STP - bridge ID


I'm reading the STP defined by 802.1d and can't understand one thing.

All bridges are is uniquely identified by 64-bit long IDs, that in fact is a concatenation of a globally-unique 48-bit bridge MAC address and 16-bit priority value. The MAC address is included to make the BridgeID unique, it's clear.

What isn't clear to me is what happens when priority field values are the same, how will the spanning protocol elect the root? MAC addresses aren't necessary to be comparable in terms of arithmetic. Could anybody explain this to me?


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MAC addresses are just 48-bit numbers. Why do you say that addresses can't be compared?

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If the path cost is equal, the bridge with the lowest ID is used. Logic dictates that if the priority is equal then the bridge with the lowest mac address is used.

For example, given two bridges:-

# Priority MAC

1 8192 01:ca:db:53:25:60 2 8192 04:53:91:4f:eb:f2

Bridge #1 will win the election.

The actual IDs are as follows ("/" is inserted between the priority and mac for readability):-

0010000000000000/000000011100101011011011010100110010010101100000 0010000000000000/000001000101001110010001010011111110101111110010

Or in decimal:-

36029289712634200 36029986260646660
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Chris Hills

You are correct that a MAC address is NOT a "number", i.e., it is a unique bit pattern that ordinarily has no numerical significance. However, for the purposes of electing a Root Bridge (and Designated Bridges, in the event of equal path cost), the algorithm treats the address as a number, and chooses the one with the numerically lowest value.

(The phrase "Not a Number" always reminds me of that great 60s British spy show, "The Prisoner". The imprisoned protagonist refuses to be treated simply as "Number Six", lamenting that he is "not a number". I once had a girlfriend so enamored of the show that she had a vanity license plate "NOT A NBR".)

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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