STP Link Cost

STP Link Cost
Speed New IEEE Cost Original IEEE Cost
10Gbps 2 1
1Gbps 4 1
100Mbps 19 10
10Mbps 100 10
Included in the BPDUs that a switch sends out is the cost of getting a frame to the root bridge.
A neighboring device receives this information and adds the cost of the link the BPDU arrived
on, and that becomes the cost for the neighboring device. For example, switch A sends out a
BPDU to switch B saying that A can reach the root with a path cost of 38. The BPDU travels
across a gigabit link between switch A and B. B receives the BPDU, giving the cost of 38, and
adds the cost of the link the BPDU arrived on, which is 4. Switch B knows that it can reach the
root by sending frames through switch A with a total path cost of 42.
After the cost is determined for all links to the root bridge, the switch decides which port has
the lowest cost. The lowest cost port is put into forwarding mode, and the other ports are placed
in blocking mode. If there are equal-cost paths, the port with the lowest port ID is put into the
forwarding state. In the previous example, if switch B had two paths to the root, both with a
cost of 42, the switch needs some other way of figuring out which single path will be used. If
switch A is accessed via gigabit port 0/3 and switch C is accessed via gigabit port 0/7, switch B
will send frames via switch A because it is attached to the lower numerical port number.