246If the PE needs to advertise to a CE the routes from other OSPF domains, it must indicate that it isthe ASBR, and advertise the routes using Type 5 LSAs.Sham linkGenerally, BGP peers carry routing information on the MPLS VPN backbone through the BGP extendedcommunity attributes. The OSPF that runs on the remote PE can use the information to create Type 3summary LSAs to be transmitted to the CEs. As shown in Figure 74, both site 1 and site 2 belong to VPN1 and OSPF area1. They are connected to different PEs, PE 1 and PE 2. There is an intra-area OSPF linkcalled backdoor link between them. In this case, the route connecting the two sites through PEs is aninter-area route. It is not preferred by OSPF because its preference is lower than that of the intra-arearoute across the backdoor link.Figure 74 Network diagram for sham linkTo resolve the problem, you can establish a sham link between the two PEs so that the routes betweenthem over the MPLS VPN backbone become an intra-area route.The sham link acts as an intra-area point-to-point link and is advertised through the Type 1 LSA. You canselect a route between the sham link and backdoor link by adjusting the metric.The sham link is considered the link between the two VPN instances with one endpoint address in eachVPN instance. The endpoint address is a loopback interface address with a 32-bit mask in the VPNaddress space on the PE. Different sham links of the same OSPF process can share an endpoint address,but that of different OSPF processes cannot.BGP advertises the endpoint addresses of sham links as VPN-IPv4 addresses. A route across the shamlink cannot be redistributed into BGP as a VPN-IPv4 route.A sham link can be configured in any area. You need to configure it manually. In addition, the local VPNinstance must have a route to the destination of the sham link.NOTE:When configuring an OSPF sham link, redistribute OSPF VPN routes to BGP, but do not redistribute BGProutes to OSPF to avoid routing loops.BGP AS number substitution and SoOBecause BGP detects routing loops by AS number, if EBGP runs between PEs and CEs, you must assigndifferent AS numbers to geographically different sites to ensure correct transmission of the routinginformation.
