Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring X.25 and LAPBLAPB Configuration Task List• LAPB N1—When you configure a connection to an X.25 network, use the N1 parameter value setby the network administrator. This value is the maximum number of bits in a LAPB frame, whichdetermines the maximum size of an X.25 packet. When you use LAPB over leased lines, the N1parameter should be eight times the hardware MTU size plus any protocol overhead. The LAPB N1range is dynamically calculated by the Cisco IOS software whenever an MTU change, a Layer2/Layer 3 modulo change, or a compression change occurs on a LAPB interface.CautionThe LAPB N1 parameter provides little benefit beyond the interface MTU, and can easily cause linkfailures if misconfigured. Cisco recommends that you leave this parameter at its default value.• LAPB N2—The transmit counter (N2) is the number of unsuccessful transmit attempts that are madebefore the link is declared down.• LAPB T1—The retransmission timer (T1) determines how long a sent frame can remainunacknowledged before the Cisco IOS software polls for an acknowledgment. For X.25 networks,the retransmission timer setting should match that of the network.For leased-line circuits, the T1 timer setting is critical because the design of LAPB assumes that aframe has been lost if it is not acknowledged within period T1. The timer setting must be largeenough to permit a maximum-sized frame to complete one round trip on the link. If the timer settingis too small, the software will poll before the acknowledgment frame can return, which may resultin duplicated frames and severe protocol problems. If the timer setting is too large, the softwarewaits longer than necessary before requesting an acknowledgment, slowing throughput.• LAPB interface outage—Another LAPB timer function that allows brief hardware failures while theprotocol is up, without requiring a protocol reset. When a brief hardware outage occurs, the linkcontinues uninterrupted if the outage corrects before the specified outage period expires.• LAPB T4—The LAPB standards define a timer to detect unsignaled link failures (T4). The T4 timerresets every time a frame is received from the partner on the link. If the T4 timer expires, a ReceiverReady frame with the Poll bit set is sent to the partner, which is required to respond. If the partnerdoes not respond, the standard polling mechanism is used to determine whether the link is down.The period of T4 must be greater than the period of T1.For an example of configuring the LAPB T1 timer, see the section “Typical LAPB ConfigurationExample” later in this chapter.Configuring Priority and Custom Queueing for LAPBLAPB uses priority and custom queueing, which improves the responsiveness of a link to a given typeof traffic by specifying the handling of that type of traffic for transmission on the link.Priority queueing is a mechanism that classifies packets based on certain criteria and then assignspackets to one of four output queues, with high, medium, normal, or low priority.Custom queueing similarly classifies packets, assigns them to one of ten output queues, and controls thepercentage of the available bandwidth of an interface that is used for a queue.For example, you can use priority queueing to ensure that all Telnet traffic is processed promptly andthat Simple Mail Transfer Protocol (SMTP) traffic is sent only when there is no other traffic to send.Priority queueing in this example can starve the non-Telnet traffic; custom queueing can be used insteadto ensure that some traffic of all categories is sent.5