CCIE Pursuit Blog

August 12, 2007

Spanning Tree Timers Reminders

Filed under: Cisco,Cisco Certification,IOS,Lab Tips,Switching,Tech Tips — cciepursuit @ 4:21 pm

This is pretty basic, but you need to remember a couple of important things when tasked with tweaking spanning-tree timers:

1) Make the changes on the root bridge.
2) The root bridge settings are the timers that are used – not the local settings on the non-root bridge(s).

You can see the timers with the “show spanning-tree vlan x” command.  The timers are set on the root. Non-root bridges will still show the local timer values, but will use the root values:

sw2#sh span v 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0012.018f.d580
             Cost        19
             Port        15 (FastEthernet0/13)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay  4 sec <-note

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0012.009c.ca00 <-sw2 is not the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec <-note
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1            Desg FWD 19        128.3    P2p
Fa0/13           Root FWD 19        128.15   P2p
Fa0/14           Altn BLK 19        128.16   P2p
Fa0/15           Altn BLK 19        128.17   P2p

Bring up a port in VLAN 1:
sw2(config)#int fa0/1
sw2(config-if)#no sh
sw2(config-if)#^Z

*Mar  1 22:33:42: %SYS-5-CONFIG_I: Configured from console by console
*Mar  1 22:33:43: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down
*Mar  1 22:33:44: set portid: VLAN0001 Fa0/1: new port id 8003
*Mar  1 22:33:44: STP: VLAN0001 Fa0/1 -> listening
*Mar  1 22:33:46: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
*Mar  1 22:33:47: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
*Mar  1 22:33:48: STP: VLAN0001 Fa0/1 -> learning  [listen to learn = 4 seconds]
*Mar  1 22:33:52: STP: VLAN0001 sent Topology Change Notice on Fa0/13
*Mar  1 22:33:52: STP: VLAN0001 Fa0/1 -> forwarding [learn to forward = 4 seconds]

The non-root bridge uses the root bridge’s Forward Delay timer of 4 seconds rather than its local timer of 15 seconds.

**************************
3 different ways to change the forward delay back to default (15 seconds)

We set the forward delay to 4 seconds (sw1 is on the root bridge):

sw1(config)#do sh sp v 1 | i ID|Forward
  Root ID    Priority    24577
             Hello Time   2 sec  Max Age 20 sec  Forward Delay  4 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay  4 sec

1) “no spanning-tree vlan 1 forward-time 4”

sw1(config)#no sp v 1 f 4
sw1(config)#do sh sp v 1 | i ID|Forward
  Root ID    Priority    24577
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

2) “default spanning-tree vlan 1 forward-time”

sw1(config)#default sp v 1 f
sw1(config)#do sh sp v 1 | i ID|Forward
  Root ID    Priority    24577
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

3) “spanning-tree vlan 1 forward-time 15”

sw1(config)#sp v 1 f 15
sw1(config)#do sh sp v 1 | i ID|Forward
  Root ID    Priority    24577
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

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