Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by Cisco systems for routing many protocols across an enterprise Cisco network. It has characteristics of both distance vector routing protocols and link state routing protocols. It is proprietary which requires that you use Cisco routers. EIGRP will route the same protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the same composite metrics as IGRP to select a best path destination. As well there is the option to load balance traffic across equal or unequal metric cost paths. Summarization is automatic at a network class address however it can be configured to summarize at subnet boundaries as well. Redistribution between IGRP and EIGRP is automatic as well. There is support for a hop count of 255 and variable length subnet masks.

Convergence

Convergence with EIGRP is faster since it uses an algorithm called dual update algorithm or DUAL, which is run when a router detects that a particular route is unavailable. The router queries its neighbors looking for a feasible successor. That is defined as a neighbor with a least cost route to a particular destination that doesn’t cause any routing loops. EIGRP will update its routing table with the new route and the associated metric. Route changes are advertised only to affected routers when changes occur. That utilizes bandwidth more efficiently than distance vector routing protocols. 

Autonomous Systems

EIGRP does recognize assignment of different autonomous systems which are processes running under the same administrative routing domain. Assigning different autonomous system numbers isn’t for defining a backbone such as with OSPF. With IGRP and EIGRP it is used to change route redistribution, filtering and summarization points.

Characteristics

Advanced Distance Vector
Routes IP, IPX, Decnet, Appletalk
Routing Advertisements: Partial When Route Changes Occur
Metrics: Bandwidth, Delay, Reliability, Load, MTU Size
Hop Count: 255
Variable Length Subnet Masks
Summarization on Network Class Address or Subnet Boundary
Load Balancing Across 6 Equal or Unequal Cost Paths (IOS 11.0)
Hello Timer: 1 second on Ethernet / 60 seconds on Non-Broadcast
Holddown Timer: 3 seconds on Ethernet / 180 seconds on Non-Broadcast
Metric Calculation = destination path minimum BW * delay (msec) * 25
Bidirectional Forwarding Detection (BFD) Support
Split Horizon
LSA Multicast Address: 224.0.0.10