EIGRP Summary

The characteristics of EIGRP follow:

  • Hybrid routing protocol (distance vector that has link-state protocol characteristics).
  • Use DUAL, first proposed by E. W. Dijkstra and C. S. Scholten, to perform distributed shortest-path routing while maintaining freedom from loops at every instant. Although many researchers have contributed to the development of DUAL, the most prominent work is that of J. J. Garcia-Luna-Aceves.
  • Cisco Proprietary created in 1994.
  • First released in IOS 9.21
  • Uses IP protocol 88.
  • Makes Automatic summarization on network Class boundary.
  • Classless protocol (supports VLSM).
  • Have the power to shut the Auto-summarization And make a configured manual Summarization.
  • Default composite metric of bandwidth and delay.
  • You can factor load, MTU and reliability into the metric.
  • Eigrp metric is the same as IGRP*256, It uses the smallest B.W,Reliablity,Load & MTU with the Comulative delay upon the path…..The MTU doesn’t actually used in the Metric calculations, But is included in the EIGRP Routing updates.
  • Sends route updates to multicast address, and nei. Reply’s back with Unicast Address.
  • Sends non-periodic, partial, and bounded updates.
  • Send Hello packets every 5 sec. and Hold down timer is 15 sec.
  • For Low speed Hello is every 60 sec. with hold down time 180 sec.
  • By default, EIGRP uses no more than 50 percent of the bandwidth of a link.
  • Support for authentication via MD5 Only.
  • Uses DUAL for loop prevention, and generating Succ./Fesible Succ.
  • Maximum paths for Load-balancing are 6 & default is 4 , maximum are 16 in IOS 12.3(2)T and later IOS releases
  • By default, Equal-Metric load balancing. If Unequal-Metric load sharing is used the router will load share inversely proportional to the metrics of the paths.
  • Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
  • Potential routing protocol for the core of a network; used in large networks.
  • For neighbor relation to be established, both routers must send and receive Hello or Ack packets from each other, they must have the same AS #, and the same Metric K values.
  • Eigrp doesn’t restrict that neighbors must have the same Hello & dead interval timers, Unlike OSPF.
  • Has a Maximum hop count of 255, the default is 100 in the last IOS releases.

The composite metric for each EIGRP route is calculated as

EIGRP metric = IGRP metric * 256

IGRP  metric = [k1*BWIGRP(min) + (k2* BWIGRP(min))/(256-LOAD) + k3*DLYIGRP(sum)]
x [k5/(RELIABILITY + k4)]

If k5 is set to zero, the [k5/(RELIABILITY+k4)] term is not used.

Given the default values for k1 through k5, the composite metric calculation used by IGRP reduces to the default metric:

IGRP metric = BWIGRP(min) + DLYIGRP(sum)

BWIGRP(min) = 107/BW(min)

DLYIGRP(sum) = DLY(sum) /10

EIGRP has four components:

Protocol-Dependent Modules (PDM): EIGRP implements modules for IP, IPX, and AppleTalk, which are responsible for the protocol-specific routing tasks. For example, the IPX EIGRP module is responsible for exchanging route information about IPX networks with other IPX EIGRP processes and for passing the information to the DUAL. Additionally, the IPX module will send and receive SAP information.

Reliable Transport Protocol (RTP): The Reliable Transport Protocol (RTP) manages the delivery and reception of EIGRP packets. Reliable delivery means that delivery is guaranteed and that packets will be delivered in order. If any packet is reliably multicast and an ACK is not received from a neighbor, the packet will be retransmitted as a unicast to that unresponding neighbor. If an ACK is not received after 16 of these unicast retransmissions, the neighbor will be declared dead. The time to wait for an ACK before switching from multicast to unicast is specified by the multicast flow timer. The time between the subsequent unicasts is specified by the retransmission timeout (RTO). Both the multicast flow timer and the RTO are calculated for each neighbor from the smooth round-trip time (SRTT). The SRTT is the average elapsed time, measured in milliseconds, between the transmission of a packet to the neighbor and the receipt of an acknowledgment. The formulas for calculating the exact values of the SRTT, the RTO, and the multicast flow timer are proprietary.

Neighbor Discovery/Recovery: Hellos are multicast every 5 sec., minus a small random time to prevent synchronization. & are unicast every 60 sec. On multipoint X.25, Frame Relay, and ATM interfaces, with access link speeds of T1 or slower also it’s the default for ATM SVCs and for ISDN PRI In all cases, the Hellos are unacknowledged.

Diffusing Update Algorithm (DUAL): Used For routing calculations, loop free & convergence.

EIGRP Configuration

config)# router eigrp <AS#>

conf-router)# network < Net ID> (classlfull)

conf-router)# metric maximum-hop <#> (Up to 255, default =224)

conf-router)#passive-interface <interface>

conf-router)#distance eigrp <AS> <internal AD> <external AD>

(Define an administrative distance, default =90 & 170)

conf-router)#maximum –paths <N> (Up to 6 Links for load sharing, 

Over Unequal metric paths default is 4 paths)

conf-router)#variance <#>

conf-router)#traffic-share < minimum / balanced >

conf-router)# eigrp log-neighbor-changes

conf-router)#no auto-summary (disable automatic summ. To the class boundary)

config-if)# ip summary-address eigrp <AS#> <Net ID + mask>

config-if)# bandwidth < # in Kbps>

config-if)#delay < msec>

config-if)# ip hello-interval eigrp <AS#> < sec >

config-if)# ip hold-time eigrp <AS#> < sec >

EIGRP Authentication MD5

The router may be configured to use more than one “key chain.” & also different keys at different times (key management).

Auth must be done in a fixed order rather than this you may have problems, The steps for setting up EIGRP authentication follow:

1-Define a key chain with a name.

2- Define the authentication key or keys on the key chain

3- Define the password of the key or keys.

4- Apply MD5 authentication on an interface (EIGRP only support MD5).

5- Apply authentication on an interface and specify the key chain to be used.

6- Optionally configure key management.

Here is the cmds:

config)# key chain <name>
  config-keychain)# key <#> 

config-keychain-key)# key-string < password> 

config-keychain-key)#accept-lifetime<h:m:s mm dd yy>duration< mm- infinite > 

config-keychain-key)#send-lifetime <h:m:s mm dd yy> duration < mm- infinite > 

config)# interface <int>
  config-if)#   ip  authentication mode eigrp <AS#><md5> 

config-if)#   ip authentication key-chain eigrp <AS#>  <name> 

# debug eigrp packets


If the service password-encryption command is not used when implementing EIGRP authentication, the key string will be stored as plain text in the router configuration. If you configure the service password-encryption command, the key string will be stored and displayed in an encrypted form; when it is displayed, there will be an encryption type of 7 specified before the encrypted key string.

The password that is accepted from other routers and the password that is used with transmitted/sent messages are managed separately. Both the accept-lifetime and the send-lifetime cmd. must have a specified start time and may have either a specified duration or end time or the keyword infinite. The key numbers are examined from the lowest to the highest, and the first valid key is used.

accept life time = accept this key

send life time = send/reply with a key for auth , based on its send lifetime.

For RIP key ID & its password isn’t important to match, they will work.

BUT for EIGRP key # must match.

Show Commands

# sh ip route

# sh ip route eigrp

# sh ip route < Net ID >

# sh ip protocols

# sh ip eigrp traffic

# sh ip eigrp traffic <AS#>

# sh ip eigrp nei

# sh ip eigrp topology

# sh ip eigrp topology <AS#> <ip add +mask > (optional)

# sh ip eigrp topology all-links

# sh ip eigrp topology < active / pending / zero-successors>

# sh ip eigrp traffic 

# sh ip eigrp events (Shows the types of packets sent and received and 

statistics on routing decisions.) 

#debug ip eigrp

#debug ip eigrp fsm

#debug ip eigrp nei (Shows the hello packets sent and received to the neighbors)

#debug ip eigrp route (Shows dynamic changes made on the routing table process)

#debug ip eigrp summary (Shows a summary of the EIGRP activity)

#debug ip eigrp packet (Shows the packets sent and received by the router.

The packet types to be monitored can be selected. 

Up to 11 types are available)

# no debug all / u all

By aelhoussiny

Tags: eigrp study guide, characteristics of eigrp, ccna eigrp notes

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