EIGRP Hybrid Routing Protocol



– Hybrid distant vector/link state Routing Protocol


Topics Discussed:

Success and Feasible successor
Basic Config
Wild Masks
Load balancing (equal / unequal) Variance cmd
EIGRP/RIPvs and auto summarization
Passive/active routes

Neighbor table – shows neighbors
Topology table – shows successor and feasible successors
Route table – only successor routes and other

Requirements for neighbors to form:

– same subnet
– same AS#
– K-value must match


EIGRP Metric

The EIGRP composite metric is computed exactly as the IGRP metric is and then multiplied by 256. Thus, the default expression for the EIGRP composite metric is:

Metric = [BandW +Delay] × 256


How EIGRP works (basics)

Why EIGRP is so good a protocol:

1 .Hybrid protocol like distant vector in that when routers first establish
Neighbor adjacencies, they exchange full routing tables then they send updates
Based off of bandwidth/delay cost.

2. IP, IPX, Apple Talk proctol compatible

3. Uses VLSM

4. Cisco proprietary is the down fall.

Cold memory:

Uses  and RTP to transmit message to/from EIGRP routers
Uses hello packets
Uses AS# must be same
Uses band/delay as cost metric

Neighbor Adjacencies can exits if:

Hello packets every 5 seconds
Same subnet
k-weights same
Same AS#


How EIGRP routing works:


Best route into routing table
Topology table hold successor, feasible successor.
Has feasible successor calculated before failure occurs
Uses DUAL algorithm, neighbor Dual query

Multicast address (hello)

Cold Memory:





Turn off split –horizon and auto-summary.

R1(config)#no ip split-horizon eigrp 100
R1(config-router)#no auto-summary


Wild Card mask used EIGRP:  subnet mask = as wild card mask. is called a host mask wild card host mask subnet mask


Equal Cost Load balancing EIGRP:


R1#show ip router EIGRP

Notice AD/Metric are the same for 2 or more rows or routes in route table.
This is Equal cost when the AD/Metric cost are the same.

Variance Command – used in unequal cost load balancing

R1#(conifg-router)#variance (value)


FD = Feasible Distance
FD x Variance = Value

Value any metric less than this value, that route will be put into routing table.

2297856 successor
2323456 feasible successor
Variance = 2

2 x FD(2298856)= 4595712

End result is both routes will be in routin table as successors. This is force

unequal cost load balancing. Any route metric in the topology table that is lower

than this variance result value will be put into the route table as a usable



Clear ip route *     /clears routing table of all routes and makes protocols recalculate routes.

Auto Summarization – what is it?

Auto summarization is when the router is configured to use auto-summary feature. What that does is insert a summary route into the routing table. This summary Route is a generalization of a bunch of routes. The bad thing about it is it wastes ip addresses. The subnet range could be so large that it encompasses many

routes that could be utilized by your inside LAN. Bottom line – it wastes ip addresses. Auto-summary is on by default and must be shut off.


R1#(config-router)no auto-summary


EIGRP and RIPv2 must turn off auto-summary. Load balancing paths command: What this is in the routing table you will see say 4 routes that are for the same destination with same AD\Metric this is equal cost load balancing. To limit the number of router possible in the equal/unequal cost load balancing, you can set the limit with these commands.


R1(config-router)#maximum-paths 1        /shuts off load balancing
R1(config-router)#maximum-paths 5       /default = 4 max of 16

R1# show ip protocol                           /shows the number of paths set


Dual Query and Active/Passive routes (Topology Table):


A EIGRP router will use DUAL to query other routers. In the topology table you

will have statuses showing ACTIVE/PASSIVE routes.

ACTIVE –  route can’t be used due to it being used by DUAL in a calculation.

PASSIVE – Good! Dual is not using it in a calculation. Ready to use!


R1#show ip eigrp topology              /shows Active/Passive routes in Topology table.

EIGRP metric Calculations:

Cold Memory:
EIGRP Metric


The EIGRP composite metric is computed exactly as the IGRP metric is and then multiplied by 256. Thus, the default expression for the EIGRP composite metric is:

Metric = [BandW +Delay] × 256
Metric = 256(10^7/band) + 256(delay+delay)
AD of EIGRP = 90


EIGRP manual forced route summary command here – Interface Earlier IOS versions treated changes in EIGRP summary address configuration (configured with the ip summary-address eigrp interface configuration command) very disruptively: all EIGRP sessions across the affected interface were cleared, sometimes resulting in a large number of routes entering active state, potentially leading to a stuck-in-active condition. Recent IOS releases are more lenient: router with a change in summary address requests a resync (logged as graceful-restart on adjacent routers). A lot of updates and queries are still sent, but the adjacencies themselves are preserved:


When configuring a summary route, all more specific prefixes on downstream routers enter active state. When a summary is removed, only the summary prefix itself enters active state and the affected router sends queries to all its neighbors, while the more specific prefixes are sent as regular EIGRP updates to the neighbors across the affected interface.


A change in EIGRP summary generates the following output on the router under configuration:

Ra1(config)#interface serial 0/0/0.100

Ra1(config-subif)#ip summary-address eigrp 1
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor (Serial0/0/0.100) is

resync: summary configured

Ra1(config-subif)#no ip summary-address eigrp 1
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor (Serial0/0/0.100) is

resync: summary configured

… and the downstream router generates log messages similar to these:

%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor (Serial0/0/0.100) is

resync: peer graceful-restart


2 Responses to “EIGRP Hybrid Routing Protocol”

  1. This is the first time I commented here and I must say that you provide genuine, and quality information for other bloggers! Great job.
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