8.3 Frame Relay Topologies
8.3.1 Connecting remote sites
Frame Relay allows you to interconnect your remote sites in a variety of ways. Example topologies, as shown in the Figure, include the following:
  • A star topology, also known as a hub-and-spoke configuration, is the most popular Frame Relay network topology. In this topology, remote sites are connected to a central site that generally provides a service or application. This is the least expensive topology because it requires the fewest PVCs. In this scenario, the central router provides a multipoint connection because it is typically using a single interface to interconnect multiple PVCs.
  • In a full-mesh topology, all routers have VCs to all other destinations. This method, although costly, provides direct connections from each site to all other sites, and allows for redundancy. When one link goes down, a router at site A can reroute traffic through site C, for example. As the number of nodes in the full-mesh topology increases, the topology becomes increasingly more expensive. The formula to calculate the total number of VCs with a fully meshed WAN is [n(n - 1)]/2.
  • In a partial-mesh topology, not all sites have direct access to each other.

By default, interfaces that support Frame Relay are multipoint connection types. This type of connection is not a problem when only one PVC is configured on an interface. However, it is a problem when multiple PVCs are configured on a single interface. However, it is a problem when multiple PVCs are supported by a single interface. In this situation, routing updates received by the central router cannot be broadcast to the other remote sites.

Depending on the traffic patterns in your network, you may want to have additional PVCs connect to remote sites that require heavy data traffic. In any case, when a single interface must be used to interconnect multiple sites, you may have reachability issues because of the nonbroadcast multiaccess (NBMA) nature of Frame Relay. With Frame Relay running multiple PVCs over a single interface, the primary issue is with split horizon.