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Fast IP

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

3Com's FastIP is a 3Com proprietary flow-driven, cut-through routing technique designed as an overlay for layer 2 switched networks. It is one of many schemes that attempt to take advantage of fast switching in layer 2 and avoid the overhead of routing every packet on a hop-by-hop basis.

For related information, see "Multilayer Switching" and "IP over ATM."

FastIP was originally devised by Cascade, IBM and 3Com together, but Cascade merged with Ascend and IBM dropped out, leaving FastIP completely in the hands of 3Com.

A cut-though route is basically a switched path that directly connects a source and destination in different virtual subnets, thus avoiding routing. In the ATM world, a cut-through route is a virtual circuit over an ATM network. Cut-though can be accomplished when multiple IP subnets exist on the same switched network. FastIP uses a modified version of the IETF NHRP protocol as its method for establishing cut-through routes over the ATM network. NHRP is discussed under its own heading.

Chuck Semeria's paper (see reference on the related entries page) describes FastIP's operations. The basic operation is paraphrased below:

  • Two end-systems (host A and host B) on different IP subnets need to exchange packets across a common switched infrastructure.
  • Host A transmits data packets to its default gateway router. In parallel, host A transmits an NHRP request (containing host A's MAC) to host B.
  • When the default gateway receives the NHRP request, it forwards the NHRP request (if policy permits) to host B.
  • Upon receipt of the NHRP request, host B sends an NHRP response back to host A using the MAC provided in the packet.
  • A cut-through path can now be constructed across the layer 2 switching network.

Fast IP is designed to work on what 3Com calls open group VLANs. According to the 3Com paper mentioned above, an open group VLAN contains traffic according to VLAN IDs but allows stations to direct traffic to VLANs other than their own.

An important feature of FastIP is policy management. Policy management is similar to quality of service. Most legacy networks don't have it. The result is that all stations on a shared network wait to transmit while another station completes its transmission. Policy management can give high-priority traffic like voice calls priority over less important traffic, like e-mail transmission. 3Com feels that policies should be set at the source, i.e., the desktop and server. The idea is to let a workstation or server request a level of priority, then tag the associated frames as appropriate. FastIP supports this approach.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.