Abstract of CCNA study guide-25 - EIGRP 1

EIGRP Features and Operation

Enhanced IGRP (EIGRP) is:

-        Classless

-        enhanced distance-vector protocol

-        used in autonomous system

-        includes the subnet mask in its route updates.

-        Allow using (VLSMs) and summarization

-        EIGRP is a hybrid routing protocol because it has characteristics of both distance-vector and link-state  protocols.

§  As distance vector : it sends distance-vector updates about networks plus the cost of reaching them

§  As link state: synchronizes routing tables between neighbors at startup and then sends specific updates only when topology changes occur.

-        EIGRP has a maximum hop count of 255 (default is 100).

There are a number of powerful features that make EIGRP a real standout from IGRP and other protocols:

-        Support for IP and IPv6  via protocol dependent modules

-        Considered classless (same as RIPv2 and OSPF)

-        Support for VLSM/CIDR

-        Support for summaries and discontiguous networks

-        Efficient neighbor discovery

-        Communication via Reliable Transport Protocol (RTP)

-        Best path selection via Diffusing Update Algorithm (DUAL)

Protocol-Dependent Modules (PDM)

EIGRP provides routing support for multiple Network layer protocols: IP, IPX, AppleTalk, and now IPv6. That routing support comes through the use of protocol-dependent modules (PDMs).

Each EIGRP PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol. What this means to you is that there will be IP/EIGRP tables and IPv6/EIGRP tables, for example.

Neighbor Discovery

There are three conditions that must be met for neighborship establishment (adjacencies):

-        Hello or ACK received

-        AS numbers match

-        Identical metrics (K values)

Link-state protocols use Hello messages to establish neighborship (also called adjacencies)

The only time EIGRP advertises its entire routing table is when it discovers a new neighbor and forms an adjacency with it through the exchange of Hello packets. When this happens, both neighbors advertise their entire routing tables to one another. After each has learned its neighbor’s routes, only changes to the routing table are propagated from then on.

When EIGRP routers receive their neighbors’ updates, they store them in a local topology table. This table contains all known routes from all known neighbors and serves as the raw material from which the best routes are selected and placed into the routing table.

Let’s define some terms before we move on:

Feasible distance: This is the best metric along all paths to a remote network, including the metric to the neighbor that is advertising that remote network

Reported/advertised distance : This is the metric of a remote network, as reported by a neighbor.

Neighbor table : Each router keeps state information about adjacent neighbors .

Topology table: It contains all destinations advertised by neighboring routers, holding each destination address and a list of neighbors that have advertised the destination.

The neighbor and topology tables are stored in RAM and maintained through the use of Hello and update packets.

Feasible successor

A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. EIGRP will keep up to six feasible successors in the topology table.

Successor :A successor route (think successful!) is the best route to a remote networkand is stored in the routing table.

Reliable Transport Protocol (RTP)

EIGRP uses a proprietary protocol called Reliable Transport Protocol (RTP) to manage the communication of messages between EIGRP-speaking routers.

When EIGRP sends multicast traffic, it uses the Class D address 224.0.0.10. For each multicast EIGRP sends out, it maintains a list of the neighbors who have replied. If EIGRP doesn’t get a reply from a neighbor, it will switch to using unicasts to resend the same data. If it still doesn’t get a reply after 16 unicast attempts, the neighbor is declared dead.

Diffusing Update Algorithm (DUAL)

EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and maintaining the best path to each remote network.

DUAL provides EIGRP with possibly the fastest route convergence time among all protocols.

The key to EIGRP’s speedy convergence is twofold:

First, EIGRP routers maintain a copy of all of their neighbors’ routes, which they use to calculate their own cost to each remote network. If the best path goes down, it examines the contents of the topology table to select the best replacement route.

Second, if there isn’t a good alternative in the local topology  table, EIGRP routers ask their neighbors for help finding one.

Using EIGRP to Support Large Networks

EIGRP includes features that make it suitable for use in large networks:

-        Support for multiple ASes on a single router

-        Support for VLSM and summarization

-        Route discovery and maintenance

Multiple ASes

EIGRP uses autonomous system numbers to identify the collection of routers that share route information. Only routers that have the same autonomous system numbers share routes.

 it’s possible to divide the network into multiple distinct EIGRP autonomous systems, or ASes. Each AS is populated by a contiguous series of routers, and route information can be shared among the different ASes via redistribution.

VLSM Support and Summarization

 EIGRP supports the use of Variable Length Subnet Masks. because the subnet mask is propagated with every route update.

EIGRP also supports the use of discontiguous subnets.

The discontiguous network: is the one that has two or more subnetworks of a Classful network connected together by different classful networks.

Figure 7.1 displays a typical discontiguous network. The subnets 172.16.10.0 and 172.16.20.0 are connected together with a 10.3.1.0 network. By default, each router thinks it has the only 172.16.0.0 classful network.

It’s important to understand that discontiguous networks just won’t work with RIPv1 or IGRP at all. And they don’t work by default on RIPv2 or EIGRP either, but discontiguous networks do work on OSPF networks by default because OSPF does not auto-summarize like EIGRP. But no worries about EIGRP—there are ways to make EIGRP work. I’ll show you how to do that a bit later in this chapter.

EIGRP automatically summarizes networks at their classful boundaries, and Figure 7.2 shows how a router running EIGRP would see the network plus the boundaries that it would auto-summarize.

Route Discovery and Maintenance

EIGRP uses a series of tables to store important information about its environment:

Neighborship table : (usually referred to as the neighbor table) records information about routers with whom neighborship relationships have been formed.

Topology table : stores the route advertisements about every route in the internetwork received from each neighbor.

Route table: stores the routes that are currently used to make routing decisions.

EIGRP Metrics

to compare routes and select the best possible path, EIGRP can use a combination of four:

-        Bandwidth

-        Delay

-        Reliability

-        Load

Like IGRP, EIGRP uses only bandwidth and delay of the line to determine the best path to a remote network by default.

there’s a fifth element, maximum transmission unit (MTU) size.

Maximum Paths and Hop Count

By default, EIGRP can provide equal-cost load balancing of up to four links. However, you can have EIGRP actually load-balance across up to six links (equal or unequal) by using the following command:

Pod1R1(config)#router eigrp 10

Pod1R1(config-router)#maximum-paths ?

<1-6> Number of paths

In addition, EIGRP has a maximum hop count of 100, but it can be set up to 255:

Pod1R1(config)#router eigrp 10

Pod1R1(config-router)#metric maximum-hops ?

<1-255> Hop count