MPLS-Enabled Applications: Emerging Developments and New Technologies

8.2. ROUTING BETWEEN CE AND PE

A key concept in the MPLS/VPN solution is that customer routes are kept in a VPN Routing and Forwarding (VRF) table. The VRF is populated with routes learned from the local CE and routes learned from remote CEs as VPN routes. In the previous sections we saw how customer routes are propagated as VPN-IPv4 routes across the provider's network from PE to PE and added to the appropriate VRF. In this section we will take a closer look at how routes are learned from the local CE.

There are several options for a PE to find out about routes from the CE attached to it: static routing, RIPv2, OSPF and BGP.^[] Regardless of how the PE finds out about the routes, it must install them in the VRF associated with the interface to the CE. Thus, a routing protocol must install routes learned over a CE-PE interface in the VRF associated with that interface. From an implementation point of view, this is accomplished by creating separate contexts for the routing protocols per VRF.

^[] At the time of this writing, a proposal to standardize IS-IS as a CE-PE protocol was under discussion in the IETF. The requirement to allow IS-IS as a CE-PE protocol has not been addressed so far, as most enterprise deployments do not use IS-IS as the IGP.

So far we have seen that the basic requirement is to have VRF aware routing. The next question is whether to use static or dynamic routing, and which routing protocol to use. One important thing to note is that this decision is local to ...