Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol based on their original IGRP. EIGRP is a distance vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of bandwidth and processing power in the router.

Some of the routing optimizations are based on the Diffusing Update Algorithm (DUAL) work from SRI, which guarantees loop-free operation. In particular, DUAL avoids the "count to infinity" behaviour of RIP when a destination becomes completely unreachable. The maximum hop count of EIGRP-routed packets is 224.

Basic operation

The data EIGRP collects is stored in three tables:

• Neighbor Table: Stores data about the neighbouring routers, i.e. those directly accessible through directly connected interfaces.

• Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains the aggregation of the routing tables gathered from all the neighbours. This table actually contains only those routes which are considered not to potentially be part of routing loops (part of the DUAL algorithm); in EIGRP terminology, they are considered "Feasible Successors".

• Routing table: Stores the actual routes to all destinations; routes may be marked either as "Passive", which is the normal state when the routing has stabilized, or "Active" when the topology has changed, and the router is in the process of updating its route to that destination.

Multiple metrics

EIGRP associates five different metrics with each route:

• Delay
• Bandwidth
• Reliability
• MTU
• Load

For the purposes of comparing routes, these are combined together in a weighted formula to produce a single metric:

[(K1*Bandwidth) + ((K2*Bandwidth)/(256-Load)) + (K3*Delay)] * [(K5/(Reliability + K4))]

where the various constants (K1 through K5) can be set by the user to produce varying behaviours. If K5 is set to zero, the K4/K5 term is not used (i.e. taken as 1). The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to:

Bandwidth + Delay

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably result.

Other details

EIGRP is able to deal with classless interdomain routing allowing the use of VLSM which is its main advantage over its predecessor. Its main disadvantage is that it only runs on Cisco equipment which may lead to an organization being locked in to this vendor.

EIGRP can run separate routing processes for IP, IPX and AppleTalk. However, this does not facilitate translation between protocols.

Further reading

• Introduction to EIGRP
• EIGRP Metric
• EIGRP

External links

• Cisco white paper