BROCADE IP PRIMER

Chapter 12: Multi Protocol Label Switching (MPLS)The customer edge router connects to the LER via a routing protocol (RIPv2,OSPF, External BGP and static routes). Note that the LER must provide connectivityfor multiple VPN customers. In order to support multiple VPN customers,the LER uses VRFs. A VRF is the routing and forwarding instance for a set ofsites with identical connectivity requirements. The following is associated withVRFs:• IP Routing Tables• Routing Protocol Contexts• Interfaces that use the VRF• Route Distinguisher• Route TargetsSince a LER can only support one routing process, routing contexts must beused. The routing contexts create virtual routing instances (IP routing tables),allowing for multiple routing instances. Each VRF (note that a VRF is not a VPN,only the routing table) is associated with a routing context.Now that we have the routing tables separated for each of the customers onthe LER, we still have the problem of possible overlapping of MPLS VPNaddress. To overcome this problem, Route Distinguisher is used. Route Distinguisher(RD) converts the non-unique IP address into a unique VPN-IPv4addresses. To determine which VPN the customer is associated and providethis information to other Peer LERs, a route target is used.A route-target extended community, or route target, is a type of BGP extendedcommunity that you use to define VPN membership. The route target appearsin a field in the update messages associated with VPN-IPv4. Route-targetimport lists and route-target export lists are created for each VRF. The routetargets that are placed in a route target export list are attached to every routeadvertised to other LERs. When a LER receives a route from another LER, itcompares the route targets attached to each route against the route-targetimport list defined for each of its VRFs. If any route target attached to a routematches the import list for a VRF, then the route is imported to that VRF. If noroute target matches the import list, then the route is rejected for that VRF.268 Brocade IP Primer