We have 4 separate sites, each site has an edge switch with VLAN 1 connected round the sites via 100mbps connections forming a loop (or square in this case), the last site was only connected a few weeks ago. The other day I had problem connecting to this site's edge switch via telnet, when I did get on I noticed errors showing mac addresses flapping between the VLAN1 'management' ports. I disabled one of the ports and the situation stablilised. Now I have investigated, it appears that three of the sites agree that the spantree root bridge is on the site that was last added, but one of the sites has elected it's own root at that site. The only anomoly I can see at this site is that the spantree ports have portfast enabled, which I am planning to remove, but I'm not convinced that this explains why one site is not taking part in the election. If anyone manages to follow this I would be grateful for any ideas in case I am missing something.
I suspect the dual root switches may be due to a diameter problem, where I have chopped one the redundant links, the 2 roots are 7 hops away from each other. Still, any other ideas please let me know. Regards, Kevin.
just saw your followup. My impression from your first post was, that we talk about 4 switches only, which seems not to be true.
For RSTP a maximum diameter of 3 hops is suggested for normal SPT a maximum diameter of 7 hops. You could configure the network diameter on the root switch, which would adjust SPT timers in all your switches to your topology.
Given the new information I would recommend using routed links between the sites, if your network does allow it.
There other switches at each site beyond the 4 edge switches, hence the diameter has reached 8 at the extreme. I have a switch of priority 30000 at on end, and discovered a switch of priority 8192 near the other end.
I have been researching hard and had just decided that udld would be on my list of trouble-shooting tools. As you say, I think disabling portfast and locating the root switch more centrally using prioority are the first steps.
Thanks again and for the config,I will probably post back in a few days with more news!
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