We have seen in this chapter that BGP-4 and OSPF can be used to provide PE-to-CE connectivity, in addition to the static and RIP Version 2 options that we saw in the previous chapter. These protocols complete the list of currently available connectivity options, although others, such as EIGRP and IS-IS, may become available over time.
Whichever protocol is used, customer VRFs will be populated based on the routes learned from the CE-routers. To identify which VRFs should receive which routes, either routing contexts (in the case of RIP Version 2 and BGP) or a separate routing process (in the case of OSPF) is used.
MP-iBGP is used to propagate VPN customer routes between PE-routers. In most cases, except with BGP-4, redistribution is necessary to allow the routes within a VRF to be placed into the BGP table for onward advertisement.
Various design options have also been reviewed, and a sample network migration from an overlay VPN solution to an MPLS/VPN-based solution has been provided. Using this migration scenario, we've also seen that the AS Override feature may be required when BGP-4 is used across the PE-to-CE links, if the same autonomous system number is used in more than one site.