EtherChannel failed when VTP domain changed

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Dec 9th, 2008
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We are running transparent mode for all the switches in our network. However, we experienced port channcel failed after we changed the VTP domain name on the switch sitting on one end of the port channel.

I am wondering if any one else experienced the same thing and any expert could explain why?


Thanks,

Cedar

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davy.timmermans Tue, 12/09/2008 - 12:57
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At first glance I see no relationship between the VTP domain and the portchannel.


Sure you didn't config something else? It would be a good idea to paste the Portchannel config and the individual ports. and maybe also an show ether summ


is the Portchannel down/down?



Giuseppe Larosa Tue, 12/09/2008 - 13:34
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Hello Cedar,

the VTP domain information is carried within DTP frames so if your etherchannel bundles are negotiating their trunk status the trunk can be disabled if the two endpoints detect a VTP domain mismatch.

This is thought as a sort of security feature: if there is a change of VTP domain the two devices can be managed by different groups so you need to configure the trunks manually while before they could be negotiated


Hope to help

Giuseppe

cedar_lee Tue, 12/09/2008 - 13:54
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Hi Giuseppe,


You mean VTP domain mismatch happens even on the switches in transparent mode?


Thanks,

Cedar

Giuseppe Larosa Tue, 12/09/2008 - 14:06
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Hello Cedar,

if the trunk status is negotiated DTP protocol carries this information the VTP domain name and the two parts can compare it and decide to not form a trunk if it is different.

I think this can happen even if the switch is in VTP transparent mode because it is another protocol DTP that makes these negotiations and probably the VTP mode of the switch is not taken in account.


see


http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080890613.shtml#topic5


Section Data Traffic Blocked between VTP Domains



Hope to help

Giuseppe



glen.grant Tue, 12/09/2008 - 14:22
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That is correct if it is a negotiated trunk connection and the vtp domain names do not match it will not stay active even in transparent mode . In this case you are forced to go to "switchport mode trunk" in the configuration .

cedar_lee Tue, 12/09/2008 - 15:07
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Thanks to Giuseppe and Glen.


But I am afraid it's the case this time. All of our switches have switchport trunk mode configured on the trunking ports.


However, I did find something interesting. We have two pair of 6509s. Each pair has its port channel. And only one of the switch in each pair has VTP V2 Mode enable while the other one disable. Does it have anything to do with this issue?

cedar_lee Tue, 12/09/2008 - 15:19
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Here is the info related to transparent mode

Transparent-VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements, but transparent switches do forward VTP advertisements that they receive out their trunk ports in VTP Version 2.

I am not sure if the port channel issue related to VTP V2 Mode enable or not.

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