MPLS L3VPN Routing Information Advertisement

In basic MPLS L3VPN networking, the advertisement of VPN routing information involves CEs and PEs. A P router maintains only the routes of the backbone and does not need to know any VPN routing information. A PE maintains only the routing information of the VPNs directly connected to it, rather than that of all VPNs. Therefore, MPLS L3VPN has excellent scalability.

The VPN routing information of a local CE is advertised in three phases:

1) Advertised from the local CE to the ingress PE.

2) Advertised from the ingress PE to the egress PE.

3) Advertised from the egress PE to the remote CE.

Then, a route is available between the local CE and the remote CE, and the VPN routing information can be advertised on the backbone.

The following describes these phases in detail.

I. Routing information exchange from the local CE to the ingress PE

After establishing an adjacency with the directly connected PE, a CE advertises its VPN routing information to the PE.

The route between the CE and the PE can be a static route, RIP route, OSPF route, IS-IS route, or BGP route. No matter which routing protocol is used, the CE always advertises standard IPv4 routes to the PE.

II. Routing information exchange from the ingress PE to the egress PE

After learning the VPN routing information from the CE, the ingress PE adds RDs and VPN targets for these standard IPv4 routes to form VPN-IPv4 routes, and maintains them for the VPN instance created for the CE.

Then, the ingress PE advertises the VPN-IPv4 routes to the egress PE through MP-BGP.

Finally, the egress PE compares the export target attribute of the VPN-IPv4 routes with the import target attribute that it maintains for the VPN instance and determines whether to add the routes to the routing table of the VPN instance.

PEs use IGP to ensure the connectivity between them

III. Routing information exchange from the egress PE to the remote CE

A remote CE can learn VPN routes from the egress PE in a number of ways. The routes can be static routes, RIP routes, OSPF routes, IS-IS routes, or EBGP routes. The exchange of routing information between the egress PE and the remote CE is the same as that between the local CE and the ingress PE.