cisco switch running both stp/pvst+

[sarahr202]

Hi everybody.

I was doing lab where two switches were connected over trunk. When i capture the traffic on the trunk, i found both stp and pvst+ traffic passing over the trunk. The switches were at their default setting.

My question is why are  cisco switches running both stp/pvst+ at the same time? I understand the role this protocol perform but why do we both i.e stp/pvst+?

thanks and have a great day.

[Peter Paluch · Accepted solution]

Hello Sarah,

The reason to run classic STP even between PVST+ switches is twofold: assuming that their interconnection may be through a shared medium (hub, WiFi, non-STP switch), to allow a STP-only switch to be connected and immediately start participating in STP at some point in the future, and to allow for native VLAN mismatch detection. Regarding the second point, you may be interested in reading the following document:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml

Best regards,

Peter