Quick Reference Guide

Spanning Tree | 153
Figure 10-111. Using the spanning-tree Command
3. Use either the spanning-tree port mode enable all command in Global Config mode to enable Spanning
Tree on all ports (as shown in Figure 10-112), or use the
spanning-tree port mode enable command in
Interface Config mode (Figure 10-113) to enable selected ports.
Figure 10-112. Using the spanning-tree port mode enable all Command
Figure 10-113. Using the spanning-tree port mode enable Command
4. Use the show spanning-tree command to verify the STP convergence (see Figure 10-119 on page 159)
and the
show spanning-tree mst port summary command (see Figure 10-125 on page 162) for behavior
of ports participating in the spanning tree.
Influencing the Spanning Tree Topology
The selection of the root port is determined in the following order:
1. Lowest port cost (determined by link speed by default)
2. Highest port priority (lowest port priority value of the port sending the BPDU; 128 by default)
3. Lowest port number. For example, 1/0/3 is higher than 1/0/1, so 1/0/1 would be preferred as the root
port. (Strictly speaking, the number is an index number. The number becomes more of an issue when
the contending ports are on separate S-Series switches and have the same port ID, such as 1/0/1.)
4. Lowest bridge ID
Note: Another configuration example is in MSTP Configuration Example on page 156.
S50-1 #config
S50-1 (Config)#spanning-tree
S50-2 #config
S50-2 (Config)#spanning-tree
S50-3 #config
S50-3 (Config)#spanning-tree
S50-1 (Config)#spanning-tree port mode enable all
S50-2 (Config)#spanning-tree port mode enable all
S50-3 (Config)#interface 1/0/1
S50-3 (Interface 1/0/1)#spanning-tree port mode enable
S50-3 (Interface 1/0/1)#exit
S50-3 (Config)#interface 1/0/2
S50-3 (Interface 1/0/2)#spanning-tree port mode enable