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VTP Pruning : Is this a bug?

I recently introduced VTP pruning on a LAN, and now I have some connectivity problems on certain VLANs.

1. Is pruning based on whether the switch has any active access ports or unpruned downstream trunks on the VLAN? Or is it based on whether there are downstream CAM entries for the VLAN?

2.Can a switch to prune a VLAN off a trunk that is the root port for that VLAN?

I have four VLANs, 21-24, that serve as point-to-point links between remote sites. On each of these VLANs, there are only two hosts: one on each site. Here is the spanning-tree for VLAN 21:


CC80#show spanning-tree vlan 21VLAN0021

Spanning tree enabled protocol rstp

Root ID Priority 24576

Address 0007.4f62.a014

Cost 15

Port 72 (Port-channel1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32789 (priority 32768 sys-id-ext 21)

Address 001b.2ae8.b280

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Gi0/16 Desg FWD 4 128.16 Edge P2p

Po1 Root FWD 3 128.72 P2p

Po2 Desg FWD 3 128.80 P2p


Po1 is the root port. G0/16 is an “up” access port. Po2 is a trunk to another access layer switch at the same level. Po2 spends its time in blocking state on the other switch. This switch prunes most VLANs off Po2.

Now let us look at the trunks:


CC80#show int trunkPort Mode Encapsulation Status Native vlan

Gi0/3 on 802.1q trunking 2

Po1 on 802.1q trunking 12

Po2 on 802.1q trunking 12Port Vlans allowed on trunk

Gi0/3 1,169

Po1 1-2,5,12,21-26,169

Po2 1-2,5,12,21-26,169Port Vlans allowed and active in management domain

Gi0/3 1,169

Po1 1-2,5,12,21-26,169

Po2 1-2,5,12,21-26,169Port Vlans in spanning tree forwarding state and not pruned

Gi0/3 1,169

Po1 1-2,5,12,22,24-26,169

Po2 1


As expected, most of the VLANs are pruned off Po2 as te other end of Po2 is on STP blocking state. Ignore G0/3; this is a server trunk. The interesting thing is that the switch has pruned VLAN 21 from the root port trunk, Po1. Why? This has effectively cut this switch off from VLAN 21.

VLAN 21 is pruned from both Po1 and Po2, and yet it has an access port on it. Now, that access port, G0/16, is apparently not receiving any MAC traffic from its connected host. There is nothing in the CAM table except the upstream switch. But it is still isolated, so it cannot see any traffic from the remote part of the VLAN, so it does not respond:


CC80#show mac-address-table dyn vlan 21

Mac Address Table

-------------------------------------------Vlan Mac Address Type Ports

---- ----------- -------- -----

21 0016.c73d.a22b DYNAMIC Po1

Total Mac Addresses for this criterion: 1


One last possible clue. We have four VLANs, each with two host connections. Two of them work, two of them don't. The difference is that they take different paths. Two of them are rooted on a 4506 running IOS 12.2(25)EWA2, and they work; they are not pruned anywhere between the two sites. The two that do not work are rooted on a 4003 running CatOS 8.4(5)GLX.

If the listings are messy, they are reproduced on my blog:

Kevin Dorrell



Re: VTP Pruning : Is this a bug?

Hi Kevin,

Definitely there are interesting clues here.

What I can help you with is the following:

VTP pruning does not prune vlan 21 itself from the trunk. It prunes the traffic for vlan 21 coming from the upstream switch.

But, it keeps the stp instance for vlan 21 in place.

My guess is the following:

Vlan 21 is pruned from Po1 because there are no active ports in vlan 21 on that switch and vlan 21 is left eligible for pruning.

(Though I don't know the very exact criteria for "active ports").

Other vlans have no active ports either, but they may have been disabled for pruning, that is the following command may have been applied:

switchport trunk pruning vlan remove 2,5,12,22,24-26,169

I hope this will give you some help.



Re: VTP Pruning : Is this a bug?

Hi Istvan,

Thanks for looking at the output from the switches. I think I have sorted it out, and I think it is a bug in the root switch for VLAN 21. Unlike our other VLANs, VLAN 21 is rooted in a CatOS switch. Due to various circumstances in our network, it tripped over a bug.

The bug is related to one I found a couple of years ago. I found that in a CatOS switch, if you manually disallow the native VLAN (in my case VLAN 12) from a trunk, then it stops the trunk passing BPDUs for VLAN 1 as well. At the time, that resulted in a 5-minute meltdown of my network.

Here are the notes I have made for this new bug. Sorry about the generalisations ... I do not know the VTP protocol very well yet.

Normally, VTP signalling is carried on the native VLAN of each trunk. By default, the native VLAN is VLAN 1, but you are allowed other values. We use VLAN 12 as native, a VLAN that is unused anywhere on the network. Now, an IOS switch will never prune VLAN 1 from a trunk. Nor will it prune the native VLAN. However, CatOS has a bug: if the (non-1) native VLAN is unused, it will prune it from the trunk regardless of the fact that it is the native. Once the native VLAN is pruned, of course, the VTP signal cannot be propagated to other switches.

It happens that we have one CatOS siwtch in our core loop. That switch is the root for VLANs 21 and 23. (And fortunately only for VLANs 21 and 23.) Because that root switch had pruned the native VLAN from its trunks, it was no longer able to send VTP unprune signals for VLANs 21 and 23 to its neighbors. Its neighbors therefore pruned VLANs 21 and 23 from the trunks to the root. The result was that there was no connectivity in VLANs 21 and 23, and every switch pruned all ports on those VLANs.

I resolved the problem by rolling back the VTP pruning.

I hope this makes sense. If François Tallet ses this posting, I wonder if you could comment please François?

Kevin Dorrell


Re: VTP Pruning : Is this a bug?

Hi Kevin,

Yes, it definitely makes sense. And thank you for your answer and rating. It is very useful for me as well to see how such problems appear and how they are resolved.

The main success is that you are trained and skilled enough and found the root cause of the problem, so the switch wasn't able to play around with you :).



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