STP - bridge ID

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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?



Re: STP - bridge ID

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

Re: STP - bridge ID

On 04/11/09 04:12, Mark wrote:
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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):-


Or in decimal:-


Re: STP - bridge ID

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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

(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".)

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