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 Roots of EIGRP: IGRP
-Developed in 1985 to overcome
RIPv1’s limited hop count
-Distance vector routing
-Metrics used by IGRP
bandwidth (used by default)
Delay (used by default)
Reliability (not used by default)
Load (not used by default)
support starting
with IOS 12.2(13)T & 12.2(R1s4)S
 OSPF requires more resources from router
 Fast convergence
 Less overhead – good for large networks
 LSAs sent in LSUs – only the changes are flooded to
the network
 Supports VLSM
 Complex to configure!
 EIGRP – Cisco proprietary routing protocol
 Key features
 Neighbor and topology tables
 Feasible Distance & Successor Routes
 EIGRP packet types
 Setting up EIGRP
 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
 The Algorithm
–EIGRP uses the Diffusing
Update Algorithm (DUAL).
–EIGRP does not send periodic
updates and route entries do not
age out.
–Only changes in the routing
information, such as a new link or
a link becoming unavailable
cause a routing update to occur.
–EIGRP routing updates are still
vectors of distances transmitted
to directly connected neighbors.
 Path Determination
DUAL maintains a topology
table separate from the routing table,
which includes both the best path to a
destination network and any backup
paths that DUAL has determined to be
a route becomes unavailable, DUAL will search its
topology table for a valid backup path.
• If one exists, that route is immediately entered into
the routing table.
• If one does not exist, DUAL performs a network
discovery process to see if there happens to be a
backup path that did not meet the requirement of
the feasibility condition.
 Convergence
–EIGRP does not use holddown
–Instead, loop-free paths are
achieved through a system of
route calculations (diffusing
computations) that are
performed in a coordinated
fashion among the routers.
–The detail of how this is done is
beyond the scope of this course,
but the result is faster
convergence than traditional
distance vector routing protocols.
EIGRP Message Format
 EIGRP Header
link frame header - contains
source and destination MAC
IP packet header - contains source
& destination IP address
EIGRP packet header - contains AS
Type/Length/Field - data portion
of EIGRP message
In the IP packet header,
 the protocol field is set to 88 to
indicate EIGRP
 the destination address is set to
the multicast
If the EIGRP packet is
encapsulated in an Ethernet frame,
 the destination MAC address is
also a multicast address: 01-005E-00-00-0A.
 EIGRP packet header contains
–Opcode field
System number
The AS number is used to track multiple
instances of EIGRP.
 EIGRP Parameters contains
• EIGRP uses for its composite metric.
• By default, only bandwidth and delay
are weighted. Both are set to 1.
• The other K values are set to zero.
–Hold time
• The amount of time the EIGRP
neighbor receiving this message
should wait before considering
the advertising router to be
 TLV: IP internal contains (EIGRP
routes within an autonomous system)
–Metric field (Delay and
• Delay is calculated as the sum of
delays from source to destination
in units of 10 microseconds.
• Bandwidth is the lowest
configured bandwidth of any
interface along the route.
–Subnet mask field
• The subnet mask is specified as
the prefix length or the number
of network bits in the subnet
• is 24
• the address of the destination network.
• Although only 24 bits are shown in this figure.
• If a network address is longer than 24 bits, then the Destination
field is extended for another 32 bits
 TLV: IP external contains
–Fields used when external
 routes are imported into
 EIGRP routing process
– import or redistribute a route into EIGRP.
Protocol Dependent Modules (PDM)
 EIGRP uses PDM to route several
different protocols i.e. IP, IPX &
 PDMs are responsible for the specific
routing task for each network layer
you can see in the figure, EIGRP uses
different EIGRP packets and maintains
separate neighbor, topology, and
routing tables for each Network layer
• The IP-EIGRP module is responsible
for sending and receiving EIGRP
packets that are encapsulated in IP
and for using DUAL to build and
maintain the IP routing table.
• The IPX EIGRP module is responsible
for exchanging routing information
about IPX networks with other IPX
EIGRP routers.
• Apple-Talk EIGRP is for Apple-talk
Reliable Transport Protocol (RTP)
 Purpose of RTP
–Used by EIGRP to transmit and receive EIGRP
– EIGRP was designed as a Network layer
independent routing protocol; therefore, it cannot
use the services of UDP or TCP because IPX and
Appletalk do not use protocols from the TCP/IP
protocol suite.
 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
EIGRP’s 5 Packet Types
 Hello packets
to discover & form adjacencies with neighbors
–EIGRP hello packets are multicasts and use unreliable
 Update packets
packets are used to
propagate routing information
–Update packets are sent only
when necessary.
–EIGRP updates are sent only to
those routers that require it.
–When a new neighbor is
discovered, unicast update packets
are sent so that the neighbor can
build up its topology table.
–In other cases, such as a link-cost
change, updates are multicast.
–Updates always are transmitted
 Acknowledgement packets
to acknowledge receipt of update, query & reply
–An acknowledgment packet is a hello packet that has no
–EIGRP acknowledgement packets are always sent as an
unreliable unicast
 Query & Reply packets
Used by DUAL for searching for
Queries and replies use reliable
Query packets can use
packet use only
(not require
acknowledgment )
(a hello packet that
has no data )
Multicast &
 Purpose of Hello Protocol
–To discover neighbors & establish adjacencies with neighbor
 Characteristics of hello protocol
–Time interval for sending hello packet
5 seconds - high bandwidth (greater than T1)
 60 seconds - multipoint circuits T1 bandwidth or slower
This is the maximum time
router should wait before
declaring a neighbor down
Default holdtime
–3 times hello interval
»15 seconds
»180 seconds
EIGRP Updates
 EIGRP only sends update when there is a change in route status
 Partial update
partial update includes only the route information that has changed
– the whole routing table is NOT sent
 Bounded update
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
Diffusing Update Algorithm (DUAL)
EIGRP’s primary method for preventing routing loops
And also hold-down timers and split horizon, too.
of using DUAL
Provides for fast convergence time by keeping a list of loop-free
backup routes
maintains a list of backup routes it has already
determined to be loop-free. If the primary route in the
routing table fails, the best backup route is immediately
added to the routing table.
 Administrative Distance (AD)
–Defined as the trustworthiness of the source route
 EIGRP default administrative distances
–Summary routes = 5
–Internal routes
= 90
–Imported routes = 170
It is good practice to authenticate
transmitted routing information.
Encrypt routing information
Authenticate routing information
This practice ensures that routers will
only accept routing information from
other routers that have been
configured with the same password or
authentication information.
Note: Authentication does not encrypt
the router's routing table.
Basic EIGRP Configuration
 EIGRP autonomous system
number actually functions as a
process ID
vast majority of companies
and institutions with IP networks
do not need an AS number
–The ISP is responsible for the
routing of packets within its
autonomous system and between
other autonomous systems.
 Process ID represents an instance
of the routing protocol running on
a router
 Example
Router(config)#router eigrp autonomous-system
EIGRP Metric Calculation
EIGRP Metrics
 Use the show interfaces
command to view metrics
 EIGRP Metrics
– EIGRP uses
a static bandwidth to
calculate metric
–Most serial interfaces use a
default bandwidth value of
1.544Mbos (T1)
–The value of the
bandwidth may or may
not reflect the actual
SPEED of the interface.
–If actual SPEED of the
link differs from the
default bandwidth
value, then you should
modify the bandwidth
The default bandwidth for ethernet is 10,000 Kbits. The default bandwidth for fastethernet is 100,000 Kbits.
EIGRP Metric Calculation
EIGRP Metrics
 Delay is the defined as the
measure of time it takes for
a packet to traverse a route
is a static value based on
link type to which interface
is connected
delay value, much like the
bandwidth value, is a default
value that can be changed by
the network administrator
EIGRP Metric Calculation
 Reliability (not a default EIGRP metric)
measure of the likelihood that a link will fail or how
often the link has experienced errors.
–Measure dynamically & expressed as a fraction of 255
• the higher the fraction the better the reliability
–Reliability is calculated on a 5-minute weighted
average to avoid the sudden impact of high (or low)
error rates.
 Condition/Network Status (not a default EIGRP
– A number that reflects how much traffic is using a
– Number is determined dynamically and is expressed
as a fraction of 255
 The lower the fraction the less the load on the link
This value is calculated on a 5-minute weighted
average to avoid the sudden impact of high (or low)
channel usage.
EIGRP Metric Calculation
 EIGRP uses the lowest bandwidth (BW)in its metric
Calculated BW = reference BW / lowest BW(kbps)
 Delay – EIGRP uses the cumulative sum of all
outgoing interfaces
Delay = the sum of outgoing interface delays
 EIGRP Metric = calculated BW + calculated delay
DUAL Concepts
 The Diffusing Update Algorithm (DUAL) is used to
prevent looping
Distance (FD)
–Feasible Successor (FS)
–Reported Distance (RD) or Advertised Distance (AD)
–Feasible Condition or Feasibility Condition (FC)
DUAL Concepts
 Successor
best least cost route to a destination found in the routing
 Feasible distance
lowest calculated metric along a path to a destination
 2 commands can be used to find the “successor” and “feasible
–show ip route
–show ip eigrp topology
What if the successor fails?
1) If feasible successor exists:
 If current successor route fails, feasible successor becomes the current
successor, i.e. the current route.
 Routing of packets continue with little delay.
2) If no feasible successor exists:
 This may be because the Reported Distance is greater than the Feasible
 Before this route can be installed, it must be placed in the active state and
 Routing of packets continue but with more of a delay.
DUAL Concepts
 Feasibility Condition (FC)
–Met when a neighbor’s reported distance (RD) is
less than the local router’s FD to the same
destination network
–The reported distance is simply an EIGRP neighbor's
feasible distance to the same destination network.
–The reported distance is the metric that a router
reports to a neighbor about its own cost to that network.
DUAL Concepts
 Reported distance (RD)
metric that a router reports to a neighbor about its own
cost to that network
DUAL Concepts
 The centerpiece of EIGRP is DUAL
and its EIGRP route-calculation
engine. The actual name of this
technology is DUAL Finite State
Machine (FSM).
 Finite Sate Machine (FSM)
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
–Selects a best loop-free path to
a destination
–Selects alternate routes by
using information in EIGRP
Fine-Tuning EIGRP
 EIGRP bandwidth utilization
–By default, EIGRP uses only up to 50% of interface
bandwidth for EIGRP information
This prevents the EIGRP process from over-utilizing a link and not
allowing enough bandwidth for the routing of normal traffic.
–Purpose of DUAL
 To prevent routing loops
 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
 Choosing the best route
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
 Offers multiprotocol operation (IP, IPX etc)
 EIGRP multiplies IGRP metric by 256
 Uses 32-bit metric c.f. IGRP 24-bit metric
 Metric = bandwidth / delay
 EIGRP BW = (107/ BW) x 256
 EIGRP delay = (delay/10) x 256
 Max hop = 224
c.f. IGRP 256
EIGRP key features
 EIGRP automatically shares routing information with IGRP
and vice versa
 EIGRP tags routes from other protocols as external
 Rapid convergence from Diffused Updating Algorithm (DUAL)
 Guaranteed no loops – all routers running EIGRP update at the
same time if change occurs
 Efficient use of BW – partial incremental updates only
 Sent only to routers that need information – not all routers
Good use of Network Bandwidth
 Partial bounded updates, unlike OSPF
 No timed routing updates
 RIP = 30s; IGRP = 90s – full routing tables
 EIGRP uses small hello packets
 Unreliable – no acknowledgements
 Supports VLSM & CIDR unlike IGRP
 Uses PDMs to support IP, IPX, Appletalk
EIGRP Router Tables
 Neighbour table lists adjacent routers
 One neighbour table per supported protocol
 Similar to OSPF adjacency database
 Holds neighbour address and interface data
 Hold time = time to wait before considering
neighbour to be ‘down’ = 3 x hello interval
Multicast to
 Smooth Round Trip Timer SRTT
Neighbour Discovery
and Rediscovery
Setting up EIGRP
Needs Autonomous System Number
Router(config)#router eigrp 123
Uses Wildcard Mask
 EIGRP similar to OSPF  Has neighbour table
 Sends hello packets
 Has topology table (1 for each protocol)
 Uses DUAL on Neighbour & Topology tables
 Elects successors and feasible successors (FS)
 FS used in times of trouble – fast selection
 Verification and debug commands