Use of Spanning-tree extend system-id for vlan verification

[paheeradan.nagulan]

I understand that the system-id extension is used to create a unique BID per vlan in PVST+. My question is 

1) Does the receiving switch will use that "System-id extension" field to verify the vlan that the frame is received?

2) What if there is a native vlan mismatch between the switches( One side is Vlan 100 and the other side is vlan 200). Does the switch use that system-id extension to figure out the issue?

[ammahend · Accepted solution]

1) Does the receiving switch will use that "System-id extension" field to verify the vlan that the frame is received?

No, its only used to identity the correct VLAN for the BPDU

2) What if there is a native vlan mismatch between the switches( One side is Vlan 100 and the other side is vlan 200). Does the switch use that system-id extension to figure out the issue?

No, I don't think so. usually its CDP and LLDP which identifies native vlan mismatch.

[paheeradan.nagulan]

Got it. Thanks for clarifying!

[paheeradan.nagulan]

Got it. Thanks for clarifying!

[Martin L · Accepted solution]

system-id extension is included in Bridge ID part of BPDU section which included in main STP (Cisco and IEEE versions) frame.  In case native vlan mismatch, I think STP will show u message and put ports in disable mode. This may be based on CDP frame that carries native vlan id I think (not 100% sure so need to check it out)

Regards, ML
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[Martin L]

Here is Wireshark STP capture; notice vlan ID "in red" ; Filed indicating vlan id is inside of TLV (6 bit towards end) part of STP frame from my picture in previous post

Regards, ML
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[paheeradan.nagulan]

That makes sense. Thank you so much for confirming!

[paheeradan.nagulan]

Thanks for the including the picture!