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MULTILAYER IP/IPX SWITCHES

We have explained how a multilayer switch can operate at both Layer 2 and Layer 3 for communications using IP—the Internet Protocol. But IP is not the only protocol that may need to be processed at both Layer 2 and Layer 3 in the LAN. In fact Novell's Internetwork Packet eXchange (IPX) protocol is currently even more widely used in enterprise LANs than IP, and therefore ideally we need a solution for IPX also.

IPX has some different characteristics from IP, particularly in the area of addressing and subnetting. With IP, we have a strictly limited number of possible hosts in a single subnet because the entire IP address has only 4 bytes which have to be split between the network or subnet number, and the host number. By contrast, the IPX address consists of 10 bytes, of which 4 bytes is the network number and the remaining 6 bytes are copied from the LAN adapter's MAC address—therefore we don't have the same limitations as to how many stations can belong to a single subnet. Also, an IP subnet is defined as a group of stations that share the same network or subnet number in their address—which is normally configured into the station's software. Whereas an IPX subnet is effectively defined as all the stations in a broadcast domain, since the stations learn their network number from a server or servers, by means of a broadcast request.

These differences mean that, in principle, we can treat IP and IPX differently in the switched LAN from the point of view of Layer 3 forwarding. We could, for example, define the entire switched LAN as a single broadcast domain which contains multiple IP subnets but which would contain only a single IPX subnet. In this case there would be no need for any Layer 3 forwarding function for IPX within the multilayer switch, since all IPX communications could be conducted with Layer 2 switching alone.

In practice, however, there may be good reasons for aligning IP and IPX subnetting. Foremost among these is the requirement to keep broadcasts under control. Since the broadcast protocols associated with IPX, the Service Advertising Protocol (SAP) and the Routing Information Protocol (RIP) are prone to generating excessive amounts of broadcast traffic, one way of dealing with this is to break the network up into a number of separate broadcast domains, each of which will contain one IP subnet and one IPX subnet. This preserves the same scheme of subnetting that is imposed when we use a conventional router to connect LAN segments together for IP and IPX communications, only with multilayer switching we can define these broadcast domains logically instead of physically. This technique is widely known under the name “Virtual LANs”—of which more later.

Clearly, if we are going to have both IP and IPX in the LAN, and we choose to divide the switched LAN into multiple separate broadcast domains, then we are going to need Layer 3 forwarding for both IP and IPX in the multilayer switch. This then becomes a “multilayer IP/IPX” switch.

IP and IPX are by no means the only protocols that are seen in today's LANs. Other protocols include both “routable” and “non-routable” protocols. IP and IPX are both routable protocols which embody the concept of hierarchical Network Layer addressing with the need for Layer 3 forwarding between subnets. Of the other common routable protocols, DECnet is probably the most widely employed, although its use is declining. When considering how to deal with lesser-used routable protocols, we should bear in mind that it isn't strictly necessary for a multilayer switch to have the capability for Layer 3 forwarding of every routable protocol in the LAN. If we are able selectively to forward these protocols at Layer 2—effectively with a bridging function—then we may have a satisfactory solution, albeit one which forces the adoption of a single subnet within the switched LAN for such protocols.

Many LANs today are also handling non-routable protocols such as NetBIOS (or NetBEUI), DEC Local Area Transport (LAT) or IBM DLC. Since none of these protocols recognizes the concept of Layer 3 forwarding, they must be bridged at Layer 2 within a multilayer switch.


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