Dynamic routing for Cisco switches, Catalyst 2950

[shen.lu]

There are three Cisco switches, Catalyst 2950 - 2950G 12 EI, which are called SW1, SW2, and SW3. They are connected as a loop by two GBIC ports of each switch: SW1-SW2-SW3-SW1.

If I want to set up automatic fault tolerance, I must use dynamic routing instead of static routing. For example, when the SW1 needs to send data to SW3, normally it can send it directly. But when the connection between the SW1 and the SW3 fails, the SW1 needs to send data to the SW2 first, then redirects to the SW3. Could you please give me some procedures or links to tell me how to implement it from the scratch?

Thanks in advance.

[liviu.gheorghe]

Catalyst 2950 is a layer 2 device, which means that it does not know anything about dynamic routing protocols which are implemented at layer 3 in OSI.

In a layer 2 device topology, loops are managed by the Spanning Tree Protocol, which ensures that at any given time, a loop free topology exists between your switches. This is acomplished by blocking certain ports to eliminate the loop and leaving the rest of the ports in forwarding state.

In case of a link failure between 2 forwarding ports, the STP recalculates the topology and transition the blocking port to forwarding to ensure connectivity again.

[shen.lu]

Yes, thanks. I found the same answer after I found some aticles from Cisco website.

[rjackson]

This isn't routing, its switching. Spanning tree controls it and in a normal state spanning tree will block one of the links so you dont have a loop, you have a tree from the root bridge on down.

Normally you would control who the root is so you indirectly control which link is blocked by SP tree. The switch that has the servers and/or internet access should be the root so users will take the shortest path most of the time.

If a link fails SPtree will unblock the backup link and use it.