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Constraining
distribution of VPN routing information
In layer 3 provider provisioned CE-based VPNs, the VPN tunnels connect CE devices. In this case, distribution of IP routing information occurs between CE devices on the customer sites. No additional constraints on the distribution of VPN routing information are necessary. In layer 3 PE-based VPNs, however, the PE devices must be aware of VPN routing information (for the VPNs to which they are attached). For scalability reasons, one does not want a scheme in which all PEs contain all routes for all VPNs. Rather, only the PEs that are attached to sites in a given VPN should contain the routing information for that VPN. This means that the distribution of VPN routing information between PE devices must be constrained. As VPN membership may change dynamically, it is necessary to have a mechanism that allows VPN route information to be distributed to any PE where there is an attached user for that VPN, and allows for the removal of this information when it is no longer needed. In the Virtual Router scheme, per-VPN tunnels must be established before any routes for a VPN are distributed, and the routes are then distributed through those tunnels. Thus by establishing the proper set of tunnels, one implicitly constrains and controls the distribution of per-VPN routing information. In this scheme, the distribution of membership information consists of the set of VPNs that exists on each PE, as well as information about the desired topology. This enables a PE to determine the set of remote PEs to which it must establish tunnels for a particular VPN. In the aggregated routing scheme, the distribution of VPN routing information is constrained by
means of
route filtering. As VPN membership changes on a PE, the route filters in use between the PE and its peers can be adjusted. Each peer may then adjust the filters in use with each of its peers in turn, and thus the changes propagate across the network. When BGP is used, this filtering may take place at route reflectors as discussed in section 4.4.4. |