| 5.2 | STP Processes | ||
| 5.2.2 | Root-bridge elections |
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The switches first need to elect a single root bridge by looking for the bridge with the lowest BID. This process of selecting the bridge with the lowest BID is often referred to as a root war. As discussed in the
"Bridge ID" section earlier, a BID is an 8-byte identifier
that is composed of two subfields: the bridge priority and a MAC
address from the supervisor or backplane. In Figure Continuing with the
example, Cat-B assumes a default BID of 32,768.BB-BB-BB-BB-BB-BB,
and Cat-C uses 32,768.CC-CC-CC-CC-CC-CC. Because all three bridges
are using the default bridge priority of 32,768, the lowest MAC
address (AA-AA-AA-AA-AA-AA) serves as the tie-breaker, and Cat-A
becomes the root bridge. Figure The bridges learn that a
Cat-A has the lowest BID through the exchange of BPDUs. As discussed earlier, BPDUs are
special packets that bridges use to exchange topology and
Spanning-Tree information with each other. By default, BPDUs are
sent out every 2 seconds. BPDUs are switch-to-switch traffic; they
do not carry end-user traffic. Figure A switch usually starts out assuming that the world revolves around itself. In other words, when a switch first boots, it always places its BID in both the Root BID and the Sender BID fields. Suppose that Cat-B boots first and starts sending out BPDUs announcing itself as the root bridge every 2 seconds. A few minutes later, Cat-C boots and boldly announces itself as the root bridge. When the Cat-C BPDU arrives at Cat-B, Cat-B discards the BPDU because it has a lower BID saved on its ports (its own BID). As soon as Cat-B transmits a BPDU, Cat-C learns that it is not quite as important as it initially assumed. At this point, Cat-C starts sending BPDUs that list Cat-B as the root BID and Cat-C as the sender BID. The network now agrees that Cat-B is the root bridge. Five minutes later Cat-A boots. As with Cat-B earlier, Cat-A initially assumes that it is the root bridge and starts advertising this fact in BPDUs. As soon as these BPDUs arrive at Cat-B and Cat-C, these switches hand over the root bridge position to Cat-A. All three switches are now sending out BPDUs that announce Cat-A as the root bridge and themselves as the sender BID.
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