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52 Dell Converged Enhanced Ethernet Administrator’s Guide
53-1002116-01
STP, RSTP, and MSTP configuration and management
6
Specifying the bridge priority
In any mode (STP, RSTP, or MSTP), use this command to specify the priority of the switch. After you
decide on the root switch, set the appropriate values to designate the switch as the root switch. If a
switch has a bridge priority that is lower than all the other switches, the other switches
automatically select the switch as the root switch.
The root switch should be centrally located and not in a “disruptive” location. Backbone switches
typically serve as the root switch because they often do not connect to end stations. All other
decisions in the network, such as which port to block and which port to put in forwarding mode, are
made from the perspective of the root switch.
Bridge protocol data units (BPDUs) carry the information exchanged between switches. When all
the switches in the network are powered up, they start the process of selecting the root switch.
Each switch transmits a BPDU to directly connected switches on a per-VLAN basis. Each switch
compares the received BPDU to the BPDU that the switch sent. In the root switch selection process,
if switch 1 advertises a root ID that is a lower number than the root ID that switch 2 advertises,
switch 2 stops the advertisement of its root ID, and accepts the root ID of switch 1. The switch with
the lowest bridge priority becomes the root switch.
NOTE
Because each VLAN is in a separate broadcast domain, each VLAN must have its own root switch.
To specify the bridge priority, perform the following steps from Privileged EXEC mode.
1. Enter the configure terminal command to access global configuration mode.
2. Enter the protocol command to enable STP, RSTP, or MSTP.
switch(config)#protocol spanning-tree rstp
3. Specify the bridge priority. The range is 0 through 61440 and the priority values can be set only
in increments of 4096. The default priority is 32678.
switch(conf-stp)#bridge-priority 20480
Specifying the bridge forward delay
In any mode (STP, RSTP, or MSTP), use this command to specify how long an interface remains in
the listening and learning states before the interface begins forwarding all spanning tree instances.
The range is 4 through 30 seconds. The default is 15 seconds. The following relationship should be
kept:
2*(forward_delay - 1)>=max_age>=2*(hello_time + 1)
To specify the bridge forward delay, perform the following steps from Privileged EXEC mode.
1. Enter the configure terminal command to access global configuration mode.
2. Enter the protocol command to enable STP, RSTP, or MSTP.
switch(config)#protocol spanning-tree stp
3. Specify the bridge forward delay.
switch(conf-stp)#forward-delay 20