The Autonomous System

EIGRP v1.31 – Aaron Balchunas

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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners.

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

1

– Enhanced Interior Gateway Routing Protocol –

EIGRP (Enhanced Interior Gateway Routing Protocol)

EIGRP is a Cisco-proprietary Hybrid routing protocol, incorporating

features of both Distance-Vector and Link-State routing protocols.

EIGRP adheres to the following Hybrid characteristics:

• EIGRP uses Diffusing Update Algorithm (DUAL) to determine the best path among all “feasible” paths. DUAL also helps ensure a loop-

free routing environment.

• EIGRP will form neighbor relationships with adjacent routers in the same Autonomous System (AS).

• EIGRP traffic is either sent as unicasts, or as multicasts on address 224.0.0.10, depending on the EIGRP packet type.

• Reliable Transport Protocol (RTP) is used to ensure delivery of most EIGRP packets.

• EIGRP routers do not send periodic, full-table routing updates. Updates are sent when a change occurs, and include only the change.

• EIGRP is a classless protocol, and thus supports VLSMs.

Other characteristics of EIGRP include:

• EIGRP supports IP, IPX, and Appletalk routing. • EIGRP applies an Administrative Distance of 90 for routes originating

within the local Autonomous System.

• EIGRP applies an Administrative Distance of 170 for external routes coming from outside the local Autonomous System

• EIGRP uses Bandwidth and Delay of the Line, by default, to calculate its distance metric. It also supports three other parameters to

calculate its metric: Reliability, Load, and MTU.

• EIGRP has a maximum hop-count of 224, though the default maximum hop-count is set to 100.

EIGRP, much like OSPF, builds three separate tables:

• Neighbor table – list of all neighboring routers. Neighbors must belong to the same Autonomous System

• Topology table – list of all routes in the Autonomous System • Routing table – contains the best route for each known network

EIGRP v1.31 – Aaron Balchunas

* * *

All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners.

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

2

EIGRP Neighbors

EIGRP forms neighbor relationships, called adjacencies, with other routers

in the same AS by exchanging Hello packets. Only after an adjacency is

formed can routers share routing information. Hello packets are sent as

multicasts to address 224.0.0.10.

By default, on LAN and high-speed WAN interfaces, EIGRP Hellos are sent

every 5 seconds. On slower WAN links (T1 speed or slower), EIGRP Hellos

are sent every 60 seconds by default.

The EIGRP Hello timer can be adjusted on a per interface basis:

Router(config-if)# ip hello-interval eigrp 10 7

The above command allows us to change the hello timer to 7 seconds for

Autonomous System 10.

In addition to the Hello timer, EIGRP neighbors are stamped with a Hold

timer. The Hold timer indicates how long a router should wait before

marking a neighbor inactive, if it stops receiving hello packets from that

neighbor.

By default, the Hold timer is three times the Hello timer. Thus, on high-

speed links the timer is set to 15 seconds, and on slower WAN links the

timer is set to 180 seconds.

The Hold timer can also be adjusted on a per interface basis:

Router(config-if)# ip hold-interval eigrp 10 21

The above command allows us to change the hold timer to 21 seconds for

Autonomous System 10.

Changing the Hello timer does not automatically change the Hold timer.

Additionally, Hello and Hold timers do not need to match between routers

for an EIGRP neighbor relationship to form.

(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#eigrp_work)

EIGRP v1.31 – Aaron Balchunas