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MULTILAYER SWITCH PERFORMANCE

LAN switches generally achieve very good price/performance by applying hardware-based frame forwarding techniques to the process of moving packets from one LAN segment to another. This is relatively easy to engineer, since the forwarding decision is based on a simple MAC address (or RIF field) look-up table, and the forwarding process involves no change to the content of the packet.

Routers, on the other hand, generally exhibit much lower forwarding rates at considerably higher cost. Three reasons for this are:

  The packet forwarding decision in a router is considerably more complex than that in a LAN switch.
  A number of changes have to be made to the contents of each packet received from one LAN segment before it can be forwarded to another—for example, the entire MAC packet header has to be replaced, and the Time-To-Live field has to be updated in the IP packet header.
  If packet filtering is used to implement network security, each packet may have to be compared with a number of complex filtering criteria.

This additional complexity has meant that many routers implement the packet forwarding process mostly or entirely in software—which accounts for the inferior price/performance characteristics of routers compared with LAN switches.

The Layer 3 forwarding function in a multilayer switch is subject to the same complexities and packet processing responsibilities as exist in a conventional router. Therefore it is not safe to assume that a product described as a “multilayer switch” automatically offers higher performance than a conventional router for forwarding packets between subnets. Performance depends very much on the manner in which the Layer 3 forwarding function has been implemented.

If the Layer 3 forwarding function is implemented entirely in hardware, then a multilayer switch should be able to show the same high performance whether it is switching at Layer 2 or Layer 3. If, as in most routers, the Layer 3 forwarding function is implemented largely in software, then the multilayer switch is likely to perform no better than a typical router when forwarding at Layer 3, and in many cases its performance may be substantially worse.

Real multilayer switch products are likely to show wide variations in the performance of their Layer 3 forwarding. In this area particularly, all switches are not created equal.

SUMMARY

In this chapter we have described multilayer IP/IPX switching as a complete solution for scalable, high performance, cost-effective LAN infrastructures. The advantages of this approach over a classical router-based backbone include much improved price/performance characteristics, greater flexibility to cope with growing traffic loads and changing traffic patterns, elimination of the need for IP address re-assignment during moves and changes, and fewer different kinds of devices to manage in the network.

The approach we described to multilayer IP/IPX switching complies with the existing standards for LAN switching and for routing, and does not introduce any new or proprietary protocols into the LAN. Therefore multilayer IP/IPX switches can be readily supported in multivendor networks, and furthermore, they are compatible with equipment that is installed in existing networks. This means both an easy migration path to a more effective networking solution—with protection of existing networking investments—and the reassurance that this solution will not create undesirable lock-in to a single vendor.

There can be little doubt that multilayer IP/IPX switching is the right way forward for large and medium-sized corporate and enterprise LANs. Where routers are already installed in the LAN, further investment in router upgrades can be frozen, and the additional capacity needed in the LAN can be provided by multilayer IP/IPX switches working alongside the routers. As traffic loads continue to grow, changing traffic patterns bring new stresses onto the backbone, and new classes of applications demand support for real-time traffic on the LAN, multilayer IP/IPX switching will provide the simplest and most cost-effective answers to the problem.


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