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Monday, December 5, 2011

VTP Pruning issues

We have just recently cut over from an older core switch/router platform (running CatOS 8.x) to a newer platform running IOS 15. So all the SVI's for our VLANs moved over from the old platform to the new, and a trunk link (4x1GB etherchannel) was configured between the two. All the other access layer switches that had trunk links to the old core were also migrated to the new core (also still trunked of course.) Most of the company's servers, the IP PBX and phones, and some other departmental access switches are still homed into the old core (they will soon be migrated to the new core as we have time.)

The old core and most of the access layer switches are running VTP v2 in a given domain. The new core is also in the same VTP domain, but is in transparent mode (we plan to do away with VTP as a part of this migration, but it hasn't happened yet for all switches.)

So, we have been having a problem since the cut with the old core switch doing VTP pruning of various VLANs off the trunk link between the old core and new core. This isolates the devices on that VLAN on the old core, because that VLAN's devices can not reach their network gateway which is now on the new core. I am familiar with the concept of VTP Pruning, but I thought that if there were other switches "down the line" from the switch that does not have ports in a given VLAN, that the switch that would otherwise do the pruning would NOT prune the VLANs from the trunk. So what I'm trying to say is like this:

[ switch 1 ] ===trunk=== [switch 2 ] ===trunk=== [ switch 3 ]
(has ports (does NOT (has ports
in V100) have ports in V100)
in V100)

So, if in this case "switch 1" is the old core, and "switch 2" is the new core, why would switch 1 prune V100 off the trunk link between itself and switch 2 if there is another switch (or switches) past switch 2 that have that same VLAN?

Note that I am not having VLAN pruning problems on any of the other trunks into the new core.

Network devices in VTP transparent mode do not send VTP join messages. On Catalyst 6500 series switches with trunk connections to network devices in VTP transparent mode, configure the VLANs that are used by the transparent-mode network devices or that need to be carried across trunks as pruning ineligible (use the clear vtp pruneeligible command). »

So in your scenarion I would use the command clear vtp pruneeligible 100, in order to exclude the vlan 100 for any pruning eligibility on the trunk and define the vlans you DO NOT want to be pruned on the trunk

Please click here for more information on how to configure a VTP.


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