AIS AI2524 user manual Point-to-point subinterfaces can be unnumbered for us, Otherwise result

Chapter 13: AI2524 Frame Relay Configuration Steps

Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual in- terface can now be forwarded out another virtual interface, even if they are configured on the same physical interface.

Subinterfaces address the limitations of Frame Relay networks by pro- viding a way to subdivide a partially meshed Frame Relay network into a number of smaller, fully meshed (or point-to-point) subnet works. Each subnetwork is assigned its own network number and ap- pears to the protocols as if it is reachable through a separate interface.

Note:	Point-to-point subinterfaces can be unnumbered for us
	with IP, reducing the addressing burden that might
	otherwise result.

For example, suppose you have a 5-node Frame Relay network (see Figure 13-2) that is partially meshed (Network A). If the entire net- work is viewed as a single subnetwork (with a single network number assigned), most protocols assume that node A can transmit a packet di- rectly to node E, when in fact it must be relayed through nodes C and D. This network can be made to work with certain protocols (for ex- ample, IP) but will not work at all with other protocols because nodes C and D will not relay the packet out the same interface on which it was received. One way to make this network function fully is to creat a fully meshed network (Network B), but doing so requires a larg number of PVCs, which may not be economically feasible.

Using subinterfaces, you can subdivide the Frame Relay network into three smaller subnetworks (Network C) with separate network num- bers. 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 two subinterfaces and can therefore forward packets without violating split horizon rules. If transparent bridging is being used, each subinterfac is viewed as a separate bridge port.

August 1997	Page 13-19
2524UM