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At the conclusion of the
root war, the switches move on to selecting root ports. The root
port of a bridge is the port that is closest to the root bridge.
Every non-root bridge must select one root port.
Again, bridges use the
concept of cost to measure closeness. Specifically, bridges track
what is referred to as root path cost, the cumulative cost of all
links to the root bridge. The Figure illustrates how this value is
calculated across multiple bridges and the resulting root port
election process.
When Cat-A (the root
bridge) sends out BPDUs, they contain a root path cost of 0 (Step
1). When Cat-B receives these BPDUs, it adds the path cost of Port
1/1 to the root path cost contained in the received BPDU. Assume the
network is running Fast Ethernet. Cat-B receives a root path cost of
0 and adds in the Port 1/1 cost of 19 (Step 2). Cat-B then uses the
value of 19 internally and sends BPDUs with a root path cost of 19
out Port 1/ 2 (Step 3).
When Cat-C receives
these BPDUs from Cat-B (Step 4), it increases the root path cost to
38 (19 + 19). However, Cat-C is also receiving BPDUs from the root
bridge on Port 1/ 1. These enter Cat-C Port-1/1 with a cost of 0,
and Cat-C increases the cost to 19 internally (Step 5). Cat-C has a
decision to make: it must select a single root port, the port that
is closest to the root bridge. Cat-C sees a root path cost of 19 on
Port 1/1 and 38 on Port 1/2. Therefore, Cat-C Port-1/1 becomes the root port
(Step 6). Cat-C then begins advertising this root path cost of 19 to
downstream switches (Step 7).
Although not detailed in
the Figure, Cat-B goes through a similar set of calculations: Cat-B
Port-1/1 can reach the root bridge at a cost of 19, whereas Cat-B
Port-1/2 calculates a cost of 38. Therefore, Port-1/1 becomes the root
port for Cat-B. Notice that costs are incremented as BPDUs are
received on a port.
Remember that STP costs
are incremented as BPDUs are received on a port, not as they are
sent out a port. For example, BPDUs arrive on Cat-B Port-1/1 with a
cost of 0 and get increased to 19 "inside" Cat-B.
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