Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring X.25 and LAPBConfiguring an X.25 Datagram TransportFigure 41 illustrates two routers sending datagrams across an X.25 PDN.Figure 41Transporting LAN Protocols Across an X.25 PDNRouter XX.25public data networkRouter Y62858To complete the X.25 configuration for your network needs, perform the tasks in the following sections:• Configuring Point-to-Point and Multipoint Subinterfaces• Mapping Protocol Addresses to X.121 Addresses• Establishing an Encapsulation PVC• Setting X.25 TCP/IP Header Compression• Configuring X.25 BridgingConfiguring the X.25 parameters and special features, including payload compression and X.25 userfacilities, is described in the section “Configuring Additional X.25 Datagram Transport Features” laterin this chapter.Configuring Point-to-Point and Multipoint SubinterfacesSubinterfaces are virtual interfaces that can be used to connect several networks to each other through asingle physical interface. Subinterfaces are made available on Cisco routers because routing protocols,especially those using the split horizon principle, may need help to determine which hosts need a routingupdate. The split horizon principle, which allows routing updates to be distributed to other routedinterfaces except the interface on which the routing update was received, works well in a LANenvironment in which other routers reached by the interface have already received the routing update.However, in a WAN environment using connection-oriented interfaces (like X.25 and Frame Relay),other routers reached by the same physical interface might not have received the routing update. Ratherthan forcing you to connect routers by separate physical interfaces, Cisco provides subinterfaces that aretreated as separate interfaces. You can separate hosts into subinterfaces on a physical interface, X.25 isunaffected, and routing processes recognize each subinterface as a separate source of routing updates,so all subinterfaces are eligible to receive routing updates.There are two types of subinterfaces: point-to-point and multipoint. Subinterfaces are implicitlymultipoint unless configured as point-to-point.A point-to-point subinterface is used to encapsulate one or more protocols between two hosts. An X.25point-to-point subinterface will accept only a single encapsulation command (such as the x25 map orx25 pvc command) for a given protocol, so there can be only one destination for the protocol. (However,you can use multiple encapsulation commands, one for each protocol, or multiple protocols for one mapor PVC.) All protocol traffic routed to a point-to-point subinterface is forwarded to the one destinationhost defined for the protocol. (Because only one destination is defined for the interface, the routingprocess need not consult the destination address in the datagrams.)A multipoint subinterface is used to connect one or more hosts for a given protocol. There is norestriction on the number of encapsulation commands that can be configured on a multipointsubinterface. Because the hosts appear on the same subinterface, they are not relying on the router todistribute routing updates among them. When a routing process forwards a datagram to a multipoint19