Frame Relay subinterfaces provide a mechanism for supporting partially meshed Frame Relay networks. Most protocols assume transitivity on a logical network; that is, if station A can communicate with station B, and station B can communicate to station C, then station A should be able to communicate to station C directly. Transitivity is true on LANs, but not on Frame Relay networks unless A is directly connected to C.
Additionally, certain protocols such as AppleTalk and transparent bridging are not supported on partially meshed networks because they require
split horizon . Split horizon is a routing technique in which a packet received on an interface cannot be sent from the same interface even if received and transmitted on different virtual circuits (VCs)
Configuring Frame Relay subinterfaces ensures that a single physical interface is considered as multiple virtual interfaces. Hence, packets received on one virtual interface can be forwarded to another virtual interface even if they are configured on the same physical interface.
Subinterfaces address the limitations of Frame Relay networks by providing an option to subdivide a partially meshed Frame Relay network into a number of smaller, fully meshed (or point-to-point) subnetworks. Each subnetwork is assigned its own network number and appears to the protocols as if it were reachable through a separate interface. (Note that point-to-point subinterfaces can be unnumbered for use with IP, thus reducing the addressing burden that might otherwise result.)
Cisco IOS XE software supports configuration of point-to-point subinterfaces.
The figure below shows a five-node Frame Relay network that is partially meshed (network A). If the entire network is viewed as a single subnetwork (with a single network number assigned), most protocols assume that node A can transmit a packet directly to node E, when, in fact it must be relayed through nodes C and D. This network can work with certain protocols (for example, IP). However, this network does not work with other protocols (for example, AppleTalk), because nodes C and D do not relay the packet out at the same interface on which it was received. To make this network fully functional, we need to created a fully meshed network (network B). However, a fully meshed network requires a large number of permanent virtual circuits (PVCs), which may not be economically feasible.
Figure 2. Using Subinterfaces to Provide Full Connectivity on a Partially Meshed Frame Relay Network
By using subinterfaces, you can divide the Frame Relay network into 3 smaller subnetworks (network C) with separate network numbers. Nodes A, B, and C are connected to a fully meshed network, and nodes C and D, as well as nodes D and E, are connected via point-to-point networks. In this configuration, nodes C and D can access 2 subinterfaces and can therefore forward packets without violating split horizon rules. If transparent bridging is being used, each subinterface is viewed as a separate bridge port.