Eigrp Facts

7/29/2019 Eigrp Facts

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EIGRP Facts

Enhanced IGRP is a Cisco-proprietary balanced hybrid routing protocol that combines the bestfeatures of distance vector and link state routing. EIGRP:

Sends the subnet mask in the routing update. It supports route summarization and VLSM.

Supports automatic classful route summarization at major network boundaries (this is the

default in EIGRP). Unlike IGRP and RIP, manual route summarization can also be

configured on arbitrary network boundaries to reduce the routing table size.

Is not susceptible to routing loops. Instead, EIGRP uses built-in loop avoidance

techniques. Under certain conditions, EIGRP will use split horizon, but not hold downs or

flush timers.

Is scalable and does not have the 16 hop limitation of RIP.

Uses hello packets to discover neighbor routers. Hello intervals on EIGRP routers do not

need to match.

Exchanges the full routing table at startup, and then partial routing updates thereafter.

Uses unicasts or multicasts to 224.0.0.10 for routing updates. Hello packets always usethe multicast address.

Uses bandwidth, delay, reliability, and load for the route metric. The metric is expressedas the number of microseconds.

o The degree to which each value is used to calculate the metric can be customized

by modifying one of five K values.

o By default, K1 and K3 are set to 1, while K2, K4, and K5 are set to 0. Thesesettings mean that with the default configuration, only delay and bandwidth have

an effect on the metric.

o On serial links, a default bandwidth of 1544 is used. EIGRP does not detect theactual bandwidth on the link. You must manually configure bandwidth values for

accurate metric calculations. Uses an autonomous system (AS) number to identify routers that are to share EIGRP

information. The AS number on both routers must match.

Maintains partial network topology information in addition to routes.

Supports load balancing on equal-cost and unequal cost links. This means that EIGRP

can keep multiple paths to a single network, even if they have a different cost. With IOS12.4 and above, EIGRP supports up to 16 paths (earlier versions supported up to 6), with

the default being 4 equal-cost paths.

Minimizes network bandwidth usage for routing updates. During normal operationEIGRP transmits only hello packets across the network. EIGRP does not send periodic

routing updates like RIP and IGRP. When change occurs, only routing table changes are

propagated in EIGRP not the entire table.

Requires less processing and memory than link state protocols.

Converges more quickly than distance vector protocols. In some cases, convergence can

be almost instantaneous because an EIGRP router stores backup routes for destinations. If

no appropriate route or backup exists in the routing table, EIGRP will query neighbor

routers to discover an alternate route. In this manner, EIGRP can quickly adapt toalternate routes when changes occur.

Uses the DUAL link-state algorithm for calculating routes.



Supports multiple protocols. EIGRP can exchange routes for IP, AppleTalk and IPX/SPX

networks.

Uses a neighbors table to keep track of neighbor routers. The neighbors table includes thefollowing for each neighbor:

o A hold time. Each hello packet includes a hold time that identifies how long the

hello information is valid. If the hold time expires without receipt of a hello packet, the neighbor is assumed to be unreachable.o Round-trip timers that help the router identify cost values to reach the neighbor

router.

Uses a topology database to keep track of all known networks.

o The topology table has a list of each destination network and all neighbor routers

that reported routes to that network.

o The best routes that will be used for routing packets are copied from the topology

table into the routing table.

o The topology table holds up to 16 known routes (previously up to 6 before IOS

version 12.4).

To understand how EIGRP can provide load balancing and fast recovery for failed links, you

need to understand the following concepts:

Term Definition

Advertised

Distance (AD)

The advertised distance (AD) is the cost to the destination network as reported

by the neighbor router. The AD is also called the reported distance (RD).

FeasibleDistance (FD)

The feasible distance (FD) is the lowest total cost to a destination network. The

feasible distance is identified for each destination network, and is determined as

follows:

1. For each neighbor, a total cost to the network through the neighbor iscalculated by adding the AD to the cost required to reach the neighbor

router (the cost of the link used to reach the neighbor router).

2. The router compares the total cost of all routes. The lowest total cost tothe destination network is the feasible distance to the network.

Note: Sometimes the total cost for each neighbor route is referred to as a feasibledistance. However, the term more correctly identifies the lowest known cost to

the network, not the total cost for each reported (possible) route.

Successor

A successor is the route to a destination network with the lowest total cost.

When a new route is first learned, the total cost to the successor route is

used as the feasible distance to that network.

The successor route is copied from the topology table into the routing

table.

You can have multiple successor routes if multiple routes to the samenetwork exist with the same lowest metric.



Feasible

Successor

A feasible successor is an alternate route to a destination network. The total cost

to the route through the feasible successor is higher than the total cost of

successor routes. A route must meet the following condition to qualify as a

feasible successor route:The advertised distance of the route through that neighbor must be less than the

feasible distance used for that network (AD < FD).Be aware of the following regarding feasible successors:

Satisfying the AD < FD condition ensures that the route is loop free. In

other words, the router knows for sure that the route does not includeitself in the path if the AD is lower than the FD. Note: Successor routes

must also meet this condition.

Feasible successor routes are kept in the topology table but are not copied

to the routing table.

Successor routes can also be classified as feasible successor routes.

When all successor routes to a network are lost, the router can

immediately begin to use the next best feasible successor route. This provides for rapid recovery in the event of a topology change.

Be aware of the following regarding the EIGRP and routes:

All known routes to a destination are kept in the topology table. Only successor routes arecopied to the routing table.

If the successor route goes down and there are no feasible successors, routes whose

advertised distance is greater than the feasible distance for the route are not used because

they might be routes that include loops.

When the last feasible successor route to a network is lost, the router recalculates allroutes for the lost neighbor. Instead of using other routes that are not feasible successor

routes, it first communicates with neighbor routers. If necessary, the router recalculatesthe feasible distance for the route.

A route whose AD is greater than the FD does not prove that a loop exists, only that a

loop might exist. After the last feasible successor route is lost, a previously unacceptableroute could be identified as a feasible successor route as long as its AD is less than the

newly-calculated FD.

By default, EIGRP uses equal-cost load balancing. To use unequal-cost load balancing,

configure the variance value. The variance is a multiplier that identifies the degree towhich alternate paths can be used.

o The variance value ranges from 1 to 255.o The default variance is 1, meaning that only routes that match the best route can

be used.o Setting the variance to 2 allows alternate routes to be used whose total costs are

within a factor of 2 (double or less) of the best cost route.

o Only feasible successor routes can be used. This means that a route whose AD isgreater than the FD cannot be used as an alternate route, even if its total cost is

within the variance amount.



For an EIGRP router to share information with a neighbor, the following conditions must be met:

Both routers are on the same subnet with the same subnet mask.

If used, authentication checks must pass.

Both routers must be configured with the same AS number.

Metric weight values (K values) must match on both routers.

EIGRP Command List

You configure EIGRP just the same as you would configure IGRP. The following table lists the

applicable commands.

Command Function

Router(config)#routereigrp number

Defines an EIGRP process.

The number must match between routers for information to be shared.

Router(config-

router)#network n.n.n.n

Router(config-

router)#network n.n.n.n

w.w.w.w

Identifies a network that participates in the routing process.

Networks can be specified with or without the wildcardmask. If you do not use a wildcard mask, the network

address you add will be automatically truncated based on

classful network boundaries.You must use a wildcard mask to identify VLSM subnets.

Router(config-router)#no

auto-summary

Turn off automatic route summarization.

With automatic route summarization, subnets are

summarized based on classful boundaries when advertisingroutes on networks with a different class boundary. You

must disable automatic summarization if you have a network address (such as 10.0.0.0) subnetted into smaller subnets and

separated by a network with a different classful network

address (such as 12.0.0.0).

Example The following commands enable EIGRP on a router and define three networks that participate in

the routing process.

Router(config)#router eigrp 2Router(config-network)#network 172.16.1.0 0.0.0.255

Router(config-network)#network 172.16.2.0 0.0.0.255

Router(config-network)#network 172.16.3.0 0.0.0.255

Use the following commands to manage and monitor EIGRP.



Command Features

show ip route View EIGRP-learned routes.

show eigrp

neighbors

View neighboring routers from which EIGRP routes can be learned. Liststhe IP address of the connected router.

show eigrpinterfaces

View the interfaces that are running EIGRP and the number of connectedrouters.

Eigrp Facts

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