Spanning Tree STP in combination of RSTP

[David VR]

Hi There,

I am doing some research on the spanning-tree mechanism. I understood that the spanning-tree 802.1d can be used in combination of the RSTP 802.1w. When two switches are sending BPDU between each other they will use the legacy STP BPDU if one is configured in 802.1d and the other one is using RSTP. This to be backwards compatible.

What I was wondering about is that for the legacy spanning tree only the root bridge is sending out BPDU.

So if you are having a mix between both Rapid and legacy spanning tree is this still the case?

I suppose no, you will have BPDU from the root bridge + BPDU from all switches using RSTP, is this correct?

But how will then a STP switch send back BPDU to a RSTP switch if the STP is not the root bridge?

Thanks for the clarification.

[Jon Marshall]

I don't think it will send it back because by definition a BPDU for a vlan would only be exchanged between a legacy STP switch and a switch running RSTP if that vlan was in use on both switches.

And if that was the case then for that vlan it would be running legacy STP so the legacy STP rules would apply.

Only if the vlan is present only on all RSTP switches would the RSTP rules apply.

Hope that makes sense because it does in my head :)

Jon

[paul driver]

Hello

stp has 3 variants 802.1d pre 1998/post 1998/ post 2004

port costs differ also but most recent iOS defaults to STP 802.1post 1998

wth pvst+ & rstp) using this and 802.1s (mst) using post 2004 STP so pvst and rstp would be compatible.

Note: you could also change the port costing in pvst+and rstp to work as post 2004 by changing the default STP cost short method to a long method as such the port costs will change to accommodate much higher interface port speeds

Also non designated ports( root or blocked ) do not send bpdus but recieive them as such the inferior port (root port) listens for the superior bpdus beng sent to it.

res

paul