Writings on various topics (mostly technical) from Oliver Hookins and Angela Collins. We have lived in Berlin since 2009, have two kids, and have far too little time to really justify having a blog.
A couple of months ago we ran into a minor issue at work when running Cisco and HP switches together. Naturally we have redundant links in place for all networking equipment so spanning tree of some sort must be used. In theory, any equipment can work together with spanning tree but the reality is not exactly this utopian:
I haven't written about it, but MSTP is a massive waste of time with current implementations. I actually list experience with MSTP on my CV, and during one interview I was actually asked about it by a potential employer and we both agreed that it was a huge nightmare which confirmed my own experiences. Basically, the administrative overheads outweigh any bandwidth gains you can potentially make, and these overheads scale up quickly. Unless you have absolutely no changes ever in your environment (right down to your list of used VLANs) I can't recommend it. So that takes MSTP out of the equation.
Cisco will drop down to STP even when configured to use one of their per-VLAN implementations (which I think are actually quite good), but it still conveys per-VLAN information in the extended system ID field of the BPDU. Most switches which aren't looking for it will pass on this information and consider it to be simply additional path weighting information.
As you can see from this rough diagram, we end up in a bit of a pickle when our upstream links from the non-Cisco switches have mismatched VLANs (but can talk on all of these VLANs between each other). The HPs blindly send on BPDU information which encapsulates the extended ID in the priority field, and when it reaches the original switch, it finds that the priority is higher (lower value) than the one it had set already by itself as the root. To get an idea of what happens, I'll describe the path weighting which comes from the above diagram if we are using 1Gbps links (usually attributed a link cost of 4):
Fortunately when I discovered this problem, we did not actually suffer from any loss of links or network problems (that I could see, anyhow) but the way the network was set up well and truly put spanning tree's behaviour into the realm of "undefined". You should not do this! But even so, it demonstrates that you can quite easily mess up your network when the network equipment can't even really standardise on a version of spanning tree. As much as the Cisco documentation will tell you they interoperate by dropping down to STP, they still maintain per-VLAN STP spanning trees so the behaviour is still very different.