© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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Chapter 1 1
EIGRP
Routing Protocols and
Concepts – Chapter 9
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Objectives
 Describe the background and history of Enhanced
Interior Gateway Routing Protocol (EIGRP).
 Examine the basic EIGRP configuration commands
and identify their purposes.
 Calculate the composite metric used by EIGRP.
 Describe the concepts and operation of DUAL.
 Describe the uses of additional configuration
commands in EIGRP.
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Introduction
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EIGRP
 Roots of EIGRP: IGRP
-Developed in 1985 to overcome RIPv1’s
limited hop count
-Distance vector routing protocol
-Metrics used by IGRP
bandwidth (used by default)
Delay (used by default)
reliability
load
-Discontinued support starting with IOS
12.2(13)T & 12.2(R1s4)S
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EIGRP
EIGRP Message Format
 EIGRP Header
Data link frame header - contains source and destination MAC
address
IP packet header - contains source & destination IP address
EIGRP packet header - contains AS number
Type/Length/Field - data portion of EIGRP message
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EIGRP
 EIGRP packet header
contains
–Opcode field
–Autonomous System number
 EIGRP Parameters contains
–Weights
–Hold time
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EIGRP
 TLV: IP internal contains
–Metric field
–Subnet mask field
–Destination field
 TLV: IP external contains
–Fields used when external
routes are imported into
EIGRP routing process
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EIGRP
Protocol Dependent
Modules (PDM)
 EIGRP uses PDM to route
several different protocols i.e.
IP, IPX & AppleTalk
 PDMs are responsible for the
specific routing task for each
network layer protocol
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EIGRP
Reliable Transport Protocol (RTP)
 Purpose of RTP
–Used by EIGRP to transmit and receive
EIGRP packets
 Characteristics of RTP
–Involves both reliable & unreliable delivery of
EIGRP packet
Reliable delivery requires
acknowledgment from destination
Unreliable delivery does not require an
acknowledgement from destination
–Packets can be sent
Unicast
Multicast
–Using address 224.0.0.10
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EIGRP
EIGRP’s 5 Packet Types
 Hello packets
–Used to discover & form adjacencies with neighbors
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EIGRP
 Update packets
–Used to propagate routing
information
 Acknowledgement
packets
–Used to acknowledge
receipt of update, query &
reply packets
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EIGRP
 Query & Reply packets
Used by DUAL for searching
for networks
Query packets
-Can use
Unicast
Multicast
Reply packet
-Use only
unicast
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EIGRP
 Purpose of Hello Protocol
–To discover & establish adjacencies with neighbor routers
 Characteristics of hello protocol
–Time interval for sending hello packet
Most networks it is every 5 seconds
Multipoint non broadcast multi-access networks
–Unicast every 60 seconds
-Holdtime
This is the maximum time
router should wait before
declaring a neighbor down
Default holdtime
–3 times hello interval
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EIGRP
EIGRP Bounded Updates
 EIGRP only sends update when there is a change in
route status
 Partial update
–A partial update includes only the route information that has
changed – the whole routing table is NOT sent
 Bounded update
–When a route changes, only those devices that are impacted
will be notified of the change
 EIGRP’s use of partial bounded updates minimizes use
of bandwidth
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EIGRP
Diffusing Update Algorithm (DUAL)
–Purpose
•EIGRP’s primary method for preventing routing loops
–Advantage of using DUAL
•Provides for fast convergence time by keeping a list of loopfree
backup routes
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EIGRP
 Administrative Distance (AD)
–Defined as the trustworthiness of the source route
 EIGRP default administrative distances
–Summary routes = 5
–Internal routes = 90
–Imported routes = 170
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EIGRP
Authentication
 EIGRP can
– Encrypt routing information
– Authenticate routing information
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EIGRP
Network Topology
 Topology used is the same as previous chapters with
the addition of an ISP router
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EIGRP
 EIGRP will automatically
summarize routes at
classful boundaries
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Basic EIGRP Configuration
 Autonomous System (AS) & Process IDs
–This is a collection of networks under the control of a single
authority (reference RFC 1930)
–AS Numbers are assigned by IANA
–Entities needing AS numbers
ISP
Internet Backbone prodiers
Institutions connecting to other institutions using AS
numbers
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Basic EIGRP Configuration
 EIGRP autonomous system
number actually functions as
a process ID
 Process ID represents an
instance of the routing
protocol running on a router
 Example
Router(config)#router
eigrp autonomous-system
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Basic EIGRP Configuration
The router eigrp command
 The global command that enables eigrp is
router eigrp autonomous-system
-All routers in the EIGRP routing domain must use
the same process ID number (autonomous-system
number)
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Basic EIGRP Configuration
The Network Command
 Functions of the network command
–Enables interfaces to transmit & receive EIGRP
updates
–Includes network or subnet in EIGRP updates
 Example
–Router(config-router)#network network-address
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Basic EIGRP Configuration
 The network Command with a Wildcard Mask
-This option is used when you want to configure EIGRP
to advertise specific subnets
-Example
Router(config-router)#network network-address [wildcard-mask]
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Basic EIGRP Configuration
Verifying EIGRP
 EIGRP routers must establish adjacencies with their
neighbors before any updates can be sent or received
 Command used to view neighbor table and verify that
EIGRP has established adjacencies with neighbors is
show ip eigrp neighbors
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EIGRP
 The show ip protocols
command is also used to
verify that EIGRP is enabled
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Basic EIGRP Configuration
Examining the Routing
Table
 The show ip route
command is also used to
verify EIGRP
 EIGRP routes are denoted
in a routing table by the
letter “D”
 By default , EIGRP
automatically summarizes
routes at major network
boundary
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Basic EIGRP Configuration
 Introducing the Null0 Summary Route
–Null0 is not a physical interface
–In the routing table summary routes are sourced from Null0
Reason: routes are used for advertisement purposes
–EIGRP will automatically include a null0 summary route as child
route when 2 conditions are met
At least one subnet is learned via EIGRP
Automatic summarization is enabled
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Basic EIGRP Configuration
 R3’s routing table shows
that the 172.16.0.0/16
network is automatically
summarized by R1 & R3
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EIGRP Metric Calculation
EIGRP Composite Metric & the K Values
 EIGRP uses the following values in its composite
metric
-Bandwidth, delay, reliability, and load
 The composite metric used by EIGRP
– formula used has values K1 K5
K1 & K3 = 1
all other K values = 0
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EIGRP Metric Calculation
 Use the sh ip protocols command to verify the K
values
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EIGRP Metric Calculation
EIGRP Metrics
 Use the show
interfaces command to
view metrics
 EIGRP Metrics
Bandwidth – EIGRP
uses a static bandwidth
to calculate metric
Most serial interfaces use
a default bandwidth value
of 1.544Mbos (T1)
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EIGRP Metric Calculation
EIGRP Metrics
 Delay is the defined as the measure of time it takes for
a packet to traverse a route
-it is a static value based on link type to which
interface is connected
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EIGRP Metric Calculation
 Reliability (not a default EIGRP metric)
-A measure of the likelihood that a link will fail
-Measure dynamically & expressed as a fraction of 255
the higher the fraction the better the reliability
 Load (not a default EIGRP metric)
– A number that reflects how much traffic is using a link
– Number is determined dynamically and is expressed as a
fraction of 255
The lower the fraction the less the load on the link
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EIGRP Metric Calculation
Using the Bandwidth Command
 Modifying the interface bandwidth
-Use the bandwidth command
-Example
Router(config-if)#bandwidth kilobits
 Verifying bandwidth
–Use the show interface command
 Note – bandwidth command
does not change the
link’s physical
bandwidth
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EIGRP Metric Calculation
 The EIGRP metric can be determined by examining the
bandwidth delay
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EIGRP Metric Calculation
 EIGRP uses the lowest bandwidth (BW)in its metric
calculation
Calculated BW = reference BW / lowest BW(kbps)
 Delay – EIGRP uses the cumulative sum of all outgoing
interfaces
Calculated Delay = the sum of outgoing interface delays
 EIGRP Metric = calculated BW + calculated delay
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EIGRP Metric Calculation
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DUAL Concepts
 The Diffusing Update Algorithm (DUAL) is used to
prevent looping
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DUAL Concepts
 Successor
The best least cost
route to a destination
found in the routing
table
 Feasible distance
The lowest
calculated metric
along a path to a
destination network
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DUAL Concepts
Feasible Successors, Feasibility Condition & Reported
Distance
 Feasible
Successor
-This is a loop
free backup
route to same
destination as
successor route
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DUAL Concepts
 Reported distance
(RD)
-The metric that a
router reports to a
neighbor about its
own cost to that
network
Feasible Successors, Feasibility Condition & Reported
Distance
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DUAL Concepts
 Feasibility
Condition (FC)
-Met when a
neighbor’s RD
is less than
the local
router’s FD to
the same
destination
network
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DUAL Concepts
 Topology Table: Successor
& Feasible Successor
 EIGRP Topology table
–Viewed using the show ip
eigrp topology command
Contents of table include:
– all successor routes
– all feasible successor
routes
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DUAL Concepts
 EIGRP
Topology
Table
dissected
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DUAL Concepts
Topology Table: No
Feasible Successor
 A feasible successor may
not be present because
the feasibility condition
may not be met
-In other words, the
reported distance of
the neighbor is greater
than or equal to the
current feasible
distance
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DUAL Concepts
 Finite Sate Machine (FSM)
–An abstract machine that defines a set of possible
states something can go through, what event
causes those states and what events result form
those states
–FSMs are used to describe how a device, computer
program, or routing algorithm will react to a set of
input events
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DUAL Concepts
 DUAL FSM
–Selects a best loopfree
path to a
destination
–Selects alternate
routes by using
information in EIGRP
tables
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DUAL Concepts
Finite State Machines (FSM)
 To examine output from EIGRP’s finite state machine
us the debug eigrp fsm command
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More EIGRP Configurations
The Null0 Summary Route
 By default, EIGRP uses the Null0 interface to discard
any packets that match the parent route but do not
match any of the child routes
 EIGRP automatically includes a null0 summary route as
a child route whenever both of the following conditions
exist
–One or subnets exists that was learned via EIGRP
–Automatic summarization is enabled
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More EIGRP Configurations
The Null0 Summary Route
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More EIGRP Configurations
Disabling Automatic Summarization
 The auto-summary command permits EIGRP to
automatically summarize at major network boundaries
 The no auto-summary command is used to disable
automatic summarization
–This causes all EIGRP neighbors to send updates
that will not be automatically summarized
this will cause changes to appear in both
-routing tables
-topology tables
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More EIGRP Configurations
Manual Summarization
 Manual summarization can include supernets
Reason: EIGRP is a classless routing protocol & include subnet
mask in update
 Command used to configure manual summarization
–Router(config-if)#ip summary-address eigrp as-number
network-address subnet-mask
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More EIGRP Configurations
 Configuring a summary route in EIGRP
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More EIGRP Configurations
EIGRP Default Routes
 “quad zero” static default route
-Can be used with any currently supported routing
protocol
-Is usually configured on a router that is connected a
network outside the EIGRP domain
 EIGRP & the “Quad zero” static default route
–Requires the use of the redistribute static command
to disseminate default route in EIGRP updates
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More EIGRP Configurations
Fine-Tuning EIGRP
 EIGRP bandwidth utilization
-By default, EIGRP uses only up to 50% of interface bandwidth
for EIGRP information
-The command to change the percentage of bandwidth used by
EIGRP is
Router(config-if)#ip bandwidth-percent eigrp asnumber
percent
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More EIGRP Configurations
 Configuring Hello Intervals and Hold Times
-Hello intervals and hold times are configurable on a per-interface
basis
-The command to configure hello interval is
Router(config-if)#ip hello-interval eigrp as-number seconds
 Changing the hello interval also requires changing the hold
time to a value greater than or equal to the hello interval
-The command to configure hold time value is
Router(config-if)#ip hold-time eigrp as-number seconds
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Summary
 Background & History
–EIGRP is a derivative of IGRP
EIGRP is a Cisco proprietary distance vector routing
protocol released in 1994
 EIGRP terms and characteristics
–EIGPR uses RTP to transmit & receive EIGRP packets
–EIGRP has 5 packet type:
Hello packets
Update packets
Acknowledgement packets
Query packets
Reply packets
–Supports VLSM & CIDR
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Summary
 EIGRP terms and characteristics
–EIGRP uses a hello protocol
Purpose of hello protocol is to discover & establish
adjacencies
–EIGRP routing updates
Aperiodic
Partial and bounded
Fast convergence
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Summary
 EIGRP commands
–The following commands are used for EIGRP
configuration
RtrA(config)#router eigrp [autonomous-system #]
RtrA(config-router)#network network-number
–The following commands can be used to verify EIGRP
Show ip protocols
Show ip eigrp neighbors
Show ip route
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Summary
 EIGRP metrics include
–Bandwidth (default)
–Delay (default)
–Reliability
–Load
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Summary
 DUAL
–Purpose of DUAL
To prevent routing loops
–Successor
Primary route to a destination
–Feasible successor
Backup route to a destination
–Feasible distance
Lowest calculated metric to a destination
–Reported distance
The distance towards a destination as advertised
by an upstream neighbor
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Summary
 Choosing the best route
–After router has received all updates from directly
connected neighbors, it can calculate its DUAL
1st metric is calculated for each route
2nd route with lowest metric is designated
successor & is placed in routing table
3rd feasible successor is found
–Criteria for feasible successor: it must have
lower reported distance to the destination than
the installed route’s feasible distance
–Feasible routes are maintained in topology
table
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Summary
 Automatic summarization
–On by default
–Summarizes routes on classful boundary
–Summarization can be disabled using the following
command
RtrA(config-if)#no auto-summary
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