Hi Taqdir, I think RFC2547 is a good source for a better understanding. As far as I see, RTs are designed for only remote PE(s) and RDs are designed for all routers in vpnv4 routing domain . P routers do not distinguish routes according to RTs they do according to RDs, and remote PE distinguish routes according to RTs (accept or not). So RTs are used in remote PEs and RD is used in MP-BGP backbone -different functions- Why does not remote PE designed do that by looking at RD? Because each route can have only one RD but multiple RTs. Using RDs instead of RTs would restrict the design goals.
Best Regards Gokhan >From FRC2547: Note that a route can only have one RD, but it can have multiple Target VPNs. In BGP, scalability is improved if one has a single route with multiple attributes, as opposed to multiple routes. One could eliminate the Target VPN attribute by creating more routes (i.e., using more RDs), but the scaling properties would be less favorable. How does a PE determine which Target VPN attributes to associate with a given route? There are a number of different possible ways. The PE might be configured to associate all routes that lead to a particular site with a particular Target VPN. Or the PE might be configured to associate certain routes leading to a particular site with one Target VPN, and certain with another.
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