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CCNA – Semester 2
Chapter 9: EIGRP
CCNA Exploration version 4.0
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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 to EIGRP
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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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IGRP to EIGRP
Enhanced Distance Vector Routing Protocol: EIGRP
• Uses the Diffusing Update Algorithm (DUAL)
• Does not age out routing entries nor uses periodic updates.
• Maintains a topology table separate from the routing table, which includes the best path and any loop-free backup paths.
• When a route becomes unavailable, DUAL will use a backup path if one exists in the topology table.
• Faster convergence due to the absence of holddown timers and a system of coordinated route calculations.
Traditional Distance Vector
Routing Protocols
• Use the Bellman-Ford or
Ford-Fulkerson algorithm.
• Age out routing entries and
uses periodic updates.
• Keep track of only the best
routes; the best path to a
destination network.
• When a route becomes
unavailable, the router must
wait for a new routing
update.
• Slower convergence due to
holddown timers.
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• EIGRP Header
EIGRP Message Format
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• EIGRP packet header
contains
– Opcode field
– Autonomous System
number
• EIGRP Parameters
contains
– Weights
– Hold time
EIGRP Message Format
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• 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
EIGRP Message Format
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Protocol Dependent Modules (PDM)
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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
RTP and EIGRP Packet Types
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EIGRP’s 5 Packet Types
• Hello packets
– Used to discover & form adjacencies with neighbors
RTP and EIGRP Packet Types
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RTP and EIGRP Packet Types
• Update packets
– Used to propagate routing
information
• Acknowledgement packets
– Used to acknowledge
receipt of update, query &
reply packets
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RTP and EIGRP Packet Types
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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• 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
Hello Protocol
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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
EIGRP Bounded Updates
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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 loop-free
backup routes
DUAL: An Introduction
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• Administrative Distance
(AD)
– Defined as the
trustworthiness of the
source route
• EIGRP default
administrative distances
– Summary routes = 5
– Internal routes = 90
– Imported routes = 170
Administrative Distance
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Authentication
• EIGRP can
– Encrypt routing
information
– Authenticate
routing information
• Note: Authentication
does not encrypt the
router's routing table
Authentication
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Basic EIGRP Configuration
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Network Topology
• Topology used is the same as previous chapters with
the addition of an ISP router
EIGRP Network Topology
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• EIGRP will
automatically
summarize routes at
classful boundaries
EIGRP
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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 providers
Institutions connecting
to other institutions
using AS numbers
Autonomous System and Process IDs
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• 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
Autonomous System and Process IDs
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The router eigrp Command
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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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
The network Command
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The network Command
• 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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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
Verifying EIGRP
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• The show ip protocols command is used to verify that EIGRP is enabled
Verifying EIGRP
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Examining the Routing Table
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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
Examining the Routing Table
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• 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
Introducting the Null0 Summary Route
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Examining the Routing Table
• 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
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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
EIGRP Metric Calculation
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• Use the show ip protocols command to verify the K
values
EIGRP Composite Metric and the K Values
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• 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)
EIGRP Metric Calculation
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EIGRP Metric Calculation
Delay
• 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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• 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
EIGRP Metric Calculation
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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
EIGRP Metric Calculation
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• The EIGRP metric can be determined by examining the:
bandwidth & delay
EIGRP Metric Calculation
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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
(Note: reference BW = 107)
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EIGRP Metric Calculation
• Calculate the metric from R2 to 192.168.1.0/24
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Diffusing Update Algorithm (DUAL)
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• The Diffusing Update Algorithm (DUAL) is used to prevent
looping:
– Successor
– Feasible Distance (FD)
– Feasible Successor (FS)
– Reported Distance (RD) or Advertised Distance (AD)
– Feasible Condition or Feasibility Condition (FC)
DUAL Concepts
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• 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
Successor and Feasible Distance
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• Feasible Successor
– This is a loop free backup route to same destination as successor route
Feasible Successor
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• Reported distance (RD)
– The metric that a router reports to a neighbor about its
own cost to that network
• Feasibility Condition (FC)
– Met when a neighbor’s RD is less than the local router’s
FD to the same destination network
Feasibility Condition and Reported Distance
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Successors and Feasible Successors
172.30.1.0
172.30.1.0
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Successors and Feasible Successors
Advertised or Destination Feasible Dist. Reported. Dist. Neighbor
172.30.1.0 40 30 X In Topology Table
172.30.1.0 31 21 Y In Routing Table
172.30.1.0 230 220 Z Not in Topology Table
Current Successor = 31
RD of RTY= 21
Feasible Successor, FC: RD30 < FD31
FD to 172.30.1.0 is
31 via Router Y
172.30.1.0
RTZ is NOT Feasible
Successor, FC:
RD220 not< FD31
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Successors and Feasible Successors
Current Successor = 31
RD of RTY= 21
Feasible Successor, FC: RD30 < FD31
FD to 172.30.1.0 is
31 via Router Y
172.30.1.0
• RTY is successor with a computed cost of 31.
• “31” is the Feasible Distance (FD).
• RTX is a feasible successor because its RD is less than or equal to the FD.
- RTX’s RD (30) is less than the FD (31).
RTZ is NOT Feasible
Successor, FC:
RD220 not< FD31
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Example of a Loop
Cost=70Cost=19
RTZ has a Reported Distance to
RTA of 220. Since its Reported
Distance is greater than RTA’s
own Feasibile Distance of 31,
RTA can’t trust that the route
RTZ takes is somehow back
through itself.
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DUAL Concepts
EIGRP Topology table
• Viewed using the
show ip eigrp
topology command
Contents of table
include:
• all successor
routes
• all feasible
successor
routes
Topology Table: Successor & Feasible Successor
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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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• 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
Finite State Machine
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• DUAL FSM
– Selects a best
loop-free path to
a destination
– Selects alternate
routes by using
information in
EIGRP tables
Finite State Machine
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Finite State Machines (FSM): with Feasible Successor
• To examine output from EIGRP’s finite state machine
us the debug eigrp fsm command
Finite State Machine
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Finite State Machine
Finite State Machines (FSM): without Feasible Successor
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More EIGRP Configuration
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The Null0 Summary Route
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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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
Automatic Summarization
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The no auto-summary Command
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Disabling Automatic Summarization
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More EIGRP Configurations
• Disabling Automatic Summarization: topology table change
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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
Manual Summarization
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• Configuring a summary route in EIGRP
Manual Summarization
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• Configuring a summary route in EIGRP
Manual Summarization
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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
EIGRP Default Route
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EIGRP Default Route
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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 as-number 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
• Describe the background and history of EIGRP
• Describe the features and operation of EIGRP
• Examine the basic EIGRP configuration commands
and identity 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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