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Routing protocol basics, part 1

A detailed look at the characteristics of IGP routing protocols RIP and IGRP/EIGRP.

Managing routers requires a knowledge of routers' basic languages, namely routing protocols. The more understanding you have of basic protocols the easier it will be to troubleshoot your network's routing schemes later. This excerpt from Informit looks at the basics of some Interior Gateway Protocols.

Routing Information Protocol (RIP) was originally designed for Xerox PARC Universal Protocol (PUP), and in many ways is still for "pups." It was called GWINFO in the Xerox Network Systems (XNS) protocol suite in 1981, and defined in RFC 1058 in 1988. It is easy to configure, and it works very well in small networks. In larger networks, however, it can be less effective; as I say to myself, "It can RIP you apart." There are alternatives to RIP for larger environments.

The following are RIP characteristics:

  • Open protocol, widely used, stable.
  • Good for small networks in that it is very easy to configure.
  • There are RIP-like distance vector routing protocols for Novell and AppleTalk.
  • Distance vector routing protocol.
  • IGP.
  • IP RIP updates are sent every 30 seconds via broadcast ( for RIPv2).
  • UDP port 520
  • Administrative distance is 120.
  • Single metric is hop count. (The limit is 15 to assist with count-to-infinity.)
  • Timers help regulate performance:
    • Update timer—Frequency of routing updates. Every 30 seconds IP RIP sends a complete copy of its routing table, subject to split horizon. (IPX RIP does this every 60 seconds.)
    • Invalid timer—Absence of refreshed content in a routing update. RIP waits 180 seconds to mark a route as invalid and immediately puts it into holddown.
    • Hold-down timers and triggered updates—Assist with stability of routes in the Cisco environment. Holddowns ensure that regular update messages do not inappropriately cause a routing loop. The router doesn't act on non-superior information for a certain period of time. RIP's hold-down time is 180 seconds.
    • Flush timer—RIP waits an additional 240 seconds after holddown before it actually removes the route from the table.
  • Other stability features to assist with routing loops include the following:
    • Split horizon—Not useful to send information about a route back in the direction from which it came.
    • Poison reverse—Updates that are sent to invalidate a route and place it in holddown.
  • Bellman-Ford algorithm.
  • RIPv2 supports VLSM and summarization. (RIPv1 doesn't.) RIPv2 always autosummarizes at the class boundary.


Interior Gateway Routing Protocol (IGRP) was developed in the mid-1980s as a Cisco proprietary protocol to help overcome some of limitations of RIP, such as the single metric of hop count. It has stability features similar to RIP—hold-down timers, split horizon, poison reverse, and triggered updates. The timers are as follows: invalid 270 seconds, holddown 280 seconds, and flush 630 seconds. It also contains mechanisms to influence route selection and unequal load sharing. I use the phrase Big Dogs Really Like Meat to remember the metrics for IGRP and EIGRP:

  • Bandwidth
  • Delay
  • Reliability
  • Load
  • MTU

IGRP is an IGP, a distance vector routing protocol based on the Bellman-Ford algorithm that broadcasts routing updates every 90 seconds over IP protocol number 9. It is fine for small and medium-size networks, but Cisco enhanced it greatly and added VLSM support to its replacement, EIGRP.

Cisco developed Enhanced IGRP (EIGRP) in the early 1990s to overcome limitations of RIP and its own IGRP. Cisco says IGRP is going to be removed from IOS. EIGRP is suitable for large networks today and supports multiple routed protocols. It consumes significantly less bandwidth because of its partial, bounded updates, and can be one of the fastest converging routing protocols there is.

The following are EIGRP characteristics:

  • Cisco proprietary protocol.
  • Good for small to large networks.
  • Very easy to configure. Uses autonomous system (AS) number.
  • Supports multiple Layer 3 routed protocol stacks, such as IP, Novell IPX, and AppleTalk.
  • Advanced distance vector routing protocol. Often called a hybrid due to its incremental updates and rapid convergence capabilities.
  • IGP.
  • Multicast triggered updates over, not periodic.
  • IP protocol number 88.
  • Internal administrative distance is 90; external is 170.
  • Metrics are bandwidth, delay, reliability, load, and MTU.
  • Supports equal- and unequal-cost load sharing.
  • Other stability features to assist with routing loops:
    • Split horizon—Not useful to send information about a route back in the direction from which it came
    • Poison reverse—Updates that are sent to remove a route and place it in holddown
  • Uses Diffusing Update algorithm (DUAL) to select loop-free paths and give it very fast convergence.
  • Supports VLSM and manual summarization (classless).
  • Automatic classful boundary summarization.
  • Manual summarization on update sent out each interface.
  • Automatic redistribution with IGRP if same AS number.
  • Route tagging for policy-based routing.

EIGRP gets its reliability from the Reliable Transport Protocol (RTP). It maintains not only a routing table, but also a neighbor and topology table. EIGRP maintains alternate routes referred to as successors (routing table) and feasible successors (topology table) to quickly converge. The following packet types are used for neighbor communications: hellos (multicast) and acks (unicast), update (multicast or unicast), query (multicast), reply (unicast), and request (multicast or unicast). Packets are held in a queue for retransmission, and there are separate neighbor tables (and entirely separate processes) for each protocol.

Next week we will look at OSPF, IS-IS and BGP. For more on network troubleshooting, check out Informit.

This was last published in May 2005

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