Spanning tree extend system -Id

mirzaakberali · ‎05-16-2007

Hello,

can any one tell me why do we configure

STP extended system-id on 3750 IOS switches, though we could set up priority with normal priority commands .

Please see the below details.

spanning-tree mode pvst

spanning-tree portfast bpduguard default

spanning-tree extend system-id

IOS 3750 switch# sh spanning-tree vlan 701

VLAN0701

Spanning tree enabled protocol ieee

Root ID Priority 33469

Address 0017.952b.9d00

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33469 (priority 32768 sys-id-ext 701)

Address 0017.952b.9d00

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 14400

Please let me know what is Extended system ID in STP.

Thanks,

Akber.

royalblues · ‎05-16-2007

Friend,

When the spanning tree extended system ID is used, the switch adds the VLAN number to the priority defined .i.e in your case the default priority + 701.

32768 + 701 = 33469

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/13ew/configuration/guide/spantree.html#wp1044385

HTH, rate if it does

Narayan

vishwancc · ‎05-16-2007

hi,

When you use PVST and other advance mode,the Extented system id come into picture,because the BPDU now have to carry pirority and the VLAN that BPDU belong to and so Extented system id is used,in normal STP Vlan informtion is not required since only one instance is running.