Cisco IOS Wide-Area Networking Configuration Guide - Free Books

X.25 Configuration Task ListConfiguring X.25 and LAPBCommandRouter(config-if)# x25 modulo {8 | 128}PurposeSets the packet-numbering modulo.NoteBecause X.25 requires the DTE and DCE devices to have identical modulos, changes you make to themodulo when the interface is up remain until X.25 restarts the packet service.The X.25 modulo and the LAPB modulo are distinct and serve different purposes. LAPB modulo 128(or extended mode) can be used to achieve higher throughput across the DTE or DCE interface, whichaffects only the local point of attachment. X.25 PLP modulo 128 can be used to achieve higherend-to-end throughput for VCs by allowing more data packets to be in transit across the X.25 network.Setting the X.121 AddressIf your router does not originate or terminate calls but only participates in X.25 switching, this task isoptional. However, if your router is attached to a PDN, you must set the interface X.121 address assignedby the X.25 network service provider. Interfaces that use the DDN or BFE mode will have an X.121address generated from the interface IP address; for correct DDN or BFE operation, any such X.121address must not be modified.To set the X.121 address, use the following command in interface configuration mode:CommandRouter(config-if)# x25 address x121-addressPurposeSets the X.121 address.For an example of configuring the X.25 interface address, see the section “Typical X.25 ConfigurationExample” later in this chapter.Configuring X.25 Switch Local AcknowledgmentX.25 switch local acknowledgment allows you the choice of configuring local or end-to-endacknowledgment on your router. End-to-end acknowledgment can result in lower overall throughput andrestrictive performance because an endpoint can only have a limited number of its packets in transit atany given time. End-to-end acknowledgment cannot send more packets until all have been acknowledgedby the transmission and receipt of the delivery-confirming packet containing the D-bit.Local acknowledgment means that the Cisco router can send acknowledgments for packets that do nothave the D-bit set, before receiving an acknowledgment from the interface to which the packet wasforwarded. This results in higher throughput of packets because acknowledgment is sent between localhops much faster and more efficiently than between end-to-end hops.Figure 39 shows the Cisco router receiving packets from DTE A destined for DTE B. Without localacknowledgment enabled, the router forwards packets to the X.25 network and then forwardsacknowledgments from the network back to DTE A. With local acknowledgment enabled, the router canacknowledge packets received from DTE A before it has received acknowledgments from the networkfor the forwarded packets. In this illustration, the X.25 network may also generate localacknowledgments.10