VTP question

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Jan 29th, 2010
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We had a switch go out this week (a layer three switch) and we replaced it with another one (layer two). Both switches are 3560 PoE. The switch that was lost was the VTP server. When we yanked it out and put the new switch in, the two other switches in that network 'took' the VTP server role, which has caused some issues. The original issue was a failed inline power. After discovering the issue of the VTP mode changing, we changed both back to clients. It keeps rolling back to server. Any thoughts on this?

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glen.grant Fri, 01/29/2010 - 14:19
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   Can't say i have heard of a switch changing vtp modes from one type to another by itself . Some of the small switches like 2950's will change from server or client to tranparent mode when inserted into a network that has more vlans than the switch can handle spanning tree wise but never  from client to server by itself .  That sounds like a software bug if its actaully doing that . Also a good reason to always have 2 servers in a network running client/server setups..

christopher_hal... Fri, 01/29/2010 - 14:26
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Thanks for the response, Glen. Would having more than one vtp server cause any kind of conflict? This is assuming that the vlans were only configured on one out of three switches.

Jon Marshall Fri, 01/29/2010 - 15:10
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Thanks for the response, Glen. Would having more than one vtp server cause any kind of conflict? This is assuming that the vlans were only configured on one out of three switches.


There is no problem with having multiple VTP servers in your network. You can use either one to modify the vlan database and the change will get propogated to the other VTP server and any VTP clients.


Jon

lamav Sat, 01/30/2010 - 10:34
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Chris:


Contrary to popular understanding -- and most Cisco documentation -- VTP client switches do originate their own VTP advertisements and propagate them out trunk interfaces every 3 minutes (using a 3550 running c3550-ipservicesk9-mz.122-35.SE5.bin) during stable times, and also every time a change is made to the vlan database.


If the client switch receives a VTP update from the VTP server, it will repackage the advertisement and place its own system MAC address in the source field of the MAC header.


If the trunk that connects the VTP server to the client is disconnected, the VTP client will no longer receive the adverts from the server switch, but it will still originate its own VTP advertisement and propagate it out its trunk interfaces. The source MAC of that update will also be its system MAC address.


So, if you are determining that the client is "switching over" to server mode when you disconnect the VTP server by the fact the the client is originating its own advertisements, this may be the explanation for it. The switch may not not necessarily be changing over from client to server mode, per se, but simply "acting" like a server by originating its own VTP adverts.


This is why I have always said that VTP is nothing but a colossal collection of confusing kaka --  and if at all possible, should never be used. It causes more problems than it solves and creates the kind of confusion that can knock out a good part of your network.


My 2 cents....


Victor


PS It is in fact a Cisco best practice to configure more than 1 VTP server on your LAN, but watch out for bouncing trunks....

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