Network Warrior by Gary A. Donahue

Author:Gary A. Donahue
Language: eng
Format: mobi, epub, pdf
Tags: Reference:Computers
ISBN: 9780596101510
Publisher: O'Reilly Media
Published: 2008-12-17T10:00:00+00:00

Frame-Relay Subinterfaces

Sometimes, having two PVCs terminating on a single interface is not what you want, but, as we've seen, having a physical interface for each PVC is not beneficial for cost reasons. For example, with the network in Figure 22-11, each PVC terminates into a single interface. If you ran a routing protocol on these routers, Router B would advertise itself, but Router A would not advertise this route out to Router C because of the split-horizon rule. Splitting the PVCs into separate interfaces would allow the routing protocol to advertise the route, because the split-horizon rule would no longer apply.

Cisco routers have a feature called subinterfaces that solves this problem. In a nutshell, you're able to configure virtual interfaces for each PVC. These virtual interfaces are named after the physical interfaces on which they are found. For example, a subinterface derived from S0/0 might be called S0/0.100. The subinterface number is user-definable, and can be within the range of 1 to 4,294,967,293. I like to name subinterfaces according to the DLCIs mapped to them.

There are two types of subinterfaces: point-to-point and multipoint. Point-to-point subinterfaces can have only one DLCI active on them, while multipoint subinterfaces can have many. Multipoint subinterfaces behave in much the same way that physical interfaces do: you can have a mix of point-to-point and multipoint subinterfaces on a physical interface. It is even possible to have some DLCIs assigned to subinterfaces, and others to the physical interface.

As mentioned earlier, one of the main benefits of frame-relay subinterfaces is the elimination of split-horizon issues with routing protocols. Creating multiple point-to-point subinterfaces, and assigning each of the PVCs to one of them, enables each PVC to be considered a different interface. Subinterfaces are created with the global interface command. Specify the name you'd like the subinterface to have, along with the keyword point-to-point or multipoint:

Router-A(config)#int s0/0.102 point-to-point

Router-A(config-subif)#

You're now in interface configuration mode for the newly created subinterface, and can configure this subinterface as you would a physical interface.

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