IPX Routing Fundamentals

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Within Cisco IOS Software, IPX can be routed using three possible routing protocols:

• IPX RIP

• IPX EIGRP

• Netware Link Services Protocol (NLSP)

NLSP is a link-state protocol developed by Novell based on the ISO Intermediate System-to-Intermediate System (IS-IS) routing protocol. NLSP is beyond the scope of this book, so the focus will be limited to IPX RIP and IPX EIGRP. Additional information regarding NLSP and its configuration can be found on CCO at www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/atipx_c/ipx/2cdipx.htm#xtocid2449537.

IPX RIP

Novell RIP, commonly known as IPX RIP, is a distance vector routing protocol and is the default routing protocol for IPX. IPX RIP uses two metrics to make routing decisions: ticks (a time measure equal to 1/18 second) and hop count (a count of each router traversed), to a maximum of 15. IPX RIP checks its two distance vector metrics by first comparing the ticks for path alternatives. If two or more paths have the same tick value, IPX RIP compares the hop count. Each IPX router periodically broadcasts copies of its IPX RIP routing table to its directly connected networks. The neighboring IPX routers increment the distance vector values as required before propagating their IPX RIP tables to other networks. The split-horizon algorithm prevents the neighbor from broadcasting IPX RIP routes back to the networks from where it received the information originally. IPX RIP also uses a hold-down timer to handle conditions in which an IPX router goes down without any explicit message to its neighbors. This prevents the downed link from being advertised to other routers.

Routing table updates are sent at 60-second intervals, by default. The default packet size is 576 bytes and can contain up to 50 route entries. This update frequency can cause excessive overhead traffic on some internetworks. Because of this overhead as well as the limit of 15 maximum hops, IPX RIP might not scale well in large internetworks or in WAN environments. For such cases, IPX EIGRP is a desirable alternative.

IPX EIGRP

EIGRP is an enhanced version of IGRP developed by Cisco. Similar to IP EIGRP, IPX EIGRP uses the same distance vector algorithm and distance information. In addition, the convergence properties and the operating efficiency of EIGRP have improved significantly over other distance vector routing protocols such as IGRP or RIP.

IPX EIGRP overcomes many of the limitations of IPX RIP through the following features:

• Fast convergence The DUAL algorithm allows routing information to converge as quickly as any currently available routing protocol.

• Partial updates EIGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature minimizes the bandwidth required for Enhanced IGRP packets.

• Less CPU usage Full update packets need not be processed each time they are received.

• Neighbor-discovery mechanism This feature is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent .

• Scaling EIGRP scales to large networks.

For a detailed review of each feature as well as how the DUAL algorithm operates, refer to Chapter 10, "Enhanced Interior Gateway Protocol (EIGRP)."

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