52 Dell Converged Enhanced Ethernet Administrator’s Guide53-1002116-01STP, RSTP, and MSTP configuration and management6Specifying the bridge priorityIn any mode (STP, RSTP, or MSTP), use this command to specify the priority of the switch. After youdecide on the root switch, set the appropriate values to designate the switch as the root switch. If aswitch has a bridge priority that is lower than all the other switches, the other switchesautomatically select the switch as the root switch.The root switch should be centrally located and not in a “disruptive” location. Backbone switchestypically serve as the root switch because they often do not connect to end stations. All otherdecisions in the network, such as which port to block and which port to put in forwarding mode, aremade from the perspective of the root switch.Bridge protocol data units (BPDUs) carry the information exchanged between switches. When allthe 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 switchcompares 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 withthe lowest bridge priority becomes the root switch.NOTEBecause 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 rstp3. Specify the bridge priority. The range is 0 through 61440 and the priority values can be set onlyin increments of 4096. The default priority is 32678.switch(conf-stp)#bridge-priority 20480Specifying the bridge forward delayIn any mode (STP, RSTP, or MSTP), use this command to specify how long an interface remains inthe 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 bekept: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 stp3. Specify the bridge forward delay.switch(conf-stp)#forward-delay 20
