From: Jonathan V Hays (jhays@jtan.com)
Date: Thu Aug 28 2003 - 10:52:19 GMT-3
For some insight on this issue, see the recent thread on the groupstudy
R&S Lab list: "Cat3550: ISL native VLAN mismatch?" (I started it Monday
8/25/03 and it is not yet in the archives.
When you connect a crossover cable between the switches, by default the
unconfigured 3550 port uses n-isl (negotiated ISL) encapsulation for
trunking. On the other hand, a Catalyst 2950 is only capable of dot1q
encapsulation.
My guess is that if you simply set the encapsulation to dot1q on the Cat
3550 port the problem should disappear.
S1_3550#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/19 desirable n-isl <===#### trunking 1
Fa0/20 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa0/19 1-4094
Fa0/20 1-4094
Port Vlans allowed and active in management domain
Fa0/19 1-4,8
Fa0/20 1-4,8
Port Vlans in spanning tree forwarding state and not pruned
Fa0/19 1-4,8
Fa0/20 1
S1_3550#conf t
Enter configuration commands, one per line. End with CNTL/Z.
S1_3550(config)#int fa0/19
S1_3550(config-if)#switchport trunk encapsulation dot1q
S1_3550(config-if)#switchport mode trunk
S1_3550(config-if)#^Z
S1_3550#sh in trunk
Port Mode Encapsulation Status Native vlan
Fa0/19 on 802.1q <===### trunking 1
Fa0/20 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa0/19 1-4094
Fa0/20 1-4094
Port Vlans allowed and active in management domain
Fa0/19 1-4,8
Fa0/20 1-4,8
Port Vlans in spanning tree forwarding state and not pruned
Fa0/19 none
Fa0/20 1
S1_3550#
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Georg Pauwen
Sent: Thursday, August 28, 2003 5:10 AM
To: ccielab@groupstudy.com
Subject: Native VLAN mismatch
Hello group,
I have a Cisco 2950 connected to a 3550 through a cross-cable on
FastEthernet0/1 and FastEthernet0/4, respectively.
On the 2950, I get the following error message:
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
FastEthernet0/1 (0), with switch FastEthernet0/4 (1).
I cannot really figure out how to get rid of this error message, both
interfaces are in VLAN1. Also, would this mismatch have any impact on
the
connectivity between these two devices ?
Thanks for your input in advance.
Regards,
Georg
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