1-8Figure 1-6 Network diagram for extranet networking schemeCECEPE 1PE 3Site 2Site 1Site 3VPN 1VPN 1VPN 2VPN 1:Import:100:1Export:100:1CEVPN 2:Import:200:1Export:200:1PE 2VPN 1:Import:100:1,200:1Export:100:1,200:1In Figure 1-6, VPN 1 and VPN 2 can access Site 3 of VPN 1.z PE 3 can receive the VPN-IPv4 routes advertised by PE 1 and PE 2.z PE 1 and PE 2 can receive the VPN-IPv4 routes advertised by PE 3.z Based on the above, Site 1 and Site 3 of VPN 1 can communicate with each other, and Site 2 ofVPN 2 and Site 3 of VPN 1 can communicate with each other.z PE 3 advertises neither the VPN-IPv4 routes received from PE 1 to PE 2, nor the VPN-IPv4 routesreceived from PE 2 to PE 1 (that is, routes learned from an IBGP neighbor will not be advertised toany other IBGP neighbor). Therefore, Site 1 of VPN 1 and Site 2 of VPN 2 cannot communicatewith each other.MPLS L3VPN Routing Information AdvertisementIn 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 routinginformation. A PE maintains only the routing information of the VPNs directly connected to it, rather thanthat 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 informationcan be advertised on the backbone.The following describes these phases in detail.Routing information exchange from the local CE to the ingress PEAfter establishing an adjacency with the directly connected PE, a CE advertises its VPN routinginformation to the PE.The route between the CE and the PE can be a static route, RIP route, OSPF route, IS-IS route, or BGProute. No matter which routing protocol is used, the CE always advertises standard IPv4 routes to thePE.
