CCNA Course

EIGRP

EIGRP Features

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol based on IGRP

Released in 1994, Unlike IGRP, which is a classful routing protocol, EIGRP supports CIDR and VLSM.

it is probably one of the two most popular routing protocols in use today.

Compared to IGRP, EIGRP boasts faster convergence times, improved scalability, and superior handling of routing loops.

EIGRP is a hybrid routing protocol, offering the best of distance vector and link-state algorithms.

EIGRP vs IGRP IGRP and EIGRP are compatible with each other.

EIGRP offers multiprotocol support, but IGRP does not.

Communication via Reliable Transport Protocol (RTP)

Best path selection via Diffusing Update Algorithm (DUAL)

Improved convergence time

Reduced network overhead

EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric.

EIGRP has a maximum hop count limit of 224.

EIGRP is a classless routing protocol

EIGRP Operation

EIGRP routers that belong to different autonomous systems (ASes) don’t automatically share routing information

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

EIGRP Processes All these processes and tables lead toward building the

IPv4 routes in the routing table, as follows:

Neighbor discovery: routers send Hello messages to

discover neighbors, and perform basic parameter checks.

Topology exchange: Neighbors exchange full topology

updates when the neighbor relationship comes up, and

then only partial updates as needed based on changes

Choosing routes: Each router analyzes its topology tables,

choosing the lowest-metric route to reach each subnet.

And place best route for each subnet into routing table

Neighbor Table

EIGRP uses hello packets to discover potential neighbors

Hello sent to multicast address 224.0.0.10

Routers check the parameters in received hello to agree:

It must pass the authentication process if used

It must use the same Autonomous System Number (ASN)

The source IP address must be in the same subnet as the local router’s interface IP address/mask

The same “k” values

Routers stores address and interface of neighbor as verification succeeded

Topology Table When two routers become neighbors, can begin exchanging

topology information using update messages.

Update messages sent to multiple routers in the same subnet using multicast address 224.0.0.10

Update can be sent to particular router using unicast address

EIGPR send updates as Reliable Transport Protocol (RTP)

Neighbors use both full routing updates and partial updates

All learned routes to a destination are maintained in the topology table.

Routing Table A successor is a route selected as the primary route to use

to reach a destination.

DUAL calculates Successor (Primary Route) and places it in

the routing table (and topology table)

up to 4 successors of equal or unequal value

DUAL calculates Feasible Successor (Backup Route) and

places it in the Topology Table.

Promoted to successor if the route goes down if it has a

lower cost than Feasible Distance (FD)

If no FS in Table - Send query

EIGRP Packets Hello Update Query Reply Acknowledge

EIGRP Metric Calculation EIGRP uses composite metric that depends on several

parameters; Bandwidth, delay, load and reliability

EIGRP advertises to neighbors all parameters including MTU

Metric calculation formula

By default two parameters are considered in calculation

Diffusing Update Algorithm “DUAL” All route computations in EIGRP are handled by DUAL

One of DUAL's tasks is maintaining a topology table of loop-free paths to every destination.

DUAL saves all paths in the topology table

The least-cost path(s) is copied from the topology table to the routing table

In the event of a failure, the topology table allows for very quick convergence if another loop-free path is available

If a loop-free path is not found in the topology table, a route recomputation must occur

DUAL queries its neighbors, who, in turn, may query their neighbors, and so on...

EIGRP Terms Feasible distance (FD) - This is the lowest calculated

metric to reach destination. This is the route that you will find in the routing table, because it is considered the best path

Reported distance (RD) - The distance reported by an adjacent neighbor to a specific destination.

Interface information - The interface through which the destination can be reached.

Route status - The status of a route. Routes are identified as being either passive, which means that the route is stable and ready for use, or active, which means that the route is in the process of being recomputed by DUAL

Equal/Unequal Cost Load balance

EIGRP supports the ability to put multiple equal-metric routes in the IPv4 routing table (Maximum paths)

EIGRP defaults to support four such routes for each subnet, and it can be configured to other values

EIGRP support unequal-cost load balancing using an EIGRP setting called variance

Variance allows routes whose metrics are relatively close in value to be considered equal

Auto Summary and Discoutiguous Network Contiguous network: A classful network in which

packets sent between every pair of subnets can pass only

through subnets of that same classful network

Discontiguous network: A classful network in which

packets sent between at least one pair of subnets must

pass through subnets of a different classful network