Bridging IPX between token ring and FDDI or Ethernet

Beware of a little-known problem when bridging IPX between token ring and FDDI or Ethernet networks. "Did I hear you say bridging?" "IPX?" "Token ring?" You heard correctly.

With the advent of gigabit routers, IP's transcendence to one of the most widely used protocols and the introduction of super-fast LANs, it may be easy to forget three veteran technologies like bridging, IPX and token ring. "Bridge where you can, route where you must" is still a rule of thumb when designing a network. Furthermore, there remains a huge installed base of NetWare and some 20 million token ring-attached desktops.

The combination of how IPX resolves its MAC addresses from its routing address, and MAC address transmission differences among token ring, Ethernet (all flavors) and FDDI, makes bridging IPX in certain instances impossible. IPX has a routing address format of "network.node address"; the node address is taken directly from the MAC address of an originating station. Conversely, the MAC address is resolved from the node address on the receiving end.

By doing this, IPX eliminates the need for a resolution protocol such as IP's "ARP" (where the router must "ask" the endstation what its MAC address is). Token ring transmits its MAC addresses with the most significant bit (MSB) of the MAC address first, known as noncanonical format. Ethernet and FDDI, on the other hand, transmit theirs with the least significant bit (LSB) first, or in canonical format. Thus, a token-ring IPX packet transmitted to Ethernet will be sent to a MAC address that doesn't exist because the MAC address will be bit-reversed and the node address will not.

If you are using IPX in a heterogeneous bridged LAN environment, speak to your bridge manufacturer about special code to make IPX bridging possible, or simply obey the adage and "route where you must."

The Tolly Group, a strategic consulting and independent testing organization, offers a full range of services designed to furnish the vendor and the end-user communities with authoritative and unbiased information. The Tolly Group is a leader in assessing leading edge technologies and provides such services as: network design, product evaluations, industry studies, and market research. For more information, visit The Tolly Group's World Wide Web site, send e-mail to info@tolly.com, call 800-933-1699 or 732-528-3300, or fax 732-528-1888.

Routing for small networks: Hardware or software?
A review. Network World, 1/12/98.

Info on NetWare 4.x Bridging
From the Novell listserv FAQ.

An example of IPX routing from Cisco

Support tips for bridges and routers from 3Com.

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