In your BCMSN / CCNP exam study, it's easy to overlook some of the details of Spanning Tree Protocol (STP). After all, you learned all of that in your CCNA studies, right? Not necessarily! While some of the BCMSN material will be a review for you, there are some details regarding familiar topics that you need to learn. That includes the timers for STP - Hello Time, MaxAge, and Forward Delay.
You may remember these timers from your CCNA studies as well, and you should also remember that these timers should not be changed lightly. What you might not have known is that if you decide to change any and all of these timers, that change must be configured on the root bridge! The root bridge will inform the nonroot switches of the change via BPDUs.
Hello Time is the interval between BPDUs, two seconds by default.
Forward Delay is the length of both the listening and learning STP stages, with a default value of 15 seconds.
Maximum Age, referred to by the switch as MaxAge, is the amount of time a switch will retain a BPDU's contents before discarding it. The default is 20 seconds.
The value of these timers can be changed with the spanning-tree vlan command shown below. Verify the changes with the show spanning-tree command.
SW1(config)#spanning-tree vlan 1 ?
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
priority Set the bridge priority for the spanning tree
root Configure switch as root
SW1(config)#spanning-tree vlan 1 hello-time 5
SW1(config)#spanning-tree vlan 1 max-age 30
SW1(config)#spanning-tree vlan 1 forward-time 20
SW1(config)#^Z
SW1#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000f.90e1.c240
This bridge is the root
Hello Time 5 sec Max Age 30 sec Forward Delay 20 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000f.90e1.c240
Hello Time 5 sec Max Age 30 sec Forward Delay 20 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
Again, you should always take great care in changing these timers. Those defaults are set for a reason - helping to prevent switching loops!
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