STP & Routing Protocols

[OmidMortazavi]

Hi Guys,

When a network is first converging, STP provides a loop free enviroment on L2.

Routing Protocols, during convergence provides a loop free L3 enviroment.

When first converging in a network, I take it L2 must converge first in order for L3 to converge, therefore some routes in L3 are dictated by the choices of STP in L2.

I.e after convergence, L2 STP is a subset of all available links, and L3 routes are a Subset of the links in L2 STP?

Thanks

Omid

[bjw]

Omid,

STP really stops at the 1st L3 device (router). When a router holds a route, it really isn't concerned with anything other than what interface to use to send packets to. STP is a L2-L2 protocol that ensures that a L2 ennvironment is loop free.

If a packet from R1 needs to get to a host on SW2, R1 only knows what interface to send it to. STP then guides the L2 path of the packet.

They do both play roles, but they have limited dependencies.

[franklaszlo]

Hi Omid,

I believe this is only true in some mls networks. If only L3 interfaces are interconnected, that is routers, then STP plays no role, because packet forwarding is based on L3 addresses and not L2 addresses.

So these things are distinct.

Regards,

Laszlo

[femalwolf]

Hi,

my understanding is L3 converging has nothing to do with L2 converging. L3 activity based on L2 which provide links. So L2 convergen first, then there is a network, then L3 begin to calculate its route on this convergened network.