Trunk Between 2950 and 3500 not Stable?

Unanswered Question
Feb 21st, 2009

Hi All;

I am back with new experience. in my LAN environment i have more the 50 different Cisco switches which includes Catalyst 2950, 3500, 3560, 3700. here i am facing issue while making trunk between any 2950 switch and 3500 switch. link become trunk, but the communication of these switches become very slow or we can say delay increases upto 3000 ms. but trunk amoung 3500 series remain stable after trunking. it happend only with 2950 switch and 3500 switch. is this any IOS bug or compatibility issue. can i resolve this issue by trying correct IOS ver?(if it is not correct).....currently i am using ver for 2950 "12.1(22)EA10" and for 3500 "12.0(5)WC3b"....even i check different versions too but the result is the same.......!!!!

I will wait for experts views



I have this problem too.
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mahmoodmkl Sat, 02/21/2009 - 02:14


2950 by default supports dot1q encap on trunk link,verfiy weather u have the same encap method on 3500.



soshomile Sat, 02/21/2009 - 03:31

hi Mehmood;

you are right, 2950 supports by default dot1q. i configure same encap on both side...that is why my trunk link is up...but response is too much delayed. in 2950 my switch doesn't support ISL. also please tel me which IOS ver of 2950 suports ISL...i will try this as well...

i will wait for your kind reply


MUHAMMAD SHAHEEN Sat, 02/21/2009 - 05:10

Hi Zargham,

Both these switches have reached their End of Life. But the IOS you are using on them are very old. You should upgrade both switches (2950 and 3500) to the latest releases.

As far as 3500 is concerned, it supports dot1q and ISl trunking and for 2950 swtch, only supports dot1q.

When you trunk them, you have must ensure 3500, port have following configuration on that port..

switchport trunk encap dot1q

switchport mode trunk

and for 2950, you only need....

switchport mode trunk

this should work perfect.

if you still have problem, then post the confing from both switches + show verson for each one and we will have a look.

Hope this helps......


Leo Laohoo Sat, 02/21/2009 - 17:14

Q: 2950 my switch doesn't support ISL. also please tel me which IOS ver of 2950 suports ISL

A: Unfortunately the 2950/2960/2975 do NOT support ISL. This series family of switches support DOT1Q. The Catalyst 1900 (End-of-Support) supports ONLY ISL.

glen.grant Sat, 02/21/2009 - 06:21

The trunk may be up but I would look at the links for things like speed/duplex mismatches , both sides have to match , either hardcode both sides or leave both as Auto . Check with show controllers fastethernet slot/port . Also check to see if someone is burying the link with traffic , on the 2950 use the show controllers utilization command to check switch fabric and port utilization. This is usually a physical layer issue such as speed/duplex mismatches . That is really old code on the 3500XL , that must be like circa 1997 for that version. 2950 does not support ISL so that is a moot point. You could post the configs if you want someone to look at them.

Leo Laohoo Sat, 02/21/2009 - 17:17

How are you connecting the 3500 to the 2950? Are you using GigaStacking cable?

Do you have multiple VLANs? If so, try pruning your VLANs allowed.


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