VPNv4 Route Propagation in the Mpls Vpn Network

The VRF separates the customer routes on the PE routers, but how are the prefixes transported across the service provider network? Because, potentially, numerous routes—perhaps hundred of thousands—could be transported, BGP is the ideal candidate because it is a proven and stable routing protocol for carrying that many routes. Just realize that BGP is the standard routing protocol for carrying the complete Internet routing table. Because the customer VPN routes are made unique by adding the RD to each IPv4 route—turning them into vpnv4 routes—all customer routes can safely be transported across the MPLS VPN network.

An overview of the route propagation in an MPLS VPN network is shown in Figure 7-7.

Figure 7-7 Route Propagation in an MPLS VPN Network

Figure 7-7 Route Propagation in an MPLS VPN Network

The PE router receives IPv4 routes from the CE router through an Interior Gateway Protocol (IGP) or external BGP (eBGP). These IPv4 routes from the VPN site are put into the VRF routing table. Which VRF is used depends on the VRF that is configured on the interface on the PE router toward the CE router. These routes are appended with the RD that is assigned to that VRF. Thus, they become vpnv4 routes, which are then put into MP-BGP. BGP takes care of distributing these vpnv4 routes to all PE routers in the MPLS VPN network. On the PE routers, the vpnv4 routes are stripped of the RDs and put into the VRF routing table as IPv4 routes. Whether the vpnv4 route, after stripping off the RD, is put into the VRF depends on whether the RTs allow the import into the VRF. These IPv4 routes are then advertised to the CE router through an IGP or eBGP that is running between the PE and CE router. Figure 7-8 shows the steps in the route propagation from CE to CE through the MPLS VPN network.

Figure 7-8 Route Propagation in an MPLS VPN Network Step by Step

IPv4 route Is redistributed Into MP-BGP RD Is added to IPv4 route to make It a VPNv4 route. RTs are added.

RTs Indicate to which VRF the route Is Imported. RD Is removed from VPNv4 route.

VPN 1

IPv4 route Is redistributed Into MP-BGP RD Is added to IPv4 route to make It a VPNv4 route. RTs are added.

RTs Indicate to which VRF the route Is Imported. RD Is removed from VPNv4 route.

VPN 1

IPv4 route is inserted into VRF routing table.

IPv4 route is inserted into VRF routing table.

IPv4 route is inserted into VRF routing table.

Because the service provider that is running the MPLS VPN network runs BGP in one autonomous system, iBGP is running between the PE routers.

The propagation from eBGP—running between the PE and CE router—to MP-iBGP in the MPLS VPN network and vice versa is automatic and needs no extra configuration. However, the redistribution of MP-iBGP into the IGP that is running between the PE and CE router is not automatic. You need to configure mutual redistribution between MP-iBGP and the IGP.

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