Selecting the Designated Port

Selecting the Designated Port
A designated port is one that is active and forwarding traffic, but doesn’t lead to the root. Often,
a designated port on one switch connects to the root port on another switch, but it doesn’t have
to. Because the root bridge doesn’t have any ports that lead to itself, and because its ports are
never dynamically turned off, all its ports are labeled as designated ports.
The selection of a designated port is fairly easy. If there are two switches that have equal-cost
paths to get to the root and are connected to each other, there must be some way of resolving
the topological loop that exists. The switches simply examine the bridge IDs, and whichever
TABLE 1 5 . 2
STP Link Cost
Speed New IEEE Cost Original IEEE Cost
10Gbps 2 1
1Gbps 4 1
100Mbps 19 10
10Mbps 100 100
492
Chapter 15 
Layer 2 Switching and the Spanning Tree Protocol (STP)
device has the lower bridge ID is the one that will be responsible for forwarding traffic from that
segment. Figure 15.4, shown earlier, illustrates this point.