Frame Relay Overview

In a Frame Relay topology, two or more branch offices are connected to a main office via permanent virtual circuits (PVCs). All traffic passes through a Frame Relay switch (WAN cloud) and is routed by a Frame Relay Access Device (FRAD) at the main office. To send traffic between branch offices, FRADs route the traffic from one PVC to the other. When a PVC becomes congested, Frame Relay sends Forward/Backward Explicit Congestion Notification (FECN and BECN) bits to FRADs at both ends of the PVC in an attempt to slow down the traffic. In extreme congestion, packets are dropped and must be retransmitted.

Packeteer units can enhance a Frame Relay network in several ways. First, the units can ensure each PVC gets the Committed Information Rate (CIR), up to the Excess Information Rate (EIR). This is accomplished with PacketWise's partition feature. CIR maps to PacketWise's partition size, and EIR maps to the excess amount by which the partition can burst. Second, PacketWise's TCP rate control can help smooth flow of TCP traffic, thereby reducing the likelihood of dropped packets. Third, PacketWise's policy feature allows network administrators to control the speed, response, and priority of individual applications running over the WAN or on a LAN. Fourth, PacketWise automatically creates a traffic class for each PVC using the dynamic IP routing information on the FRAD. This saves you from having to manually specify all the subnets/IP addresses for each PVC. Finally, PacketWise can throttle back traffic in response to the presence of FECN/BECN bits in the flow to avoid further congestion of the PVC.

Packeteer units are installed behind the FRAD at the main office and, optionally, between the FRAD and LAN at each branch office. The PacketWise software uses SNMP polling to gather the data it needs from the local FRAD. Based upon this information, PacketWise can determine how much traffic is occurring on the PVCs and can discover how many FECN/BECN bits have been generated on the PVCs. By having knowledge of all traffic traversing the access link, PacketWise can manage the link's bandwidth effectively.

PacketWise Frame Relay Support

PacketWise's Frame Relay support performs the following functions:

1. After you configure the FRADs, PacketWise creates an inbound and outbound traffic class for each PVC and automatically creates a partition for each of these classes.
   
   Initially, the partition size is set to 0, burstable to the link size. You can use the frame override command to set the CIR and EIR for each PVC partition.
   
   
2. PacketWise updates its traffic tree and Frame Relay configuration with information it collects from regular polling of the FRAD.
   
   
3. PacketWise automatically adjusts the available bandwidth of its inbound and outbound PVC partitions, based on current bandwidth consumption and levels of congestion on these PVCs. For example, PacketWise can increase the partition size to optimize use of the PVC if data congestion occurs at the FRAD.
   
   Note: PacketWise makes these adjustments to the PVC partition sizes, regardless of whether shaping is turned on or off. However, shaping must turned be on for the partition adjustments to actually take effect and be enforced.

PacketWise polls the FRAD immediately after a reset, and then every 15 minutes and looks for the following types of configuration updates:

• Data Link Control identifiers (DLCIs) of new PVCs configured on the FRAD
• Changes in the name and number of the WAN interface(s) on the FRAD
• Changes in PVC status — for example, a PVC has gone down or a new one has been added
• Changes to IP routing tables, which PacketWise uses to associate destination addresses with PVCs

PacketWise polls the FRAD more frequently for updates on bandwidth usage and congestion level.

Prerequisite Checklist

PacketWise's Frame Relay support should only be used in specific environments. If a unit is configured with Frame Relay support, you cannot use PolicyCenter to share its tree or Frame Relay configuration because Frame Relay PVC classes must each have a unique name. You can, however, subscribe it to PolicyCenter for other reasons, like image distribution or sharing setup parameters.

Before implementing the Frame Relay feature, verify the following:

• If using a Cisco router, the IOS version is 11.3 or later.
• If using a non-Cisco router, the router software supports the following RFCs: 1315 or 2115, 1573, and 1156.
• The FRAD supports the MIB variables used by Packeteer's Frame Relay feature.
• Router does not have more than 1000 routes associated with the PVCs.
• Router relies on dynamic routing for the IP routes associated with their PVCs; static routes may not be picked up by PacketWise.
• You are not using the Easy Configure feature. It is recommended that you not use Easy Configure with Frame Relay networks.

See also:

Configure Frame Relay Support

PVC Traffic Classes

Frame Relay Tips

Frame Relay Notes