Question on PVST & router sub-interface status

rsgamage1 · ‎07-23-2008

Hi,

What is the behavior a PVST environment when a router "sub-interface" configured with a particular VLAN is shutdown/brought up. The router connects to a c2950 dot1Q trunk port configured for few VLANs.

When there's a change of the physical interface state(up/down) I see the STP state transition. However the change of status of sub-interface has no impact on STP status of the connected switch interface(remains in forwarding even for the disabled VLAN).

How is this behavior explained? Have I overlooked something here?

Thanks in advance for your time/thoughts.

lgijssel · ‎07-23-2008

There would only be impact when the sub-interface was actually sending BPDU's.

A router is a layer3 device. It will not participate in STP by default. On the switch-side the router will be seen as an edge port.

regards,

Leo

rsgamage1 · ‎07-23-2008

I understand that the router does not participate in STP.

What actually makes the difference in STP/PVST when the physical router interface is shut compared to the router sub-interface(dot1Q VLAN)?

lgijssel · ‎07-23-2008

The difference is at the switch' side. The switch has no way to know the status of a sub-interface on a connected device when it does not receive bpdu's from it. As long as the vlan is allowed on the trunk, all traffic for it will be forwarded. (either broad-, multi- or unicast) The receiving end will silently discard this traffic and that's all there is to it.

On the other hand, a change in the status of the physical interface is all too easily noticed and will result in an STP-state change.