32 bit number
• Neighbors are always identified by Router ID

New LSA Types

• Link LSA
  Informs neighbors of link local address Informs neighbors of IPv6 prefixes on link
• Intra-Area Prefix LSA
  Associates IPv6 prefixes with a network or router

Agenda

• Starting from OSPFv2
• LSA Details
• OSPFv3 Revisited Concepts
• Configuring OSPFv3
• Troubleshooting Commands

OSPFv3 packet types

• Share a common protocol header
• Routing protocol packets are sent with type of service (TOS) of 0
• Five types of OSPF routing protocol packets
  Hello
  Database description
  Link-state request
  Link-state update
  Link-state acknowledgment

Addition of Flooding Scope

• Flooding scope for LSAs has been generalized Explicitly coded in the LS type field of the LSA
• Three flooding scopes for LSAs
  Link-local
  Area
  AS
• Coded in first three bits of LS Type field
  0x0008 – link LSA has link-local flooding scope
  0x2001 – router LSA has area flooding scope
• Supports flooding of LSAs with an unknown type

New LSA Type Field

• U: indicates how the LSA should be handled by a router that does not recognize the LSA's function code
• S2,S1: indicate the flooding scope of the LSA (link local, area, AS local)
• Each LSA function code also implies a specific setting for the U, S2, and S1 bits

LSA Type Review

LSA Format Change

• New LSA added to distribute IPv6 address and data for next-hop resolution
• Change in LSA names for more consistency
  Option field removed from header and increased in size
  LSA type field expanded to 16 bits; contains flooding scope bits
  Addresses expressed as prefix/length Fragmented Router-LSA

LSA Header Comparison

• OSPFv2 20 bytes
• OSPFv3 20 bytes

LSA Options

Follow the LSA header when it makes sense

• V6: router/link should be excluded from IPv6 routing calculations if this is clear
• E: AS-external-LSAs are not supported if this is clear
• MC: describes whether IP multicast datagrams are forwarded
• N: set if attached to a NSSA
• R: indicates whether the originator is an active router
  If the router bit is clear, then routes that transmit the advertising node cannot be computed
• DC: describes the router's handling of demand circuits

Different Types of LSAs

Sample network for each LSA

Router LSA

• Fragments possible, distinguished by their “Link State ID” field
• Taken together, the collection of router- LSAs originated by the router for an area describes the collected states of all the router's interfaces to that area
• Area flooding scope

Router LSA Structure Comparison

Router LSA of R3 for Area 1

R3#show ipv6 ospf database router Router Link States (Area 1)

Router LSA of R3 for Area 0

R3#show ipv6 ospf database router Router Link States (Area 1)

Router LSA Types

Network LSA Structure Comparison

Network LSA for LAN 1

R3#show ipv6 ospf database network Intra Area Prefix Link States (Area 1)

Intra-Area LSA

• Area flooding scope
• 1- associate prefix when a transit network references a Network-LSA
• 2-associate prefix when a router or stub references a Router-LSA
• Multiple instances for each router or transit network distinguished by their Link State ID

LSA Options

Part of the (prefix, options, prefix length) triple

• NU: if set, the prefix should be excluded from IPv6 Unicast calculations
• LA: "local address" capability bit If set, the /128 prefix is actually an IPv6 interface address of the advertising router
• MC: the "multicast" capability bit;
  if set, the prefix should be included in IPv6 multicast routing calculations
• P: The "propagate" bit; set on NSSA area prefixes that should be re-advertised at the NSSA area border

Intra area LSA structure

Intra-Area LSA Stub

R1#show ipv6 ospf database prefix Intra Area Prefix Link States (Area 1)

Intra-Area LSA Transit

R3#show ipv6 ospf database prefix Net Link States (Area 1)

Inter-Area Prefix LSA

• Describes the destination outside the area but still in the AS
• Summary is created for one area, which is flooded out in all other areas
• Originated by an ABR
• Only intra-area routes are advertised into the backbone
• Link State ID simply serves to distinguish inter-areaprefix- LSAs originated by the same router
• Link-local addresses must never be advertised in inter-area prefix-LSAs

Inter-area Prefix LSA Structure Comparison

Inter-area Prefix LSA

Inter-area Router LSA

• Describes the path to a Router outside the area, but still in the AS
• The Options field in an inter-area-router- LSA should be set equal to the Options field contained in the destination router's own router-LSA
• Link State ID simply serves to distinguish inter-area-prefix-LSAs originated by the same router

Inter-area Router LSA

Inter-area Router LSA Details on R3

External LSA

• Defines routes to destination external to the AS
• Two types of external LSA: E1: consider the total cost up to the external destination E2: consider only the external cost advertised
• Default route can be injected as an external LSA

External LSA (Cont.)

• Link State ID simply serves to distinguish inter-area-prefix-LSAs originated by the same router
• Link-local addresses not advertised in ASexternal- LSAs
• Optionally reference another LSA
• Forwarding address and tag now optional

External LSA Structure

External LSA Details

NSSA External LSA

NSSA External LSA (Type 7) RFC1587

• NSSA was created to inject external routes from stub area into OSPF domain
• Redistribution in NSSA creates Type 7 LSA
• Generated by the NSSA ASBR
• Type 7 can only exists in NSSA area
• NSSA ABR does the translation from 7–5

NSSA External LSA Details

Link LSA

• A link LSA per link
• Link local scope flooding on the link with which they are associated
• Provide router link local address
• List all IPv6 prefixes attached to the link
• Assert a collection of option bit for the Router-LSA

Link LSA structure

Link LSA of R3 For LAN1

Agenda

• Starting from OSPFv2
• LSA Details
• OSPFv3 Revisited Concepts
• Configuring OSPFv3
• Troubleshooting Commands

OSPFv3 vs. OSPFv2 differences

• Interface ID is now a 32 bit ID
• Hello Packet no longer contains an IP network mask
• DR and BDR indicated by Router ID and Interface ID E-bit: the interface attaches to a non-stub area N-bit: the interface attaches to an NSSA area DC- bit: the router wishes to suppress the sending of future Hellos over the interface

Hello Packet

• OSPF packet type 1
• Sent periodically on all interfaces in order to establish and maintain neighbor relationships
• Hello Packets are multicast on links that have a multicast or broadcast capability, enabling dynamic discovery of neighboring routers
• All routers connected to a common link must agree on certain parameters (HelloInterval and RouterDeadInterval)

Hello Structure

Handling Unknown LSA

• More flexible handling of unknown LSA types
• Unknown LSA types are either treated as having linklocal flooding scope (U=0), or are stored and flooded as if they were understood (U=1)

This distinction is based on LS type

• IPv4 OSPF behavior of simply discarding unknown types is unsupported, because of the desire to mix router capabilities on a single link Discarding unknown types causes problems when the Designate Router supports fewer options than the other routers on the link

Stub Area Support

• Stub areas carry only router-LSAs, network-LSAs, Inter-Area-Prefix-LSAs, Link-LSAs, and Intra-Area- Prefix-LSAs
• IPv6 allows LSAs with unrecognized LS types to be labeled "Store and flood the LSA, as if type understood"
• Uncontrolled introduction of such LSAs could cause a stub area's link-state database to grow larger than its component routers' capacities
• LSA whose LS type is unrecognized may only be flooded into/throughout a stub area if LSA has area or Link-local flooding scope, AND LSA has U-bit set to 0

OSPFv3 Virtual Link Handling

• LSA with AS flooding scope are not flooded over a virtual adjacency
• Site local or global scope address Used as IPv6 source for packets sent over the virtual link
• Virtual link interface IPv6 address advertises in Intra-area-prefix-LSA to enable its discovery
• Virtual links are assigned with an unique interface ID

Unchanged Mechanisms

• Designated router election
• Database description exchange
• Area support
• SPF calculations
• LSA aging

Agenda

• Starting from OSPFv2
• LSA Details
• OSPFv3 Revisited Concepts
• Configuring OSPFv3
• Troubleshooting Commands

Configuring OSPFv3 in Cisco IOS® Software

• Similar to OSPFv2 Prefixing existing Interface and Exec mode commands with “ipv6”
• Interfaces configured directly Replaces network command
• “Native” IPv6 router mode Not a sub-mode of router ospf

Configuration Modes in OSPFv3

• Entering router mode [no] ipv6 router ospf
• Entering interface mode [no] ipv6 ospf area < area ID>
• Exec mode [no] show ipv6 ospf [] clear ipv6 ospf []

Cisco IOS OSPFv3 Specific Attributes

• Configuring area range [no] area < area ID> range /
• Configuring area range [no] area < area ID> range /

Enable OSPFv3 globally

Enable OSPFv3 On An Interface

ABR Configuration

ASBR Configuration

Agenda

• Starting from OSPFv2
• LSA Details
• OSPFv3 Revisited Concepts
• Configuring OSPFv3
• Troubleshooting Commands

Show ipv6 ospf

Show ipv6 ospf (Cont.)

Number of areas in this router is 2. 1 normal 0 stub 1 nssa

Show ipv6 ospf interface

Show ipv6 ospf neighbor

Show ipv6 ospf database

Show ipv6 ospf database

Show ipv6 ospf database database-summary

R3#show ipv6 ospf database database-summary Area 0 database summary

Process 1 database summary

Show ipv6 ospf virtual-links

Show ipv6 ospf stat

Requires Enable mode

Show ipv6 ospf borders-routers

Other Show Commands

Other Show Commands (Cont.)

OSPFv3 Debug Commands

• Adjacency is not appearing

• SPF is running constantly

• General purpose

Debug ipv6 ospf adj (PtoP)

Debug ipv6 ospf (LAN)

OSPF Log-Adjacency-Changes

Summary: What Have We Learned?

• Key differences between OSPFv2 and OSPFv3
• How to look at the specific LSAs in the database in detail
• OSPFv3 specificities
• Different troubleshooting commands and what to look for in those commands

Cisco IOS Software Availability

• Release 12.2(15)T
• Release 12.2(RLS3)S for Cisco 7x00 Series Routers and Cisco Catalyst 6000 Series Switches
• Release 12.0(24)S the Cisco 12000 Series Internet Routers

References

• RFC 2740 “OSPF for IPv6”
• RFC 2328 “OSPF version 2”
• RFC 1587 “NSSA”
• RFC 2373 “IP Version 6 Addressing Architecture”
• RFC 2460 “Internet Protocol, Version 6 (IPv6) Spec“
• RFC 3041 “Extensions to IPv6 Address Autoconfiguration”

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