Adaptive Shaping to ForeSight
Adaptive shaping to ForeSight allows a Cisco router to respond to ForeSight messages and to react to network congestion by enabling VC-level traffic shaping and rate limiting in a timely manner. Adaptive shaping to ForeSight is a component of Frame Relay Traffic Shaping. On Cisco switches, router ForeSight is a network traffic control software that allows the Frame Relay switches to extend ForeSight messages over a User-to-Network (UNI) interface and passes the BECN bit for VCs.
To enable adaptive shaping to ForeSight on a Cisco router, Frame Relay Traffic Shaping must be configured on the main interface. Additionally, the Frame Relay switch must be able to support adaptive shaping to ForeSight.
NOTE
By default, adaptive shaping to ForeSight is enabled on a Cisco router when Frame Relay Traffic Shaping is configured. However, it is not applied to any VC until the frame-relay adaptive-shaping foresight command is applied to the specified VC's map class.
When adaptive shaping to ForeSight is configured on a Frame Relay switch, the switch periodically sends out ForeSight messages based on a configured time interval (which ranges from 40 to 5000 milliseconds). On receiving ForeSight messages that carry congestion information from the switch, the router becomes aware that certain data-link connection identifiers (DLCIs) are congested. The router responds to the ForeSight message by applying traffic shaping to slow down the output rate on the congested VCs.
Compared with adaptive shaping to BECN, the router reacts in the same way as if it had received a BECN tagged packet. Therefore, both adaptive shaping to BECN and ForeSight reduce the output traffic rate during periods of network congestion. A notable difference between BECN and ForeSight is that BECN requires a user packet to be sent in the direction of the congested DLCI to inform a congested state. However, sending user packets is unpredictable and thus is generally not a reliable mechanism. On the other hand, timed ForeSight messages allow a router to always receive a congestion notification message before congestion escalates to become a significant problem. With adaptive shaping to ForeSight, traffic shaping is applied timely to slow down the output traffic rate on the congested DLCIs.
Frame Relay Traffic Shaping is available as of IOS Software Release 11.2 with adaptive shaping to BECN support. In Cisco IOS Software Release 11.3, adaptive shaping to ForeSight message support has been added to the traffic shaping feature's list of functionalities.
NOTE
The frame-relay adaptive-shaping { becn | foresight } command introduced in IOS Release 11.3 has replaced the frame relay becn-response-enable command.
To enable adaptive shaping to ForeSight on a Cisco router, Frame Relay Traffic Shaping must be configured on the main interface. Additionally, the Frame Relay switch must be able to support adaptive shaping to ForeSight.
NOTE
By default, adaptive shaping to ForeSight is enabled on a Cisco router when Frame Relay Traffic Shaping is configured. However, it is not applied to any VC until the frame-relay adaptive-shaping foresight command is applied to the specified VC's map class.
When adaptive shaping to ForeSight is configured on a Frame Relay switch, the switch periodically sends out ForeSight messages based on a configured time interval (which ranges from 40 to 5000 milliseconds). On receiving ForeSight messages that carry congestion information from the switch, the router becomes aware that certain data-link connection identifiers (DLCIs) are congested. The router responds to the ForeSight message by applying traffic shaping to slow down the output rate on the congested VCs.
Compared with adaptive shaping to BECN, the router reacts in the same way as if it had received a BECN tagged packet. Therefore, both adaptive shaping to BECN and ForeSight reduce the output traffic rate during periods of network congestion. A notable difference between BECN and ForeSight is that BECN requires a user packet to be sent in the direction of the congested DLCI to inform a congested state. However, sending user packets is unpredictable and thus is generally not a reliable mechanism. On the other hand, timed ForeSight messages allow a router to always receive a congestion notification message before congestion escalates to become a significant problem. With adaptive shaping to ForeSight, traffic shaping is applied timely to slow down the output traffic rate on the congested DLCIs.
Frame Relay Traffic Shaping is available as of IOS Software Release 11.2 with adaptive shaping to BECN support. In Cisco IOS Software Release 11.3, adaptive shaping to ForeSight message support has been added to the traffic shaping feature's list of functionalities.
NOTE
The frame-relay adaptive-shaping { becn | foresight } command introduced in IOS Release 11.3 has replaced the frame relay becn-response-enable command.
