IBM SC30-3681-08 manual Congestion Monitoring, Congestion Notication and Avoidance

Using Frame Relay

monitoring. You can also dynamically enable and disable CIR monitoring using the

enable cir-monitor and disable cir-monitor console commands.

Congestion Monitoring

Congestion monitoring is an optional feature, set per interface, that allows the VIR

of VCs to vary in response to network congestion. The VIR assumes values

between the minimum IR and a maximum IR of the line speed. Congestion

monitoring is enabled by default. It can be disabled with the disable

congestion-monitorcon®guration command and re-enabled with theenable

congestion-monitorcommand. You can also dynamically enable and disable

congestion monitoring using the enable congestion-monitorand disable

congestion-monitor console commands.

CIR monitoring, if enabled, overrides congestion monitoring. If both CIR monitoring

and congestion monitoring are disabled, the VIR for each VC on the interface is set

to the line speed and does not decrease in response to network congestion.

Note: Even with compression enabled, the device uses the uncompressed size of

frames to determine if the VIR is being exceeded.

Congestion Noti®cation and Avoidance

When congestion occurs, the FR backbone network is responsible for notifying the

sender and receiver by sending out a FECN or a BECN signal. FECN and BECN

are bits that are set in a frame to notify the DTEs at each end of a VC that

congestion is occurring. FECN indicates that congestion is occurring in the same

direction from which the frame was received; the sender is causing the congestion.

BECN indicates that the frames sent by this DTE are causing network congestion.

Optionally, the network can use CLLM messages to convey congestion information

for PVCs. CLLM messages are sent only to the congestion source and should be

treated similarly to BECN messages by the DTE.

The example in Figure 25 on page 401 shows a congestion condition at switch B

when frames are sent from router X to router Y. The FR backbone network noti®es

router X that frames it sends are encountering congestion by setting the BECN bit

in frames sent to router X. The FR backbone network also noti®es router Y that

frames it receives encountered congestion by setting the FECN bit.

When the router receives a frame containing BECN, it is the router's responsibility

to throttle down the VC's VIR (variable information rate) if either CIR monitoring or

congestion monitoring is enabled. The router does this gradually as it receives

consecutive frames with BECN until either the minimum IR is reached or a frame

without BECN arrives. FR switches often set BECN in multiple frames after

reaching a congestion threshold. In order for FR to avoid overreacting to network

congestion when the network is setting multiple frames with BECN, FR will

decrease a VC's VIR at most once every second. This allows the VIR to decrease

gradually. As the router receives consecutive frames without BECN, the VIR

gradually rises to the maximum IR.

Depending on the operation of the FR network, it may be necessary for the device

to throttle down the VC's VIR when the device receives a FECN to minimize the

overall amount of traffic being offered to the network as quickly as possible.

Reducing the overall load on the network reduces the number of packets discarded

400MRS V3.2 Software User's Guide