MPLS L3VPN route advertisement
In a basic MPLS L3VPN, CEs and PEs are responsible for advertising VPN routing information. P routers maintain only the routes within the backbone. A PE maintains only routing information for directly connected VPNs, rather than for all VPNs.
VPN routing information is advertised from the local CE to the remote CE by using the following process:
From the local CE to the ingress PE:
The CE advertises standard IPv4 routing information to the ingress PE over a static route, RIP route, OSPF route, IS-IS route, EBGP route, or IBGP route.
From the ingress PE to the egress PE:
The ingress PE adds RD and route target attributes to these standard IPv4 routes to create VPN-IPv4 routes, saves them to the routing table of the VPN instance created for the CE, and advertises the VPN-IPv4 routes to the egress PE through MP-BGP.
From the egress PE to the remote CE:
After receiving the VPN-IPv4 routes, the egress PE compares their export target attribute with the local import target attribute, and, if they match, adds the routes to the routing table of the VPN instance. Then the egress PE restores the VPN-IPv4 routes to the original IPv4 routes and advertises those routes to the connected CE over a static route, RIP route, OSPF route, IS-IS route, EBGP route, or IBGP route.