EIGRP

The Enhanced Interior Gateway Routing Protocol (Enhanced IGRP) is a routing protocol developed by Cisco Systems and introduced with Software Release 9.21 and Cisco Internetworking Operating System (Cisco IOS) Software Release 10.0. Enhanced IGRP combines the advantages of distance vector protocols, such as IGRP, with the advantages of link-state protocols, such as Open Shortest Path First (OSPF). Enhanced IGRP uses the Diffusing Update Algorithm (DUAL) to achieve convergence quickly.

Enhanced IGRP includes support for IP, Novell NetWare, and AppleTalk. The discussion on Enhanced IGRP covers the following topics:

·         Enhanced IGRP Network Topology

·         Enhanced IGRP Addressing

·         Enhanced IGRP Route Summarization

·         Enhanced IGRP Route Selection

·         Enhanced IGRP Convergence

·         Enhanced IGRP Network Scalability

·         Enhanced IGRP Security

Enhanced IGRP Network Topology

Enhanced IGRP uses a nonhierarchical (or flat) topology by default. Enhanced IGRP automatically summarizes subnet routes of directly connected networks at a network number boundary. This automatic summarization is sufficient for most IP networks. See the section "Enhanced IGRP Route Summarization" later in this chapter for more detail.

Enhanced IGRP Addressing

The first step in designing an Enhanced IGRP network is to decide on how to address the network. In many cases, a company is assigned a single NIC address (such as a Class B network address) to be allocated in a corporate internetwork. Bit-wise subnetting and variable-length subnetwork masks (VLSM’s) can be used in combination to save address space. Enhanced IGRP for IP supports the use of VLSM’s.

Enhanced IGRP Route Summarization

With Enhanced IGRP, subnet routes of directly connected networks are automatically summarized at network number boundaries. In addition, a network administrator can configure route summarization at any interface with any bit boundary, allowing ranges of networks to be summarized arbitrarily.

Enhanced IGRP Route Selection

Routing protocols compare route metrics to select the best route from a group of possible routes. The following factors are important to understand when designing an Enhanced IGRP internetwork.

Enhanced IGRP uses the same vector of metrics as IGRP. Separate metric values are assigned for bandwidth, delay, reliability and load. By default, Enhanced IGRP computes the metric for a route by using the minimum bandwidth of each hop in the path and adding a media-specific delay for each hop. The metrics used by Enhanced IGRP are as follows:

·         Bandwidth-Bandwidth is deduced from the interface type. Bandwidth can be modified with the bandwidth command.

·         Delay-Each media type has a propagation delay associated with it. Modifying delay is very useful to optimize routing in network with satellite links. Delay can be modified with the delay command.

·         Reliability-Reliability is dynamically computed as a rolling weighted average over five seconds.

·         Load-Load is dynamically computed as a rolling weighted average over five seconds.

·         MTU-MTU is the Maximum Transmission Unit or the maximum size of the packets that can be transmitted on a particular media.  Larger MTUs produce more efficient transport.

When Enhanced IGRP summarizes a group of routes, it uses the metric of the best route in the summary as the metric for the summary.

Enhanced IGRP Convergence

Enhanced IGRP implements a new convergence algorithm known as DUAL (Diffusing Update Algorithm). DUAL uses two techniques that allow Enhanced IGRP to converge very quickly. First, each Enhanced IGRP router stores its neighbors routing tables. This allows the router to use a new route to a destination instantly if another feasible route is known. If no feasible route is known based upon the routing information previously learned from its neighbors, a router running Enhanced IGRP becomes active for that destination and sends a query to each of its neighbors asking for an alternate route to the destination. These queries propagate until an alternate route is found. Routers that are not affected by a topology change remain passive and do not need to be involved in the query and response.

A router using Enhanced IGRP receives full routing tables from its neighbors when it first communicates with the neighbors. Thereafter, only changes to the routing tables are sent and only to routers that are affected by the change. A successor is a neighboring router that is currently being used for packet forwarding, provides the least cost route to the destination, and is not part of a routing loop. Information in the routing table is based on feasible successors. Feasible successor routes can be used in case the existing route fails. Feasible successors provide the next least-cost path without introducing routing loops.

The routing table keeps a list of the computed costs of reaching networks. The topology table keeps a list of all routes advertised by neighbors. For each network, the router keeps the real cost of getting to that network and also keeps the advertised cost from its neighbor. In the event of a failure, convergence is instant if a feasible successor can be found. A neighbor is a feasible successor if it meets the feasibility condition set by DUAL. DUAL finds feasible successors by the performing the following computations:

Enhanced IGRP Network Scalability

Network scalability is limited by two factors: operational issues and technical issues. Operationally, Enhanced IGRP provides easy configuration and growth. Technically, Enhanced IGRP uses resources at less than a linear rate with the growth of a network.

Memory

A router running Enhanced IGRP stores all routes advertised by neighbors so that it can adapt quickly to alternate routes. The more neighbors a router has, the more memory a router uses. Enhanced IGRP automatic route aggregation bounds the routing table growth naturally. Additional bounding is possible with manual route aggregation.

CPU

Enhanced IGRP uses the DUAL algorithm to provide fast convergence. DUAL re-computes only routes, which are affected by a topology change. DUAL is not computationally complex, so it does not require a lot of CPU.

Bandwidth

Enhanced IGRP uses partial updates. Partial updates are generated only when a change occurs; only the changed information is sent, and this changed information is sent only to the routers affected. Because of this, Enhanced IGRP is very efficient in its usage of bandwidth. Some additional bandwidth is used by Enhanced IGRP's HELLO protocol to maintain adjacencies between neighboring routers.

Enhanced IGRP Security

Enhanced IGRP is available only on Cisco routers. This prevents accidental or malicious routing disruption caused by hosts in a network.

In addition, route filters can be set up on any interface to prevent learning or propagating routing information inappropriately.