Frame Relay Components, part 2 - Page 4

 By Cisco Press
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Cisco also implements a traffic control mechanism called ForeSight. ForeSight is the network traffic control software used in some Cisco switches. The Cisco Frame Relay switch can extend ForeSight messages over a UNI, passing the backward congestion notification for VCs.

ForeSight allows Cisco Frame Relay routers to process and react to ForeSight messages and adjust VC level traffic shaping in a timely manner. ForeSight must be configured explicitly on both the Cisco router and the Cisco switch. ForeSight is enabled on the Cisco router when Frame Relay traffic shaping is configured. The router's response to ForeSight is not applied to any VC until the frame-relay adaptive-shaping foresight command is added to the VC's map-class. When ForeSight is enabled on the switch, the switch will periodically send out a ForeSight message based on the time value configured. The time interval can range from 40 to 5,000 milliseconds (ms).

For 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 must be adapted to ForeSight.
In addition, the UNI connecting to the router consolidated link layer management (CLLM) must be enabled, with the proper time interval specified.

Frame Relay Router ForeSight is enabled automatically when the frame-relay traffic-shaping command is used. However, the map-class frame-relay command and the frame-relay adaptive-shaping foresight command must both be issued before the router will respond to ForeSight and apply the traffic shaping effect on a specific interface, subinterface, or VC.

When a Cisco router receives a ForeSight message indicating that certain DLCIs are experiencing congestion, 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 frame with the BECN bit set.

Our next segment from Cisco Press' Network Consultants Handbook will look at Frame Relay Virtual Circuit (VC) Parameters.

This article was originally published on Jan 16, 2002
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