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CSR is a "route at the edge, switch in the core" technology designed by Toshiba for ATM networks. It is similar to other multilayer integration technologies, as discussed in the sections "Label Switching" and "Multilayer Switching." CSR was one of the first attempts at multilayer switching on ATM switches. Ipsilon's IP Switching was another. Both control an ATM device using IP protocols, making the ATM switch operate like a switch router.

CSR uses a data-driven model, in which individual traffic flows are detected and a binding is made to a virtual circuit if the flow is sufficient to warrant it. This is called cut-through or shortcut routing. Normal routing is used otherwise. This flow-driven approach limits the number of labels to the flows detected. However, switches must detect flows, which takes time. In some cases, by the time a flow is detected and a shortcut is set up, the flow may actually be ending. In this case, only the end of that flow may gain the benefit of being switched.

A CSR network is composed of CSR multilayer devices with ATM label-switching capabilities, and layer 3 packet routing and forwarding capabilities. The network is seen as a cloud with edge devices (ingress and egress nodes) that connect to legacy networks (standard IP networks running on top of LANs such as Ethernet).

A cut-through path is established from the ingress node to an egress node across intermediate CSR multilayer devices. The cut-through path is established by the ingress CSR and maintained by all the CSRs that make up the path.

Note that CSR solves many problems related to communicating between logical IP subnets (as is the case in overlay models like MPOA), since each device in the network has routing capabilities-not just the edge devices. At the same time, forwarding is handled with fast layer 2 switching.

Like IP switching, CSR was not considered scalable for service provider networks and was limited to use in enterprise LANs. For additional information, refer to the following RFCs:

• RFC 2098 (Toshiba's Router Architecture Extensions for ATM Overview, February 1997)


• RFC 2129 (Toshiba's Flow Attribute Notification Protocol (FANP) Specification, April 1997)

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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