I have an environment with 5 Foundry FastIron switches and two Cisco 3750s. All of the Foundrys and one of the Cisco switches (3750-2) recognizes one switch (Foundry5) as the root. The other 3750 (3750-1) considers itself to be the root, and while passing BPDUs to the Foundry attached to it, doesn't seem to recognize it in STP. All of the Foundrys have the same priority, 32768, the root election was based off of the lowest MAC. The two Cisco switches have the same priorities; when the two Cisco's are dual attached, a bridging loop occurs. My thought is to raise the priorities for the two VLANs on the one Cisco switch (3750-1) that thinks it is the root above the other Cisco, which would also be above the Foundry then dual attaching. I would like to get rid of the Foundry, but until then, I have to live with it. Any thoughts from anyone in similar situations? Also, all run RSTP. Attached is a diagram. Thanks in advance.
I'm not a fan of Foundry switches but I've occasionally had to deal with them. I haven't had a STP problem with them.
First, I would never leave a STP root bridge election to any sort of chance. Set your 'Main' bridge's priority to something low so it will win out, and your backup root bridge almost as low(4096 for primary and 8192 for secondary is what I use).
Check the ports connecting 3750-1 and Foundry4. One of them must be set to portfast, hence it's not actualy getting the BPDUs and the cause of the loop when you plug 3750-1 into 3750-2.
I would never leave it to chance either, this is an environment that was in place when I came to the organization. Leaving port fast enabled when interconnecting bridges would be a networking 101 mistake, it is not enabled on any uplinks.
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