ATM Overview

In an Asynchronous Transfer Mode (ATM) topology, two or more branch offices are connected to a main office via virtual circuits (VCs). All traffic passes through an ATM WAN cloud and can be routed by a router at the main office. To send traffic between branch offices, some routers may route the traffic from one virtual circuit to the other.

PacketShapers can enhance an ATM network in several ways. First, the units can ensure each virtual circuit gets its Peak Cell Rate (PCR). This is accomplished with Packeteer's partition feature. Second, Packeteer's TCP rate control can help smooth the flow of TCP traffic, thereby reducing the likelihood of dropped packets. Third, Packeteer'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 VC using the dynamic IP routing information on the router. This saves you from having to manually specify all the subnets/IP addresses for each VC. Finally, PacketWise automatically adjusts the available bandwidth of its inbound and outbound VC partitions when it detects compression or contention on the router.

PacketShapers are installed behind the router at the main office and, optionally, between the router and LAN at each branch office. The PacketWise software uses SNMP polling to gather the data it needs from the local router. Based upon this information, PacketWise can determine how much traffic is occurring on the virtual circuits. By having knowledge of all traffic traversing the access link, PacketWise can manage the link's bandwidth effectively.

Note: The ATM feature is only compatible with Cisco routers using IOS version 12.0 or later.

PacketWise ATM Support

Packeteer's ATM support performs the following functions:

1. After you configure the routers, PacketWise creates an inbound and outbound traffic class for each virtual circuit and automatically creates a partition for each of these classes.
   
   Initially, the partition size is set to 0, burstable to the Peak Cell Rate (PCR); if PacketWise is unable to determine the PCR by polling the router, it uses the link size. You can use the atm override command to set the size for each VC partition.
   
   
2. PacketWise updates its traffic tree and ATM configuration with information it collects from regular SNMP polling of the router.
   
   
3. PacketWise automatically adjusts the available bandwidth of its inbound and outbound VC partitions when it detects compression or contention on the router. For example, if PacketWise detects that it sent 100k to the router but only 80k went on the circuit, it will increase the partition size to optimize use of the VC. Or, if PacketWise sent 100k to the router and the router actually sent 120k out (in other words, something else was using 20k of the link), PacketWise would reduce the partition size by 20k.
   
   Note: PacketWise makes these adjustments to the VC 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 router immediately after a reset, and then every 15 minutes and looks for the following types of configuration updates:

• Virtual Path Identifiers (VPIs) and Virtual Channel Identifiers (VCIs) of new VCs configured on the router
• Changes in the name and number of the WAN interface(s) on the router
• Changes in VC status — for example, a VC has gone down or a new one has been added
• Changes to IP routing tables, which PacketWise uses to associate destination addresses with VCs

PacketWise polls the router more frequently for updates on bandwidth usage.

ATM Feature Requirements

Before implementing the ATM feature, verify the following:

• You must be using a Cisco router.
• The IOS version must be 12.0 or later.
• The router must support the MIB variables used by Packeteer's ATM feature. To see Cisco's router MIB support lists, go to ftp://ftp.cisco.com/pub/mibs/supportlists/.
• The router does not have more than 2000 routes associated with the VCs.
• Router relies on dynamic routing for the IP routes associated with their VCs; static routes may not be picked up by PacketWise.

Note: The ATM feature is not available on PacketShaper ISP, PacketShaper 1200, and PacketShaper 1400 Lite models.

See also:

Configure ATM Support

VC Traffic Classes for ATM

ATM Tips