One of the cool things about MPLS is its versatility.
In this post i will show how its possible for a service provider to support legacy frame-relay installations without actually having any frame-relay switches.
I will establish an MPLS core and show how a customer with three sites, one hub site and two spoke sites, will never even know that the core is running MPLS and not end-to-end frame-relay.
The hub site will have a single access-interface with two frame-relay PVC’s, one to each spoke site, where as the spoke sites will only have a connection to the hub site. So a partial mesh topology.
Seen from the customers perspective, it will logically look like this:
In order to accomplish this behavior, i will be using AToM (Any Transport over MPLS), which is configured with pseudo-wires.
This will be configured on the PE routers attached to each site.
Each PE router will then establish a targeted LDP session with the remote PE router and exchange labels for each of them to use.
In practice this means R2 and R3 establishing a targeted session with R1 and R1 having two sessions, one going to each site.
The physical topology will be setup as this: