STP Root Guard

[ralijaonaa1]

Hi all, could someone explain the meaning of "superior BPDU " in the following context :

" If a root-guard-enabled port receives BPDUs that are superior to those that the current root bridge is sending, that port is moved to a root-inconsistent state. "

In my understanding, for a new switch to be considered as the new root bridge, it has to send a BPDU message which the BID is lower than that of the existing Root Bridge. Thus, so far, superior BPDU seems controversial to my understanding. I'm sure there must be a clear explaination to this however I can't figure it out.

I'd be pleased if anyone could clear it to me.

Thanks

[Sam Byers · Accepted solution]

Hello!

You are correct, the meaning of superior BPDU is that the BID is lower than the current root. As you probably know, the BID is a combination of the bridge priority and the system MAC. 

In context of STP, superior or better means a lower value (similar to route costs or metrics).

[Dennis Mink]

yes Sam is right, on the "superiority" of BID's.  what are you perceiving as being controversial, maybe we can clear that out of the way?

Please rate if useful.

[ralijaonaa1]

I thought that what they mean with superior BPDU is superior or higher BID as well, which is contradictory to the eligibility requirement for becoming STP root bridge. But Sam has cleared it, "better" instead of "superior" is more easily understandable, which is then in the context of STP, a BPDU message with lower BID but not superior.

Thanks.

[ralijaonaa1]

Hi,

Thanks a lot !

Once you used "better" as a synonym to "superior", immediately, my concern has flown away.

Thank you Sam Byers.