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New Member

Mixing ISL and dot1q trunking


We have the following environment:

Cat5000 <--isl trunk--> Cat2924 <--dot1q trunk--> Cat2950

We had to use ISL trunking on the first leg because even though the CatOS on the Cat5000 supports dot1q, the particular module (WS-X5111) only supports ISL.

We had to use dot1q trunking on the second leg because the 2950 series switches only support dot1q.

This was done on a live environment, with no VLANs enabled previously. I know best practice is not to use VLAN 1, but this cannot be helped.

The problem is VLAN 1 traffic works across the two trunks, but we are having trouble establishing connectivity from a device in VLAN 2 on the Cat5000 to a device in VLAN 2 on the Cat2950. Can anyone shed some light on the situation?

Also, VTP is running okay between all three switches.

Thanks for any help.



New Member

Re: Mixing ISL and dot1q trunking

Make sure VLAN 2 is enabled on the trunks:

5000 - show port trunk slot/module

Not familar with 29xx but will be something like:

29xx - show interface f0/n trunk

Check spanning tree is forwarding on trunks:

5000 - show spant mod/port

show spant 2

29xx - show span int f0/n

show span vlan 2

Try connnecting the devices on VLAN 2 into the 5000 & 2924 to see if that works. Then try the 2924 & 2950 to see if it works. Might help work out which trunk/switch has the problem.

Otherwise post your configs so we can have a look.

New Member

Re: Mixing ISL and dot1q trunking

Traffic in VLAN 2 is now working. We have placed a PC on the Cat5000 and Cat2950 ends and they can talk to each other.

The problem experienced previously is most likely to do with a misconfiguration (physically or software-wise) of the firewall that acts as the layer 3 device for the VLAN.

Thanks for your least this appears to confirm that a mixed environment should work.

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