06-07-2012 03:18 PM - edited 03-07-2019 07:08 AM
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
06-07-2012 11:36 PM
I Daud,
I guess, PVST/PVST+ won't be possile without using extended IDs...
HTH,
Smitesh
PS: Please rate if helpful...
06-08-2012 03:16 AM
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
06-08-2012 07:28 AM
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,
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