Questions about RPVST+ and RSTP System IDs

[a1111]

Hello,

I’d like to ask two questions about Wendell Odom’s explanation of RPVST+/PVST. The relevant pages are 245-246. Can someone please help me figure it out?

1.

He says:

“

* RPVST+ adds an extra type-length value (TLV) to the BPDU that identifies the VLAN ID, while RSTP does not (because it does not need to, as RSTP ignores VLANs.)

* Both view the 16-bit priority as having a 12-bit System ID Extension, with RSTP setting the value to 0000.0000.0000, meaning “no VLAN,” while RPVST+ uses the VLAN ID.

“

Why does RPVST+ add the TLV to identify the VLAN if the VLAN information is already included in the System ID Extension? Adding the TLV doesn’t add any useful information.

2.

He gives this output:

Again quoting what he wrote:

“Both view the 16-bit priority as having a 12-bit System ID Extension, with RSTP setting the value to 0000.0000.0000, meaning “no VLAN,” while RPVST+ uses the VLAN ID.”

If RSTP sets the  value to 0000.0000.0000, then why doesn’t the sys-id ext say 0 instead of 9?

Thanks.

Attila

[MHM Cisco World · Accepted solution]

if you run multi STP modes run between SW's
the sys-id is 0 but the BPDU carry the TLV of VLAN ID 
so not all case the sys-id is same TLV 

[guhanece]

As Per VLAN, RSTP allows to restruct the spanning tree with respective to VLAN , the sys-id ext denotes the VLAN ID for which the associated Bridge is configured irrespective of choosing Root bridge stages.

In Per-VLAN RSTP, if a VLAN is not properly configured or there is no STP instance running for that VLAN, you might see:

• Bridge ID present

• VLAN ID correct (sys-id ext retained)

• Root Bridge as 0000.0000.0000

The VLAN ID stays visible because:

• It's part of the system ID extension, and

• It’s tied to the VLAN instance, not dependent on BPDU reception

• It's part of the local configuration, not learned from others