8.8 Scalability

The only real problem is scaling. All others inherent are from that one. If you can scale, everything else must be working.

Mike O Dell, Chief Scientist, UUNet
MPLS Conference, Nov 1998

We already discussed some of the scaling properties of the BGP/MPLS VPN approach in Section 8.4, where we observed that the amount of routing peering that a CE router has to maintain is constant and therefore independent of the total number of sites within a VPN, thus allowing support of VPNs with sites numbering in the hundreds or thousands. We also observed that the amount of configuration changes needed when adding or deleting a site is constant and therefore is independent of the total number of sites within a VPN.

Now let s take a look at the scaling properties with respect to handling routing information. First of all, by employing the MPLS hierarchy of routing knowledge, we keep all the P routers completely free of any VPN routing information. That is, P routers don t maintain any VPN routing information the VPN routing information is maintained only on the PE routers.

Second, although a PE router has to maintain VPN routing information, it has to maintain only the routing information for the VPNs whose sites are directly connected to that PE router it doesn t have to maintain routes for all the VPNs supported by a service provider. If the volume of VPN routing information on a particular PE router gets too high for that router, we can add a new...