Routing & EIGRP
CCNA 3.0
Enhanced Interior Gateway
Routing Protocol (EIGRP)
• Really just an enhanced version of IGRP
• A Cisco proprietary routing protocol
• Called a hybrid protocol, but really just an advanced distance
vector protocol.
• Fast convergence
• Variable length subnet masks
• Partial updates - only when the metric for a route changes
(bounded updates)
• Multiple network layer support - IP, IPX, and AppleTalk
• A router running EIGRP stores all its neighbor’s routing tables
so that it can quickly adapt or alternate routes.
Features of EIGRP
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

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Classless Routing Protocol (VLSM, CIDR)
Faster convergence times and improved scalability
Multiprotocol support: TCP/IP, IPX/SPX, Appletalk
– There is no IPX/SPX or Appletalk in CCNA or CCNP
Rapid Convergence and Better handling of routing loops –
(DUAL)
Efficient Use of Bandwidth
– Partial, bounded updates: Incremental updates only to the
routers that need them.
– Minimal bandwidth consumption: Hello packets and by
default uses no more that 50% of link’s bandwidth EIGRP
packets.
PDM (Protocol Dependent Module)
– Keeps EIGRP modular
– Different PDMs can be added to EIGRP as new routed protocols
are enhanced or developed: IPv4, IPv6, IPX, and AppleTalk
IGRP & EIGRP
They work together and routes are redistributed
automatically
RTB(config)# router igrp 2446
RTB(config-router)#network 192.168.1.0
RTB(config)#router eigrp 2446
RTB(config-router)# network 10.1.1.0
RTB(config-router)# network 172.16.1.0
IGRP & EIGRP
Metric calculation: IGRP/EIGRP
metric = [K1 * bandwidth + ((K2 * bandwidth) / (256 * load)) +
(K3 * delay)] * [K5/(reliability + K4)]
(with the following default constant values):
Constant Value
K1
1
K2
0
K3
1
K4
0
K5
0
Notes
 k2 metric effects LOAD
 k4 and k5 effects RELIABILITY
Metric Calculation
metric = [K1 * bandwidth + ((K2 * bandwidth) / (256 * load)) +
(K3 * delay)] * [K5/(reliability + K4)]
When K2, K4 and K5 are 0, these portions of the
equation is not factored in to the metric.
Thus, with the default constant values, K1=1 and
K3=1, the metric equation boils down to this:
metric = [(1 * bandwidth) + (1 * delay)]
metric = bandwidth + delay
Actually:
metric = slowest bandwidth + sum of all delays
Metric Calculation
The metrics used by EIGRP in making routing decisions are (lower the
metric the better):
 bandwidth
 delay
 load
 reliability
By default, EIGRP uses only:
 Bandwidth
 Delay
Analogies:
Think of bandwidth as the width of the pipe
and
delay as the length of the pipe.


Bandwidth is a the carrying capacity
Delay is the end-to-end travel time.
Metric Calculation
If these are the default:
 bandwidth (default)
 delay (default)
When are these used?
 load
 reliability
Only when configured by the network administrator to
do so!
Use show interface command to view the metrics
used on a specific interface that is routing EIGRP.
Metric Calculation – show
interfaces
Router> show interface s0/0
Serial0/0 is up, line protocol is up
Hardware is QUICC Serial
Bandwidth
Delay
Description: Out to VERIO
Internet address is 207.21.113.186/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted>
Reliability
Load
shows reliability as a fraction of 255, shows load as a fraction of 255, for
for example (higher is better):
example (lower is better):
rely 190/255 (or 74% reliability)
load 10/255 (or 3% loaded link)
rely 234/255 (or 92% reliability)
load 40/255 (or 16% loaded link)
rely 255/255 (or 100%
load 255/255 (or 100% loaded link)
reliability)
IGRP & EIGRP
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The routers in the EIGRP network will convert
the 24-bit IGRP metric that IGRP uses to its
32-bit metric
EIGRP Metric = IGRP Metric x 256 (32 bit
vs. 24 bit)
EIGRP Terminology
Term
Neighbor table
Definition
Lists adjacent routers (like
adjacency DB in OSPF
Topology table
Each router has one for each
network protocol routed
Routing table
Chooses routes from topology
table – maintains one for each
network protocol
Successor
Has these for
EACH routed
protocol
Feasible successor
Route selected as the primary
route to use to reach a
destination
A backup route to above –
multiple feasible successors for
a destination can be kept in the
topology table
Improvements from IGRP
1.
Neighbor discovery & recovery
–
–
2.
Use small “hello” packets to estab. adjacencies
– sent every 5 seconds
Dynamically learn routes that way
Reliable Transport Protocol (RTP)
–
–
A transport layer protocol (Layer 4) that
guarantees delivery order
EIGRP is protocol independent, so has its own
guarantee (compare to TCP)
Improvements from IGRP
Dual finite-state machine
3.
–
–
4.
An algorithm that EIGRP uses to calculate routes
Tracks all routes advertised by neighbors and uses a
composite metric of each route to compare them
Protocol-dependent modules
–
–
Each module is responsible for all functions
related to its specific routed protocol
In other words, there is an IP PDM, an IPX PDM,
an AppleTalk PDM, etc.
EIGRP Neighbor Table
•This table is the basis for all EIGRP routing
updates & convergence
•Has IP info about neighbor routers
•Smooth Round Trip Timer (SRTT) The average
time it takes to send and receive packets from
a neighbor.
•Queue count The number of packets waiting in
queue to be sent.
EIGRP Topology Table
RouterB#show ip eigrp topology
IP-EIGRP Topology Table for process 44
Codes: P - Passive, A - Active, U - Update, Q - Query, R Reply, r - Reply status
P 206.202.17.0/24, 1 successors, FD is 2195456
via 206.202.16.1 (2195456/2169856), Ethernet0
P 206.202.18.0/24, 2 successors, FD is 2198016
via 192.168.0.2 (2198016/284160), Serial0
via 206.202.16.1 (2198016/2172416), Ethernet0
Each EIGRP router maintains a topology table for each configured
network protocol.
This table includes the current routes (successors) and back-up routes
(feasible successors).
P = Passive (good), A = Active (not ready, DUAL running)
EIGRP uses its topology table to store all the information it needs to
calculate a set of distances and vectors to all reachable
destinations.
Info for the destination route: routing protocol,
feasible distance of the route, the route cost
Terms
RD=5
RD=5
10
FD=15
FD=15
10
Coming soon:
RD = Reported Distance
FD = Feasible Distance
14
15
15
20
FD=20
6
RD=6
Successor – Current Route
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A successor is a route selected as the primary route to use to reach a
destination.
Successors are the entries kept in the routing table.
Feasible Successor - A backup route
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These routes are selected at the same time the successors are identified, but
they are kept in the topology table.
Multiple feasible successors for a destination can be retained in the topology
table.
Terms
RD=5
RD=5
10
FD=15
FD=15
10
14
15
15
20
FD=20
6
RD=6
Feasible distance (FD) is the minimum distance (metric) along a path
to a destination network. (“This Router’s Distance”)
Reported distance (RD) is the distance (metric) towards a destination
as advertised by an upstream neighbor. (“The Neighbor Router’s
Distance”)
172.30.1.0/24
RouterX’s FD = 30 to 172.30.1.0/24 (Sent as RD to RouterA)
(20)
(10)
Router H
RouterX
(10)
Router G
RouterY’s FD = 21 to 172.30.1.0/24 (Sent as RD to RouterA)
(1)
Best Route
FDDI Ring
Router A
FD = 31
172.30.1.0 is 31
via RouterY
Router B
RouterY
Router C
(10)
RouterZ’s FD =220 to 172.30.1.0/24 (Sent as RD to RouterA)
Router D
RouterZ
(100)
Router E
(100)
Router F
The Feasible Distance to a network is sent to other
routers, as this router’s Reported Distance.
Feasible Successor: RD= 30, FC: RD30 < FD31
172.30.1.0/24
(20)
(10)
Router H
RouterX
Router G
(10)
(1)
(Current) Successor: RD= 21
FDDI Ring
Router A
Router B
Router C
RouterY
FD = 31
(10)
172.30.1.0 is 31
NOT a Feasible Successor: RD = 220, FC not met: RD220 > FD31
via RouterY
Router D
RouterZ
(100)
Router E
(100)
Router F
A neighbor meets the feasible condition (FC) if the reported distance by the
neighbor is smaller than the current feasible distance (FD) of this router.
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A distance-vector routing protocol not allowing possible paths with loops
paths.
"If a neighbors metric is less than mine, then I know the neighbor doesn't
have a loop going through me."
A feasible successor (FS) is a neighbor whose reported distance (RD) is less
than the current feasible distance (FD).

Feasible successor is one who meets the feasible condition (FC).
DUAL – Diffusing Update Algorithm
The centerpiece of EIGRP is DUAL fsm (finite state
machine), the EIGRP route-calculation engine.
DUAL selects alternate routes quickly by using the
information in the EIGRP neighbor and topology table.
If a link goes down, DUAL looks for a feasible successor
in its topology table.
Feasible successors provide the next lowest-cost path
without introducing routing loops.
Selects a best loop-free path to a destination, the next
hop being known as the successor.
All other routers to the same destination, that also meet
the feasible condition, meaning they are also loopfree, become feasible successors, or back-up routes.
Router# debug eigrp fsm
EIGRP Packet Types
Packet
Hello – multicast –
every 5 seconds
Acknowledgement
(unicast)
Update
Definition
Discovers, verifies, rediscovers
neighbor routers
Indicates receipt of an EIGRP
during a reliable packet
When a router discovers a new
neighbor or a topology change
Query
Needing specific info from a
neighbor
Reply
Response to a query
Looking for a New Route
RtrD
RtrB
Queries
Replies
RtrE
RtrA
X
RtrF
RtrC
RtrG
•
If there are no Feasible Successors, the router must ask neighbors for help in hope of
finding a new, loop-free path to the destination.
•
Neighbor routers are compelled to reply to this query.
If a neighbor has a route, it will reply with information about the successor(s).
If not, the neighbor notifies the sender that it doesn’t have a route to the destination either.
This command
shows only
EIGRP routing
table entries
EIGRP Routing Table
Note that the default administrative distance is 90
with EIGRP internal routes
•Contains routes installed by DUAL FSM as the
best loop-free paths
•Can maintain up to 4 routes per destination
•Maintains a separate routing table for each
protocol
Routing table has the primary route to a destination –
this primary route is ALSO in the topology table
Basic EIGRP Configuration
EIGRP Commands
Router(config)# router eigrp AS
 AS must be the same on all routers.
Router(config-router)# network network-number
Interface Commands
Router(config-if)# bandwidth kilobits
Serial links should reflect actual link bandwidth instead of the
default of 1544 kbps – or the network may not be able to
converge
 Bandwidth should be set because a less desirable route could
be chosen as the best path if the bandwidth setting is higher
than the actual bandwidth of the link
Router(config-if)# eigrp log-neighbor-changes
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This command enables the logging of neighbor adjacency
changes to monitor the stability of the routing system and to
help detect problems.
Configuring EIGRP Summarization
EIGRP Commands
Router(config-router)# no auto-summary
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Like RIP and IGRP, EIGRP automatically summarizes as major
network boundaries.
This command turns off automatic summarization, useful for
discontiguous networks (see RIPv2 example).
Interface Commands
Router(config-if)# ip summary-address eigrp
autonomous-system-number ip-address mask
[administrative-distance]
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Summarizes addresses being advertised out of this interface.
Administrative distance for EIGRP Summary Routes is 5, but
can be modified (optional).
Configuring EIGRP Summarization
EIGRP Commands
Router(config-router)# no auto-summary


Like RIP and IGRP, EIGRP automatically summarizes as major
network boundaries.
This command turns off automatic summarization, useful for
discontiguous networks
Interface Commands
Router(config-if)# ip summary-address eigrp
autonomous-system-number ip-address mask
[administrative-distance]


Summarizes addresses being advertised out of this interface.
Administrative distance for EIGRP Summary Routes is 5, but
can be modified (optional).
Configuring a Default Route
Method 1: Quad-Zero Static Route
Gateway Router
ip route 0.0.0.0 0.0.0.0 serial0
!
router eigrp 100
redistribute static
<text omitted>
Method 2: ip default-network command
Gateway Router
router igrp 24
<text omitted>
network 207.21.20.0
ip default-network 207.21.20.0
ip route 0.0.0.0 0.0.0.0 207.21.20.1
Troubleshooting IGRP
commands when network is
unreachable
1.
2.
3.
show ip route to see if the network
has been installed in the routing table
show ip protocols to verify that
IGRP is indeed advertising that
network.
show running-configuration to
check the network statements.
Troubleshooting
 Use
show commands to
monitor normal router
behavior
 Use debug for finding
specific problems
Routing & EIGRP
CCNA 3.0