Trunking Between Catalyst
4500/4000, 5500/5000, and
6500/6000 Series Switches Using
802.1Q Encapsulation with
Cisco CatOS System SoftwarePrintable Pdf
Document ID: 14970
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Contents
Introduction Prerequisites
Requirements
Components Used
Conventions What is a Trunk? Basic Characteristics of 802.1Q Trunking
Tagging Mechanism
Spanning Tree Consideration
Cisco's Implementation Configuring 802.1Q Trunks
Hardware/Software Requirements
DTP Modes
Step-by-Step Example Common Errors
Different Native VLANs
Different VTP Domains
Trunking Mode Incompatible with the Encapsulation Type Commands Used in the Document
Command Summary NetPro Discussion Forums - Featured Conversations Related Information
Introduction
This document introduces the concept of trunking between two Ethernet switches and focuses on the
IEEE 802.1Q trunking standard. After a brief description of the 802.1Q trunking mechanism, the
implementation on the Catalyst 4500/4000, 5500/5000, and 6500/6000 range of switches will be
described. A full example is provided, along with some common errors related to 802.1Q trunking
configuration using Catalyst OS (CatOS) system software. For examples of 802.1Q trunking using Cisco
IOS® System Software, refer to Configuring 802.1Q Trunking Between a Catalyst 3550/3560/3750 and
Catalyst Switches That Run Cisco IOS Software.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
Conventions
Refer to Cisco Technical Tips Conventions for more information on document conventions.
What is a Trunk?
Using Cisco's terminology, a trunk is a point-to-point link carrying several VLANs. The purpose of a
trunk is to save ports when creating a link between two devices implementing VLANs, typically two
switches. In the following diagram, we can see two VLANs that we want available on two switches, Sa
and Sb. The first easy method to implement is to create two physical links between the devices, each one
carrying the traffic for a VLAN:
Of course, this solution does not scale. If we wanted to add a third VLAN, we would need to sacrifice
two additional ports. This design is also inefficient in terms of load sharing; the traffic on some VLANs
may not justify a dedicated link. A trunk will bundle virtual links over one physical link, as shown in the
next diagram:
Here, the unique physical link between the two switches is able to carry traffic for any VLAN. In order
to achieve this, each frame sent on the link is tagged by Sa so that Sb knows which VLAN it belongs to.
Different tagging schemes exist. The most common for Ethernet segments are:
ISL (this is the original Cisco proprietary Inter-Switch Link protocol)
802.1Q (the IEEE standard we will focus on in this document)
Basic Characteristics of 802.1Q Trunking
Tagging Mechanism
802.1Q uses an internal tagging mechanism. Internal means that a tag is inserted within the frame (with
ISL, the frame is encapsulated instead):
Note that on an 802.1Q trunk, one VLAN is NOT tagged. This VLAN, named the native VLAN, must
be configured the same on each side of the trunk. This way, we can deduce to which VLAN a frame
belongs when we receive a frame with no tag.
The tagging mechanism implies a modification of the frame; the trunking device inserts a 4-byte tag and
recomputes the frame check sequence (FCS):
The EtherType field identifying the 802.1Q frame is 0x8100. In addition to the 12-bit VLAN-ID, 3 bits
are reserved for 802.1p priority tagging.
Also, note that inserting a tag into a frame that already has the maximum Ethernet size creates a 1522
byte frame that can be considered as a "baby giant" by the receiving equipment. The 802.3 committee is
extending the maximum standard frame size to address this issue.
Spanning Tree Consideration
802.1Q standard is more than just a tagging mechanism. It also defines a unique spanning tree instance
running on the native VLAN for all the VLANs in the network. Such a Mono Spanning Tree (MST)
network lacks some flexibility compared to a Per VLAN Spanning Tree network (PVST) that runs one
instance of Spanning Tree Protocol (STP) per VLAN. Cisco developed PVST+ to allow running several
STP instances (even over an 802.1Q network) by using a tunneling mechanism. Although beyond the
scope of this document, it can be briefly described as utilizing a Cisco device to connect a MST zone
(typically another vendor's 802.1Q-based network) to a PVST zone (typically a Cisco ISL-based
network). There is no specific configuration to enter in order to achieve this. Ideally, a mixed
environment should look like the following diagram:
Cisco's Implementation
In the current implementation, Cisco devices support only VLAN numbers up to 1005. This restriction,
introduced to match the number of VLANs available with ISL, is allowed by the 802.1Q standard. Cisco
implemented a VLAN mapping feature in CatOS 5.1 to simplify interoperability with other vendors'
devices, but it is seldom necessary.
Cisco also adapted its Dynamic ISL (DISL) protocol and turned it into Dynamic Trunking Protocol
(DTP). DISL can negotiate ISL trunking on a link between two devices; DTP can, in addition, negotiate
the type of trunking encapsulation (802.1Q or ISL) that will be used as well. This is an interesting
feature as some Cisco devices support only ISL or 802.1Q, whereas some are able to run both.
In Cisco implementation, a trunk is a point-to-point link, although it is possible to use the 802.1Q
encapsulation on an Ethernet segment shared by more than two devices. Such a configuration is seldom
needed but is still possible by disabling DTP negotiation.
Configuring 802.1Q Trunks
Hardware/Software Requirements
From a software point of view, the first appearance of 802.1Q encapsulation was with CatOS software
4.1. In this release, trunking configuration had to be hardcoded; DTP only appeared with CatOS 4.2. See
the next section, dedicated to DTP.
Not all Catalyst ports support 802.1Q encapsulation. Currently, while Catalyst 4500/4000 switches only
support 802.1Q, ports of the Catalyst 6500/6000 families are able to use 802.1Q or ISL encapsulation.
Depending on the module, Catalyst 5500/5000 trunk capable ports are able to use 802.1Q encapsulation,
ISL encapsulation, or both. The best way to check this out is to use the show port capabilities
command. The trunking capacity is explicitly stated:
Sa> (enable) show port capabilities 1/1
Model WS-X5530
Port 1/1
Type 1000BaseSX
Speed 1000
Duplex full
Trunk encap type 802.1Q,ISL
Trunk mode on,off,desirable,auto,nonegotiate
Channel no
Broadcast suppression percentage(0-100)
Flow control receive-(off,on,desired),send-(off,on,desired)
Security no
Membership static
Fast start yes
Rewrite no
DTP Modes
When configuring a port for trunking, two parameters can be set: the trunking mode and the
encapsulation type (if DTP is supported on that port).
The trunking mode defines how the port will negotiate the setup of a trunk with its peer port.
Here is a list of the possible settings:
Be careful that some modes (on, nonegotiate, off) explicitly specify in which state the port will
end up. A bad configuration can lead to a dangerous inconsistent state where one side is trunking
and the other is not.
A port in on, auto, or desirable sends DTP frames periodically. A trunking port in auto or
desirable goes back to non-trunking if it does not receive a DTP update from its neighbor in five
minutes.
Note that if you are running CatOS software 4.1, you will need to disable any form of negotiation
by using the off or nonegotiate mode when configuring 802.1Q trunking.
The encapsulation type allows the user to specify whether 802.1Q or ISL should be used when
setting up the trunk. Of course, the parameter is only relevant if the module you are using is able
to use both. The parameter can have three different values:
See Results of Possible Fast Ethernet and Gigabit Ethernet Trunk Configurations for a list of all the
possible resulting configurations.
Note: No negotiation will take place between two switches in different VTP domain (VLAN Trunk
Protocol).
Step-by-Step Example
Network Diagram
The following example is based on a very simple lab setup involving two Catalyst 5500/5000 switches
linked together via trunk capable ports. You need a crossover cable in order to interconnect two
switches.
Minimal Setup of a 802.1Q Trunk with Connectivity Tests
Follow the instructions provided below.
Check that the ports' status are up but not trunking.
Connect a terminal to the console of your switches. See the document Connecting a Terminal to
the Console Port on Catalyst Switches if necessary. First, check the status of the port involved in
the setup. Use the command show port 5/24 on Sa ( show port 2/24 on Sb) and check that the
status is connected:
Sa> (enable) show port 5/24
Port Name Status Vlan Level Duplex Speed Type
----- --------- ------ ----- ------ ------ ----- -------
5/24 connected 1 normal a-full a-100 10/100BaseTX
< snip>
We have default value for that kind of port. It came negotiating 100 MB full-duplex and it is
assigned to VLAN 1. Entering a show trunk 5/24 command clearly tells us that the port is not
trunking and has a default mode auto and encapsulation negotiate.
Sa> (enable) show trunk 5/24
Port Mode Encapsulation Status Native vlan
---- ----- ----------- --------- --------
5/24 auto negotiate not-trunking 1
Set an IP address on the sc0 management interfaces.
Use the set interface sc0 10.0.0.1 command on switch Sa and set interface sc0 10.0.0.2 on switch
Sb to assign an IP address to our two switches. A show interface confirms that the management
interface is now correctly set in the default VLAN 1:
Sa> (enable) set interface sc0 10.0.0.1
Interface sc0 IP address set.
If you have the output of a show interface command from your Cisco device, you can use Output
Interpreter ( registered customers only) to display potential issues and fixes.
Check connectivity between Sa and Sb.
A ping 10.0.0.2 command from switch Sa proves that switch Sb can now be reached:
Sa> (enable) ping 10.0.0.2
10.0.0.2 is alive
Sa> (enable)
Configure the same VTP domain on both switches.
Now, assign the same VTP domain to both switches. As we saw, having the same VTP domain is
mandatory to use DTP negotiation. Enter the set vtp domain cisco command on both switches to
configure them with the domain name "cisco":
Enter the command set vlan 2 on both switches in order to create the VLAN 2. If the switches
were already linked by a trunk, we would only need to enter the command on one switch and the
other would learn it automatically via VTP. As we don't have a trunk yet, there is no VTP
communication between Sa and Sb:
We are now going to move the management interface of both switches into VLAN 2. This way,
we are going to show that there is no communication between Sa and Sb before a trunk is
established. Enter the command set interface sc0 2 on each switch to move the sc0 interface in
VLAN 2. Issue a show interface to check that the command is effective:
Check if connectivity is broken between the two switches.
Now, the ping 10.0.0.2 to Sb fails from Sa, proving that there is no connectivity in VLAN 2
between the switches:
Sa> (enable) ping 10.0.0.2
no answer from 10.0.0.2
Sa> (enable)
Check the port capabilities.
Before we start configuring a trunk, we can check with the show port capabilities command that
both ports are able to implement 802.1Q trunking:
Sa> (enable) show port capabilities 5/24
Model WS-X5225R
Port 5/24
Type 10/100BaseTX
Speed auto,10,100
Duplex half,full
Trunk encap type 802.1Q,ISL
Trunk mode on,off,desirable,auto,nonegotiate
Channel 5/23-24,5/21-24
Broadcast suppression percentage(0-100)
Flow control receive-(off,on),send-(off,on)
Security yes
Membership static,dynamic
Fast start yes
Rewrite yes
Sa> (enable)
Configure the trunk encapsulation to be 802.1Q.
Now the trunk on Sa must be configured. We have seen in Step 1 that both ports were in the
default trunking mode auto, encapsulation type negotiate. A combination auto-auto does not bring
a trunk up. This is normal; each side is willing to become trunk, but will only do it if the remote
requests it. Considering the default configuration:
We just need to change the trunk mode to desirable on one side to bring the trunk up. This is
because a port in desirable mode notifies its neighbor that it wants to go trunking. As the
remote (in auto mode) goes to trunking if prompted to, this is enough to bring the trunk up.
We also need to specify which encapsulation we want to use. This is because both ports are
ISL capable and this encapsulation is chosen first when both ends are in negotiate mode.
The syntax of the command is: set trunk module/port [on | off | desirable | auto | nonegotiate]
[vlan_range] [isl | dot1q | negotiate]. Enter set trunk 5/24 dot1q desirable on switch Sa:
Sa> (enable) set trunk 5/24 dot1q desirable
Port(s) 5/24 trunk mode set to desirable.
Port(s) 5/24 trunk type set to dot1q.
1997 May 07 17:32:01 %DTP-5-TRUNKPORTON:Port 5/24 has become dot1q trunk
1997 May 07 17:32:02 %PAGP-5-PORTFROMSTP:Port 5/24 left bridge port 5/24
1997 May 07 17:32:13 %PAGP-5-PORTTOSTP:Port 5/24 joined bridge port 5/24
Verify the trunk is up.
The console log of the previous command clearly shows that the port moved to trunking, but we
can also check by issuing a show trunk 5/24 command on Sa and a show trunk 2/24 command
on Sb. You can see a subtle difference between the two outputs:
The port on Sa is in desirable mode, whereas the Sb port is in auto mode.
More interesting, the encapsulation is dot1q on Sa whereas it is n-dot1q on Sb. This is to
show that Sb negotiated its encapsulation to dot1q. If we did not specify an encapsulation
on Sa, both ports would have ended up in n-isl encapsulation:
Sa> (enable) show trunk 5/24
Port Mode Encapsulation Status Native vlan
---- ------- ----------- ------- -----------
5/24 desirable dot1q trunking 1
Port Vlans allowed on trunk
-------- ------------------------
5/24 1-1005
Port Vlans allowed and active in management domain
-------- -----------------------------------------------------------
5/24 1-2
Port Vlans in spanning tree forwarding state and not pruned
------ -----------------------------------------------------------
5/24 1-2
Sa> (enable)
Sb> (enable) show trunk 2/24
Port Mode Encapsulation Status Native vlan
---- ------ ------------ ------- -----------
2/24 auto n-dot1q trunking 1
< snip>
If you have the output of a show trunk command from your Cisco device, you can use
Output Interpreter ( registered customers only) to display potential issues and fixes.
Check connectivity.
Now, we can check that VLAN 2 is now going through our trunk, simply pinging Sb from Sa:
Sa> (enable) ping 10.0.0.2
10.0.0.2 is alive
Sa> (enable)
Setting the Native VLAN
Follow the instructions provided below.
1. Use the set vlan command.
The command set vlan 2 5/24 is used to assign a port to a specific VLAN. In the case of a
trunking port, it changes the native VLAN to VLAN 2. Of course, we need to do the same on Sb
with set vlan 2 2/24 :
Before we change the native VLAN on Sb, there is now an inconsistency between Sa and Sb
configuration. The two ends of the trunk don't have the same native VLAN configuration. Here,
some warning messages are displayed on Sb console. Note that the switch reporting the
inconsistency may vary depending on which one is the root bridge for VLANs 1 and 2.
Sb> (enable) 2000 Dec 07 16:31:24 %SPANTREE-2-RX_1QPVIDERR: Rcved
pvid_inc BPDU on 1Q port 2/24 vlan 1.
2000 Dec 07 16:31:24 %SPANTREE-2-TX_BLKPORTPVID: Block 2/24 on xmtting
vlan 2 for inc peer vlan.
2000 Dec 07 16:31:24 %SPANTREE-2-RX_BLKPORTPVID: Block 2/24 on rcving
vlan 1 for inc peer vlan 2.
Sb> (enable)
Sb> (enable) set vlan 2 2/24
VLAN 2 modified.
VLAN 1 modified.
VLAN Mod/Ports
---- ------------
2 2/24
Sb> (enable) 2000 Dec 07 16:31:46 %SPANTREE-2-PORTUNBLK: Unblock
previously inc port 2/24 on vlan 1.
2000 Dec 07 16:31:48 %SPANTREE-2-PORTUNBLK: Unblock previously inc
port 2/24 on vlan 2.
The native VLAN mismatch has been corrected and everything goes back to normal.
Check the result.
Now, let's simply check the result of these commands on our trunk, using the show trunk 5/24
command:
Sa> (enable) show trunk 5/24
Port Mode Encapsulation Status Native vlan
---- ------- ----------- ------- -----------
5/24 desirable dot1q trunking 2
Specifying VLANs Allowed on the Trunk
Follow the instructions provided below:
Create additional VLANs.
When you create a new trunk, it carries by default all the existing VLANs in the network. We are
going to see how to restrict the list of allowed VLANs on a trunk. First, we need to create two
additional VLANs (3 and 4), entering set vlan 3 and set vlan 4 commands on Sa for instance. It is
only necessary to enter the command on one switch; VTP will propagate this information to the
other.
Note: This part of the configuration is absolutely the same whether 802.1Q or ISL encapsulation
is used.
Sa> (enable) set vlan 3
Vlan 3 configuration successful
Sa> (enable) set vlan 4
Vlan 4 configuration successful
Remove VLANs from the trunk.
The command clear trunk module/port -list allows you to remove one or several VLANs from a
given trunk. Here, the four VLANs we created were defined on our trunk. Remove VLAN 2 and
VLAN 3 using the commands clear trunk 5/24 2-3 on Sa, and clear trunk 2/24 2-3 on Sb. You
can check the result of the clear command using the show trunk 5/24 command. Only VLANs 1
and 4 are now crossing the trunk between Sa and Sa. A ping between Sa and Sb will then now
fail:
Sa> (enable) clear trunk 5/24 2-3
Removing Vlan(s) 2-3 from allowed list.
Port 5/24 allowed vlans modified to 1,4-1005.
Sa> (enable) show trunk 5/24
Port Mode Encapsulation Status Native vlan
---- ------ ----------- -------- -----------
5/24 desirable dot1q trunking 2
Port Vlans allowed on trunk
----- ---------------------------
5/24 1,4-1005
Port Vlans allowed and active in management domain
----- -------------------------------------------
5/24 1,4
Port Vlans in spanning tree forwarding state and not pruned
----- -----------------------------------------------------
5/24 1,4
Reactivate a VLAN.
To add a VLAN back on a trunk, use the set trunk module/port vlan-list command.
Sa> (enable) set trunk 5/24 2
Adding vlans 2 to allowed list.
Port(s) 5/24 allowed vlans modified to 1-2,4-1005.
Sa> (enable) show trunk
Port Mode Encapsulation Status Native vlan
---- -------- ---------- -------- -----------
5/24 desirable dot1q trunking 2
Port Vlans allowed on trunk
---- ----------------------
5/24 1-2,4-1005
Port Vlans allowed and active in management domain
----- --------------------------------------------
5/24 1-2,4
Port Vlans in spanning tree forwarding state and not pruned
----- ---------------------------------------------------
5/24 1-2,4
VLAN 2 is now flowing again on our trunk (ping Sa to Sb possible).
Common Errors
Different Native VLANs
This is a frequent configuration error. The native VLAN configured on each end of a 802.1Q trunk must
be the same. Remember that a switch receiving a non-tagged frame will assign it to the native VLAN of
the trunk. If one end is configured for native VLAN 1 and the other to native VLAN 2, a frame sent in
VLAN 1 on one side will be received on VLAN 2 on the other. You are then merging VLAN 1 and 2.
There is no reason why you would want that and it may imply some connectivity issues in your network.
A Cisco device will usually warn you on a native VLAN mismatch. See Step 1 for the kind of error
messages you will get on the console in this case. Always check that the native VLAN is the same on
your switches' trunk configuration.
Different VTP Domains
When you create a trunk between two switches and you are using DTP negotiation, double check that
the VTP domain configured on both switches is the same. Negotiation will not take place between two
switches that are in different VTP domains. In the following example, we took the working trunking
configuration described above:
Note: Even if two switches are in different VTP domains, you can make these switches communicate
with each other if you add VLANs manually on each switch. Although there is a VTP domain mismatch,
the VLAN communication works fine. However, VTP updates are not propagated through this link on
that VLAN because the domains are different.
Sa in trunking mode desirable, encapsulation dot1q.
Sb in trunking mode auto, encapsulation negotiate.
Same native VLAN, same VLANs allowed on each side.
The only difference is that we assigned VTP domain "c" on Sa and VTP domain "cisco" on Sb:
Sa> (enable) show trunk
No ports trunking.
Sa> (enable) show trunk 5/24
Port Mode Encapsulation Status Native vlan
---- ------ ---------- --------- -----------
5/24 desirable dot1q not-trunking 1
Port Vlans allowed on trunk
---- -----------------------
5/24 1-1005
Port Vlans allowed and active in management domain
---- ---------------------------------------------
5/24 1
Port Vlans in spanning tree forwarding state and not pruned
----- -----------------------------------------------------
5/24
Sb> (enable) show trunk
No ports trunking.
Sb> (enable) sh trunk 2/24
Port Mode Encapsulation Status Native vlan
---- ------ ----------- ------- -----------
2/24 auto negotiate not-trunking 1
Port Vlans allowed on trunk
----- ----------------------
2/24 1-1005
Port Vlans allowed and active in management domain
---- --------------------------------------------
2/24 1
Port Vlans in spanning tree forwarding state and not pruned
---- -----------------------------------------------------
2/24
Sb> (enable)
We can see that the trunk did not come up. When you are seeing that kind of issue, check the VTP
domain configured on the switches using the show vtp domain command:
Sa> (enable) show vtp domain
Domain Name Domain Index VTP Version Local Mode Password
------- ------------ ----------- ----------- ----------
c 1 2 server -
Vlan-count Max-vlan-storage Config Revision Notifications
---------- ---------------- --------------- -------------
8 1023 0 disabled
Last Updater V2 Mode Pruning PruneEligible on Vlans
------------ ------- ------- ----------------------
10.0.0.1 disabled disabled 2-1000
Sb> (enable) show vtp domain
Domain Name Domain Index VTP Version Local Mode Password
----------- ------------ ----------- ---------- ----------
cisco 1 2 server -
Vlan-count Max-vlan-storage Config Revision Notifications
---------- ---------------- --------------- -------------
8 1023 20 disabled
Last Updater V2 Mode Pruning PruneEligible on Vlans
------------ ------- ------- ----------------------
10.0.0.1 disabled disabled 2-1000
Now, we will put switch Sa in VTP domain "cisco", using the set vtp domain cisco command. After a
few seconds, the trunk is negotiated and up again:
Sa> (enable) set vtp domain cisco
VTP domain cisco modified
Sa> (enable) 1997 May 13 13:59:22 %DTP-5-TRUNKPORTON:Port 5/24 has become dot1q trunk
1997 May 13 13:59:22 %PAGP-5-PORTFROMSTP:Port 5/24 left bridge port 5/24
1997 May 13 13:59:33 %PAGP-5-PORTTOSTP:Port 5/24 joined bridge port 5/24
If you want to keep different VTP domains, but still create a trunk between two switches, then you have
to hardcode trunking on each side of the trunk (using nonegotiate/on).
Trunking Mode Incompatible with the Encapsulation Type
This is a common issue that started to be raised to Cisco Technical Support when the first modules able
to support both 802.1Q and ISL shipped. People were used to configuring a trunk by entering set trunk
module/port on or set trunk module/port nonegotiate. The problem is that by default the encapsulation
type is set to negotiate. The negotiate encapsulation type is only supported by auto or desirable trunking
modes. The on and nonegotiate encapsulation types do not perform any negotiations between switches
and must be hard set to ISL or 802.1Q encapsulation when they are configured. The following is a log of
what is happening on the switch in that case:
Sa> (enable) set trunk 5/24 on
Failed to set port 5/24 to trunk mode on.
Trunk mode 'on' not allowed with trunk encapsulation type 'negotiate'.
Sa> (enable) set trunk 5/24 nonegotiate
Failed to set port 5/24 to trunk mode nonegotiate.
Trunk mode 'nonegotiate' not allowed with trunk encapsulation type
'negotiate'.
Sa> (enable)
This makes sense because if you do not negotiate with the remote, how would you know which kind of
encapsulation (802.1Q or ISL) to use in order to bring up the trunk? Two possibilities:
Use the desirable mode. In this case, you will negotiate the encapsulation mode with the remote:
Sa> (enable) set trunk 5/24 desirable
Port(s) 5/24 trunk mode set to desirable.
Sa> (enable) 1997 May 09 17:49:19 %DTP-5-TRUNKPORTON:Port 5/24 has become
isl trunk
Specify the encapsulation you want to use:
Sa> (enable) set trunk 5/24 isl on
Port(s) 5/24 trunk mode set to on.
Port(s) 5/24 trunk type set to isl.
Sa> (enable) 1997 May 09 17:50:16 %DTP-5-TRUNKPORTON:Port 5/24 has become
isl trunk
Commands Used in the Document
Command Summary
ping
set interface
set trunk
set vlan
set vtp domain
show interface
show port
show port capabilities
show trunk
show vtp domain
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Related Information
Configuring ISL Trunking on Catalyst 5500/5000 and 6500/6000 Family Switches
Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports (Catalyst 5000
Cisco Documentation)
Understanding and Configuring VLAN Trunk Protocol (VTP)
