Cisco IOS Wide-Area Networking Configuration Guide - Free Books

X.25 Configuration Task ListConfiguring X.25 and LAPBbut allows values outside the target range to be considered as long as the range complies with proceduresdefined in the ITU-T Recommendation X.25. With this feature, the Cisco router allows different flowcontrol value configurations and acceptable window and packet size formats for both DTE devices.The ability to disable flow control parameter negotiation provides compatibility with equipment thatdoes not support flow control parameter negotiation. Similarly, forcing flow control parameternegotiation provides compatibility with devices that require the flow control parameter negotiationfacilities to be present in all calls.To control packet transmission flow values on the interface, use one or more of the flow controlcommands—x25 subscribe flow-control, x25 subscribe windowsize, or x25 subscribe packetsize—ininterface configuration mode.CommandRouter(config-if)# x25 subscribe flow-control {always | never}Router(config-if)# x25 subscribe windowsize {permit wmin wmax |target wmin wmax}Router(config-if)# x25 subscribe packetsize {permit pmin pmax |target pmin pmax}PurposeDetermines flow control parameternegotiation behavior.Sets permitted and target ranges for windowsize negotiation.Sets permitted and target ranges for packetsize negotiation.The flow control subscription commands may be applied to an X.25 interface, to an X.25 profile, or toa LAN interface on which the cmns enable command has been configured. For X.25 over TCP (XOT),the flow control parameter negotiation facilities are always included (the equivalent of x25 subscribeflow-control always).For an example of setting flow control parameter negotiation, see the sections “Setting AsymmetricalWindow and Packet Sizes Flow Control Never Example” and “Configuring Flow Control AlwaysExample” later in this chapter, and for verification see the following section, “Verifying Flow ControlParameter Negotiation.”Verifying Flow Control Parameter NegotiationTo verify flow control parameter settings, use the show running-configuration command in EXECmode. In the following example, X.25 encapsulation has been set on serial interface 1/4 with flow controlnegotiation set to “always.” Permitted packet sizes are set at 64 (minimum) and 1024 (maximum), withtarget packet sizes set at 128 (minimum) and 1024 (maximum). Permitted window sizes are set at 1(minimum) and 7 (maximum), with target window sizes set at 2 (minimum) and 4 (maximum).Router# show running-configurationx25 subscribe flow-control alwaysx25 subscribe packetsize permit 64 1024 target 128 1024x25 subscribe windowsize permit 1 7 target 2 4Setting Default Flow Control ValuesSetting correct default flow control parameters of window size and packet size is essential for correctoperation of the link because X.25 is a strongly flow controlled protocol. However, it is easy to overlookthis task because many networks use standard default values. Mismatched default flow control valueswill cause X.25 local procedure errors, evidenced by Clear and Reset events.12