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New Member

Disabling VTP ?

Hello All!

I try to disable VTP on Catalyst 2950 [ IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(13)EA1 ] :

WS2950-12(config)#vtp mode transparent

WS2950-12#show vtp status

VTP Version : 2

Configuration Revision : 0

Maximum VLANs supported locally : 64

Number of existing VLANs : 8

VTP Operating Mode : Transparent

VTP Domain Name :

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

Seems to be ok? Lets see.

WS2950-12#show run interface fastEthernet 0/3

interface FastEthernet0/3

switchport access vlan 3

no cdp enable

spanning-tree portfast

WS2950-12#show interfaces fastEthernet 0/3

FastEthernet0/3 is up, line protocol is up (connected)

Hardware is Fast Ethernet, address is 0007.8431.4883

<last skipped>

I connect to port 3 sniffer (tcpdump):

<time> 0:7:84:31:48:83 1:0:c:cc:cc:cc 002c 60: sap aa ui/C len=35

<hex data skipped>

<time> 0:7:84:31:48:83 1:0:c:0:0:0 8100 94: 802.1Q vlan#1 P7 sap aa ui/C len=65

<hex data skipped>

I see some traffic with Source_MAC of my WS2950-12. And as far as I understand this traffic is a part of VTP (Ethernet_SNAP, OUI= 00:00:0C, OUI belongs to Cisco).

Where is my mistake or How can I disable VTP on 2950 really?


PS. Disabling VTP on 2900XL with similar configuration does not cause a problem.


Re: Disabling VTP ?


AFAIK, VTP transparent doesn't turn off VTP process on the switch.

The switch is still looking at the VTP frames and forwarding them.

In VTP ver.1 VTP frames are forwarded only if VTP domain name and version matches, VTP ver. 2 doesn't provide this version check.

VTP frames are not forwarded by normal bridging procedures.

BUT VTP frames should not be forwarded on access ports at all!!

If you want to disable VTP you can use set vtp mode off. But this command is available since CatOS 7.5(1) and is not available on IOS based switches yet.



New Member

Re: Disabling VTP ?

In my opinion I've made significant downgrade of software (C2950-C3H2S-M, Version 12.0(5)WC2b) and problem disappear.

I've tested c2950-i6q4l2-mz.121-12c.EA1.bin and c2950-i6q4l2-mz.121-9.EA1d.bin - they are affected by this bug too :(

People who wish may easy reproduce and report a bug :)