VLAN identification without Extended System ID (802.1t) in PVST/PVST+

daudparvez · ‎06-07-2012

Hi All,

When we use extended system id, the vlans can be identified because, Bridge Priority = Configured Value + VLAN Number. Thus the switch recieving a BPDU from another switch identifies the VLAN for which this BPDU is recieved.

However, when we do not use extended system id , but instead, use the traditional 802.1d system id, then there is no such VLAN portion in this. How does then a switch performs VLAN to BPDU mapping/identification for recieved BPDUs?

Best Regards,

Daud Parvez

smitesh kharecha · ‎06-07-2012

I Daud,

I guess, PVST/PVST+ won't be possile without using extended IDs...

HTH,

Smitesh

PS: Please rate if helpful...

Giuseppe Larosa · ‎06-08-2012

Hello Daud,

before introduction of extended system-id Cisco PVST+ used a different MAC address for each Vlan/STP instance.

These MAC addresses were taken from a block of consecutive MAC addresses that are associated to the device.

You can see them in the output of show module on a C6500 or C4500 devices.

The main reason for extended system-id is to avoid this waste of MAC addresses.

Also PVST+ checks the 802.1Q external vlan-id with an internal field in the BPDU for vlan consistency.

Hope to help

Giuseppe

daudparvez · ‎06-08-2012

Thanks all for your kind support.

Dear Giuseppe,

Can you please explain a bit deeper that even, if a switch uses a different MAC Address for every STP instance, how does it help the neighbors to recognize the VLAN from which it belongs?

Best Regards,