Trunking issue

[damesac]

Anyone have any issues setting up trunking between a 3550 running 12.0 and a 2950 running 12.1? Earlier today I built a 3550 running 12.0 and successfully set up trunking with another 3550 of the like. VTP information was passed and I was able to configure ports for each of the different VLANs without a problem. I removed the freshly built 3550 and moved on to the next project of building this 2950. One of the differences between IOSs that I noticed right off the bat was that you can no longer configure the encapsulation type for a trunked link in 12.1. I'm assuming that it defaults to auto negotiate instead of being able to set it to isl or dot1q???

Anyways, without trunking configured, the switches can talk and the 2950 can see the world (can get to the router on the other side of the 3550). As soon as I configure trunking on the link, they can no longer see each other. A show cdp nei command is successful, but no vtp information is passed. The parent 3550 is set to vtp server and the new 2950 is set to client.

So I'm just putting my feelers out there to see if anyone has run across perhaps this same issue.

"Be the packet."

 

[damesac]

One of my co-workers suggested this theory...

The parent 3550 has a maximum of 254 VLANs.
The new 2950 has a maximum of 64 VLANs.

If the VLAN numbers that are in the parent 3550's database are all....for example 100 and higher, does that have anything to do with the 2950's in-ability to see them?

What he's saying is that the 3550 would be able to see VLANs 1-254, whereas the 2950 can only see VLANs 1-64 so the only VLANs that could be shared between these two switches would be any VLAN numbered 1-64.

No right? The maximum number of VLANs pertains to a physical count, irregardless of the actual VLAN number correct?

"Be the packet."

 

[baddos]

Please post your 3550 and 2950 trunk configurations.

You can number the VLANs over 100 and their fine. Your 2950 (SMI image) supports a maximum of 64 vlans, but as long as you don't go over id 1001 you should be fine.

 

[damesac]

Unfortunately, I won't be able to post the configs due to legal reasons, but I believe I found the answer last night digging around on Cisco. It appears as though the 2950s are only capable of using 801.q encapsulation, conforming to me finding that I was unable to set the encapsulation type......it defaults to 801.q.....being a cisco product I assumed it would default to isl. --------Brainstorm--------If when producing this 2950, the designers decided to only allow one encapsulation type, it would make sense to only allow 801.q so that the device could still communicate with non-cisco devices because if you hooked it up to another cisco device, you could always set that one to 801.q, but if you hooked it up to a non-cisco device, you would not be able to set that device to isl. So the default encapsulation type on the 3550 is isl. I'm headed to work now and I'll set the encap type on the 3550 to 801.q and see if that does it.

Is 801.q standard better than isl? What's the detailed difference? Anyone know offhand?

"Be the packet."

 

[baddos]

802.1q and ISL both have their strengths and weaknesses. I beleive Cisco is trying to phase out ISL and switch over to 802.1q to be standardized with other vendors.

 

[damesac]

Just to let everyone know, it was as I presumed earlier. Once I configured the 3550 to use dot1q, it worked....interesting.

"Be the packet."

 

[lui3]

ISL adds length to the packet header. I think 30 to 40 bytes. Its cisco proprietary and causes headaches with none cisco devices.