The distribution layer switches often serve as the spanning-tree root because these switches
typically do not connect to end stations. In addition, moves and changes within the network
are less likely to affect these switches.
Example 2-7 STP Protocol Veri?¬?cation
SwitchX#sshhooww ssppaannnniinngg--ttrreeee vvllaann 3300
VLAN0030
Spanning tree enabled protocol rstp
Root ID Priority 24606
Address 00d0.047b.2800
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24606 (priority 24576 sys-id-ext 30)
Address 00d0.047b.2800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
-------- ----- --- --- -------- ----
Gi1/1 Desg FWD 4 128.1 P2p
Gi1/2 Desg FWD 4 128.2 P2p
Gi5/1 Desg FWD 4 128.257 P2p
Improving Performance with Spanning Tree 63
By increasing the priority (lowering the numerical value) of the preferred switch so that it
becomes the root bridge, you force spanning tree to perform a recalculation that re?¬‚ects a
new topology with the preferred switch as the root.
The switch with the lowest BID becomes the root bridge for spanning tree for a VLAN.
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