Spanning Tree Interface Costs
|
Bandwidth |
Cost |
|
4 Mbps |
250 |
|
10 Mbps |
100 |
|
16 Mbps |
62 |
|
45 Mbps |
39 |
|
100 Mbps |
19 |
|
155 Mbps |
14 |
|
622 Mbps |
6 |
|
1 Gbps |
4 |
|
10 Gbps |
2 |
As shown in Figure 1-16,
Switch3 has two paths to the root. To prevent a loop on the network, it must
decide to block one of its ports. The algorithm used to make the decision is
based on three choices:
In this example, the lowest path cost to the root will decide
which port will be forwarding and which one will be blocking. Because the cost
is less through Switch2 path, 38, Switch3 will be forwarding out of this port
and blocking on the other. This behavior of blocking a port allows the spanning
tree to be loop free and provide redundancy should one of the ports go down.
Each active port can have a specific role to play in the
spanning-tree algorithm:
-
Designated Port (DP)— The port
responsible for sending BPDUs on the segment
-
Non-Designated Port (N-DP)—
Does not send BPDUs on the segment
-
Root Port (RP)— The closest
port to the root
STP sends configuration messages out every port of the bridge.
These messages are called bridge protocol data units (BPDUs). BPDUs contain the
appropriate information for STP configuration. The Type field for BPDU message
is 0x00, and it uses the multicast MAC address 01-80-C2-00-00-00. The BPDU
packet is shown in Figure 1-17.
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