PVST or PVST+

[mcat84]

Customer have 2 4503 running PVST. Have trunk link to new/old Cisco access switch using dot1q and isl encap.

Customer introduce a third party IPS devices which support PVST+ only. Can i change my Cisco 4503 PVST to PVST+? i have isl and dot1q encap on my trunk link? Thanks.

[Francois Tallet]

Hi,

Cisco switch were only running PVST when ISL encapsulation was the only trunking mechanism available. As soon as dot1Q encapsulation was supported, all Cisco switch became PVST+ switches. What I'm writing here is an approximation, but you can consider that a PVST+ switch runs PVST+ on dot1Q trunks and PVST on ISL links.

So there is no configuration that changes between a PVST+ and a PVST mode.

A trunk can only be in ISL or dot1Q trunk. As long as both the IPS and the 4503 are able to form a trunk, whether dot1Q or ISL, you are fine.

Regards,

Francois

[mcat84]

This is my 4503 configuration

!

no file verify auto

spanning-tree mode pvst

spanning-tree extend system-id

power redundancy-mode redundant

!

This switch has trunk line (either isl and dot1q) to other access switch.

What is the correct way to configure my 4503? is it

i) spanning-tree mode pvst or

ii) spanning-tree mode pvst+

Should i go to each Interface (trunk connection) to configure the spanning-tree mode ?

Thanks in advance.

[Francois Tallet]

I guess the confusion comes from our CLI. The pvst spanning tree mode here is to be compared with rapid-pvst and mst modes, not with an hypothetical pvst+ mode. In fact, when you enter "spanning-tree mode pvst", your switch is running pvst+.

I'm sorry if I was not clear, I'm just trying to avoid a long writeup about the difference between pvst and pvst+ because I don't think it would help answering your question. Just rest assured that there is only one mode that all cisco switches support nowadays, and this mode is pvst+.

To add to the confusion, CatOS uses a syntax close to your ii) when IOS uses i). But both are the same and IOS will only accept i).

Regards,

Francois