Cisco IOS Wide-Area Networking Configuration Guide - Free Books

Configuring Frame Relay Traffic ShapingConfiguring Frame Relaycommand is added to the VCs map-class. When ForeSight is enabled on the switch, the switch willperiodically send out a ForeSight message based on the time value configured. The time interval canrange from 40 to 5000 milliseconds.When a Cisco router receives a ForeSight message indicating that certain DLCIs are experiencingcongestion, the Cisco router reacts by activating its traffic-shaping function to slow down the output rate.The router reacts as it would if it were to detect the congestion by receiving a packet with the backwardexplicit congestion notification (BECN) bit set.When ForeSight is enabled, Frame Relay traffic shaping will adapt to ForeSight messages and BECNmessages.For an example of configuring Foresight, see the section “Traffic Shaping with ForeSight Example” laterin this chapter.Frame Relay ForeSight PrerequisitesFor router ForeSight to work, the following conditions must exist on the Cisco router:• Frame Relay traffic shaping must be enabled on the interface.• The traffic shaping for a circuit is adapted to ForeSight.The following additional condition must exist on the Cisco switch:• The UNI connecting to the router is Consolidated Link Layer Management (CLLM) enabled, withthe proper time interval specified.Frame Relay router ForeSight is enabled automatically when you use the frame-relay traffic-shapingcommand. However, you must issue the map-class frame-relay command and the frame-relayadaptive-shaping foresight command before the router will respond to ForeSight and apply thetraffic-shaping effect on a specific interface, subinterface, or VC.Frame Relay Congestion Notification MethodsThe difference between the BECN and ForeSight congestion notification methods is that BECN requiresa user packet to be sent in the direction of the congested DLCI to convey the signal. The sending of userpackets is not predictable and, therefore, not reliable as a notification mechanism. Rather than waitingfor user packets to provide the congestion notification, timed ForeSight messages guarantee that therouter receives notification before congestion becomes a problem. Traffic can be slowed down in thedirection of the congested DLCI.Configuring Enhanced Local Management InterfaceEnhanced Local Management Interface (ELMI) allows the router to learn QoS parameters andconnectivity information from the Cisco switch and to use this information for traffic shaping,configuration, or management purposes. ELMI simplifies the process of configuring traffic shaping onthe router and reduces chances of specifying inconsistent or incorrect values when configuring therouter. ELMI works between Cisco routers and Cisco switches (BPX and IGX platforms).14