- Enterasys Security Router User's Guide

Controlling Congestion in Frame Relay Networks
9-4 Configuring Frame Relay
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-relay traffic-shaping command on the XSR. Adaptive
shaping is the ability to further reduce CIR to alleviate network congestion, enabled by the
frame-
relay adaptive-shaping
command 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.