Saeed Darvish Pazoki – MCSE, CCNA

Abstracted From: •Cisco Press – ICND 2 – 10

EIGRP

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EIGRPLike OSPF, EIGRP follows three general steps to be

able to add routes to the IP routing table:Neighbor discovery:

EIGRP routers send Hello messages to discover potential neighboring EIGRP routers and perform basic parameter checks to determine which routers should become neighbors. (show ip eigrp neighbor)

Topology exchange: Neighbors exchange full topology updates when the neighbor

relationship comes up, and then only partial updates as needed based on changes to the network topology. (show ip eigrp topology)

Choosing routes: Each router analyzes its respective EIGRP topology tables,

choosing the lowest-metric route to reach each subnet.(show ip route)

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EIGRPEIGRP uses EIGRP Hello messages, sent to

multicast IP address 224.0.0.10, to dynamically discover potential neighbors. A router learns of potential neighbors by receiving a Hello.

A potential neighbor is a router from which an EIGRP Hello has been received. Then the router checks the following settings to determine if the router should be allowed to be a neighbor:It must pass the authentication process.It must use the same configured AS number.The source IP address used by the neighbor’s

Hello must be in the same subnet.

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EIGRPThe EIGRP neighbor relationship is much simpler than

OSPF. EIGRP does not have an additional concept of being fully

adjacent like OSPF, and there are no neighbor states like OSPF.

As soon as an EIGRP neighbor is discovered and passes the basic verification checks, the router becomes a neighbor.

At that point, the two routers can begin exchanging topology information.

The neighbors send Hellos every EIGRP Hello interval. A router considers its EIGRP neighbor to no longer be

reachable after the neighbor’s Hellos cease to occur for the number of seconds defined by the EIGRP Hold Timer the rough equivalent of the OSPF Dead Interval.

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EIGRPExchanging EIGRP Topology Information

EIGRP uses EIGRP Update messages to send topology information to neighbors.

These Update messages can be sent to multicast IP address 224.0.0.10 if the sending router needs to update multiple routers on the same subnet; otherwise, the updates are sent to the unicast IP address of the particular neighbor. (Hello messages are always sent to the 224.0.0.10 multicast address.)

Unlike OSPF, there is no concept of a Designated Router (DR) or Backup Designated Router (BDR), but the use of multicast packets on LANs allows EIGRP to exchange routing information with all neighbors on the LAN efficiently.

The update messages are sent using Reliable Transport Protocol (RTP).

The significance of RTP is that, like OSPF, EIGRP resends routing updates that are lost in transit.5

EIGRPExchanging EIGRP Topology Information

Neighbors use both full routing updates and partial updates.

A full update means that a router sends information about all known routes, whereas a partial update includes only information about recently changed routes.

Full updates occur when neighbors first come up. After that, the neighbors send only partial updates in reaction to changes to a route.

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EIGRPExchanging EIGRP Topology Information

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EIGRPCalculating the Best Routes for the Routing

TableEIGRP uses a composite metric, calculated as

a function of bandwidth and delay by default.

The calculation can also include interface load and interface reliability, although Cisco recommends against using either.

EIGRP calculates the metric for each possible route by inserting the values of the composite metric into a formula.

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EIGRPCalculating the Best Routes for the Routing Table

The term least-bandwidth represents the lowest-bandwidth link in the route, using a unit of kilobits per second.

If the slowest link in a route is a 10-Mbps Ethernet link, the first part of the formula is 107 / 104, which equals 1000. You use 104 in the formula because 10 Mbps is equal to 10,000 kbps (104 kbps).

The cumulative-delay value used in the formula is the sum of all the delay values for all links in the route, with a unit of “tens of microseconds.”

You can set both bandwidth and delay for each link, using the cleverly named bandwidth and delay interface subcommands.

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EIGRPCalculating the Best Routes for the Routing

TableEIGRP updates list the subnet number and

mask, along with the cumulative delay, minimum bandwidth, along with the other typically unused portions of the composite metric.

The router then considers the bandwidth and delay settings on the interface on which the update was received and calculates a new metric.

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EIGRPCalculating the Best Routes for the Routing

Table

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EIGRP

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Feasible Distance and Reported DistanceFeasible Distance (FD):

The metric of the best route to reach a subnet, as calculated on a router

Reported Distance (RD): The metric as calculated on a neighboring router

and then reported and learned in an EIGRP Update.

EIGRP

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Caveats with Bandwidth on Serial LinksEIGRP’s robust metric gives it the ability to

choose routes that include more router hops but with faster links.

However, to ensure that the right routes are chosen, engineers must take care to configure meaningful bandwidth and delay settings.

In particular, serial links default to a bandwidth of 1544 and a delay of 20,000 microseconds.

EIGRP

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Caveats with Bandwidth on Serial Links

EIGRP

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EIGRP ConvergenceLoop avoidance poses one of the most difficult

problems with any dynamic routing protocol. Distance vector protocols overcome this

problem with a variety of tools, some of which create a large portion of the minutes-long convergence time after a link failure.

Link-state protocols overcome this problem by having each router keep a full topology of the network, so by running a rather involved mathematical model, a router can avoid any loops.

EIGRP

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EIGRP ConvergenceEIGRP avoids loops by keeping some basic topological

information, but it avoids spending too much CPU and memory by keeping the information brief.

When a router learns multiple routes to the same subnet, it puts the best route in the IP routing table.

EIGRP keeps some topological information for the same reason as OSPF so that it can very quickly converge and use a new route without causing a loop.

Essentially, EIGRP keeps a record of each possible next-hop router, and some details related to those routes, but no information about the topology beyond the next-hop routers.

This sparser topology information does not require the sophisticated SPF algorithm, resulting in quick convergence and less overhead, with no loops.

EIGRP

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EIGRP ConvergenceThe EIGRP convergence process uses one of

two branches in its logic, based on whether the failed route does or does not have a feasible successor route. If a feasible successor route exists, the router can

immediately use that route. If not, the router must use a query and response

process to find a loop-free alternative route. Both processes result in fast convergence, typically quicker than 10 seconds, but the query and response process takes slightly longer.

EIGRP

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EIGRP Successors and Feasible SuccessorsEIGRP calculates the metric for each route to reach each

subnet. For a particular subnet, the route with the best metric is

called the successor, with the router filling the IP routing table with this route.

Of the other routes to reach that same subnet—routes whose metrics were larger than the FD for the route—EIGRP needs to determine which can be used immediately if the currently best route fails, without causing a routing loop.

These alternative, immediately usable routes are called feasible successor routes, because they can feasibly be used when the successor route fails.

A router determines if a route is a feasible successor based on the feasibility condition: If a non-successor route’s RD is less than the FD, the route is a

feasible successor route.

EIGRP

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EIGRP Successors and Feasible Successors

EIGRP

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The Query and Reply ProcessWhen a route fails and has no feasible

successor, EIGRP uses a distributed algorithm called Diffusing Update Algorithm (DUAL).

DUAL sends queries looking for a loop-free route to the subnet in question. When the new route is found, DUAL adds it to the routing table.

The EIGRP DUAL process simply uses messages to confirm that a route exists, and would not create a loop, before deciding to replace a failed route with an alternative route.

EIGRP

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EIGRP Summary and Comparisons with OSPF