VTP connecting Cisco Trunk Ports accross non Cisco Switches

Written by James McDonald

April 15, 2009

I have been trying to get VLAN’s running between 2 Cisco Catalyst 2900XL switches linked by a Linksys SD208.

I had some difficultly (I could ping through the Linksys to either Cat2900 switch but not access higher level protocols) until I Googled and read the following.

Both ISL and IEEE 802.1Q tagging perform explicit tagging as the frame is tagged with VLAN information explicitly. ISL (the default on the Cat2900 switch) uses an external tagging process that does not modify the existing Ethernet frame whereas 802.1Q uses an internal tagging process that does modify the Ethernet frame. This internal tagging process is what allows IEEE 802.1Q tagging to work on both access and trunk links, because the frame appears to be a standard Ethernet frame.

From this I deduced that the Ethernet frames (ISL) of a default VLAN Trunk port are incompatible with the Linksys SD208 because they add to the Ethernet frame. By changing to dot1q encapsulation on my trunk interface the VLAN trunk then worked even though it was passing through the intermediate SD208.

To enable 802.1Q encapsulation you need to run the following commands change the interface to be the correct port e.g for port 22 you would enter “interface fa0/22” after going into configure mode.

>enable
Password: ******
Router#config t
Router(config)#interface fastEthernet 0/12
Router(config-)#switchport trunk encapsulation dot1q
Router(config-)#switchport mode trunk
Router(config-)#end
Router#copy running-config startup-config
Router#copy running-config tftp: <= always do this to get a hard copy of the switch config

Router#show interfaces fastEthernet 0/12 switchport
Name: Fa0/12
Switchport: Enabled
Administrative mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1-3
Pruning VLANs Enabled: 2-1001

Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none
Appliance trust: none

I would like to run my VLAN accross a WAP54G wireless link as well (see diagram). I haven't as yet tried it but I will post if it's successfully.

Update: Just hooked up my Catalyst VLAN trunk ports to the ethernet ports on two WAP54G in Wireless Bridge mode. After discovering I hadn't copied my running-config to the startup-config on one of the switches and that one trunk port was still in ISL VTP encapsulation mode I changed that problem and away we went.

So yes a VTP trunk running dot1q encapsulation can traverse a Linksys WAP54G Wireless Link.

4 Comments

  1. David Bullock

    I was just asking myself the very question “will the WAP54G pass-through 802.1Q tagged frames?” and I found that you’d answered the question already! Thanks for writing this informative article.

    regards,
    David.

    Reply
    • james

      Someone somewhere has the answer… only problem is digging it out of the global digital jungle. Glad to know the post helped you out.

      Reply
  2. ed

    thanks i had the same question as bullock and you had another informative post about creating web based user interfaces- that helped alot! Thanks!

    Reply
  3. JOSE ALFREDO FERNANDEZ

    hi james, i´ve been trying to run a voice vlan across 2 wap54g and i have not been able to do that, is there any way you could help me via email ?

    Reply

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