MPLS-Enabled Applications: Emerging Developments and New Technologies

8.6. CONVERGENCE TIMES IN A VPN NETWORK

It is not enough to build a scalable VPN network. One must also make sure that the network meets the customers' expectation: to have the same convergence times as the alternative of buying circuits and running an IGP-based network over these circuits. When discussing convergence for VPN networks, there are two distinct scenarios: (a) a route delete/add in a customer site and (b) a failure in the provider's network, affecting connectivity between customer sites. Let us examine these separately below.

8.6.1. Convergence time for a customer route change

The propagation of a route add/delete in a customer site includes the following steps:

• Propagation of the route information between the CE and the local PE.

• Propagation of the route information between the local PE and the remote PEs. This step includes:

  ❿ the update generation on the PE;

  ❿ the processing and propagation of the update at the route reflector (if used);

  ❿ the import of the route into the correct VRF on the remote PEs.

• Propagation of the route information between the remote PE and the CEs attached to it.

It is important to understand that in order to provide comparable service to an IGP-based network, the above steps must be performed in an event-driven way rather than a timer-driven way. Some implementations use periodic timers to scan for changes and process them (also known as scan timers). Such implementations are open to a maximum delay of the sum of the scan timer intervals, ...