Enterasys Networks X-PeditionTM manual Controlling Congestion in Frame Relay Networks

Controlling Congestion in Frame Relay Networks

Address Resolution

The XSR supports dynamic resolution via Inverse ARP to map virtual circuits (DLCI) to remote protocol addresses, as defined in RFC-2390.

Dynamic Resolution Using Inverse ARP

Inverse ARP lets a network node request a next hop IP address corresponding to a given hardware address. Technically, this applies to FR nodes that may have a Data Link Connection Identifier (DLCI), the FR equivalent of a hardware address, associated with an established Permanent Virtual Circuit (PVC), but do not know the IP address of the node on the other side of the link.

Controlling Congestion in Frame Relay Networks

While FR provides dedicated, logical channels throughout the network, these channels share physical resources - links and FR switches, for example. When a DLCI is provisioned, the network assigns a Committed Information Rate (CIR), Committed burst (Bc) and Excess burst (Be) values for the virtual circuit.

Both CIR and Bc values are guaranteed under normal conditions. Excess burst bandwidth, though, is not guaranteed at all times. You can set the CIR rate on the XSR with the frame-relay cir command.

FR network design assumes that not all users will need all of their provisioned bandwidth all the time, and that any unused excess capacity can be borrowed by other customers to send bursts of data exceeding their Committed burst rate. In this environment, it is possible for multiple users to contend for the same resources at the same time causing congestion.

If congestion does occur, FR provides several reactive mechanisms, including explicit congestion notifications that inform end stations that congestion exists on the network.

One issue with reactive congestion controls is that congestion has already occurred. Although congestion is eventually cleared, frames may be lost and response times reduced. This problem can be solved if network traffic is limited to avoid congestion in the first place and that is accomplished with enforced CIR for a PVC.

CIR enforcement also prevents a PVC from hogging all the bandwidth on the access link - the connection between the access device and the FR switch. Without this feature, one VC can use all the access-link bandwidth before FR congestion techniques even start up.

Rate Enforcement (CIR) - Generic Traffic Shaping

Traffic shaping is a high level mechanism of throttling bursty output traffic to address congestion on the network, enabled by the frame-relaytraffic-shapingcommand on the XSR. Adaptive shaping is the ability to further reduce CIR to alleviate network congestion, enabled by the frame- relay adaptive-shapingcommand on the XSR.

CIR is the minimum rate of service that a public FR provider guarantees for a given PVC under normal conditions. FR provides the ability to burst beyond the CIR if bandwidth is available.

You can transmit traffic at a rate exceeding the CIR using Excess Information Rate (EIR), but excess traffic might be discarded in the event of congestion. Traffic shaping prevents traffic from being sent in excess of a value such as CIR, which considerably reduces the likelihood of network congestion. Without this feature, one VC could use all the access-link bandwidth before FR congestion techniques even begin.

9-4 Configuring Frame Relay