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Mysterious spanning -tree updates on network

I have been using Ethereal at at Customer site in order to try and correct some traffic issues.

One thing I do see on a consistent basis across the customers switch fabric is some Spanning-tree updates. The problem is that I can not find the Source MAC address showing up in the sniffer on any of the switches in the switch fabric.

The Ethereal output (decode) shows the destination of the packets as going to PVST+.

Some of the switches on our network are 3550's ; these seem to be configured with PVST, i am not certain if there is a correlation.

Fundamentally i would like to make the unnecessary traffic from STP stop. We do not have any redundant connections.

In one specific VLAN, I see 4 updates from STP every 2 seconds ( I think that is the default BPDU timer).

How can I track this down and make it stop???

thx

3 REPLIES
Purple

Re: Mysterious spanning -tree updates on network

Hi,

Since the BPDUs have to be generated by the root switch, I would check the Root Bridge ID from the Ethereal trace. That would tell you the source MAC of those BPDUs.

Paresh

Community Member

Re: Mysterious spanning -tree updates on network

thanks for the suggestion; I had manually searched each switch (INcluding my Core switch) for the MAC's in question. They don's see to show up on any MAC tables on dist or access layer, nor do they show up in the CAM tables on the 4006 Core...

Re: Mysterious spanning -tree updates on network

What are you looking for exactly? Identifying the root bridge generating the BPDUs or a problem in your network? What is the problem and why are you linking it to BPDUs? This question is especially relevant if you don't have any redundancy in your network, in which case STP is not going to do anything special. If you have an STP related issue, it's generally a loss of connectivity or a high link utilization.

Now, as to how to find the root bridge: the simplest is to ask STP. STP will tell you where is the root port, leading to the root bridge (show spanning-tree is explicit). Follow the root ports until you find the root bridge.

The source mac address of BPDUs is not learnt. This is because a BPDU is not data traffic, it's control traffic, relayed from bridge to bridge. The source mac address of a BPDU is the mac address of the port that generated it. What you are looking for in term of address is the root bridge address. You will find this inside your ethereal decode. If you don't have access to any console, you can find the root by comparing the sender bridge ID and the root ID in a BPDU. If they are the same, the sender bridge is the root.

If you don't want to see any bpdu, disable STP (no span vlan 1-4094). However, I don't recommend that even in the case where STP is not needed. STP is an important safety mechanism and should not disturb your network operation.

Regards,

Francois

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