CCNA3 – module 3 EIGRP
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EIGRP
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Cisco proprietary, released in 1994
Based on IGRP
EIGRP is an advanced distance-vector
routing protocol that relies on features
commonly associated with link-state
protocols. (sometimes called a hybrid
routing protocol)
Supports VLSM and CIDR
Allows for multiple routed protocols
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EIGRP
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Uses Hello protocols to create and maintain neighbor
relationships and to determine when a link is down.
(Like link-state)
 Every 5 seconds, with a hold timer of 15 seconds
 Links less than T1 speed, Hellos are every 60
seconds, hold timer of 180 seconds
Partial routing updates in response to topology
changes
When a change in topology occurs, EIGRP does not
flood updates like link-state protocols, but immediately
sends those changes to its neighbors.
Has speed and efficiency of routing updates like a
link-state protocol, along with a topology database.
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EIGRP Benefits
Rapid Convergence – DUAL routing algorithm
Efficient use of bandwidth
partial, bounded updates – only sent to routers
that need the information & not entire routing
table
small “Hello” packets to keep in touch with other
routers – minimal use of bandwidth
Supports VLSM and CIDR
Multiple network-layer support (multiprotocol)
IP, IPX, and AppleTalk
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EIGRP and IGRP
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Automatically redistributes when using same AS
number.
Topology example….
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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
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Metric Calculation
Simplify the previous equation by
applying the default constants…
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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 (carrying capacity)
 Delay (end-to-end travel time)
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Metric Calculation
If these are the default:
 bandwidth (default)
 delay (default)
When are these used?
 load
 reliability
Use show interface command to view the
metrics used on a specific interface that is
routing EIGRP.
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Metric Calculation – show interfaces
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
Hardware is QUICC Serial
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>
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Metric Calculation – Bandwidth
Bandwidth
 Expressed in kilobits (show interface)
 This is a static number and used for metric
calculations only.
 Does not necessarily reflect the actual bandwidth of
the link.
 You can adjust the bandwidth metric on an interface
using what command?
 What command restores the interface bandwidth to
the original default?
The default values:
 What is the default bandwidth of serial interfaces on
CISCO routers?
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Metric Calculation – Delay
Delay
 Like bandwidth, delay it is a static number.
 Expressed in microseconds, millionths of a
second
 (Uses the Greek letter mu with an S, S, NOT “ms”
which is millisecond or thousandths of a second)
The default values:
 What is the default delay of a Cisco serial
interface?
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IGRP/EIGRP metric uses the sum of all of the
delays of all of the outbound interfaces to the
destination network.
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Metric Calculation – Delay
Changing the delay informational parameter:
The delay can be changed using:
Router(config-if)# delay tens-of-S
(microseconds)
Example of changing the delay on a serial interface to
30,000 microseconds:
Router(config-if)# delay 3000
To restore the 20,000 microsecond default value:
Router(config-if)# no delay
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IGRP vs EIGRP Metric Calculation
The difference:
 IGRP metric is 24 bits long
 EIGRP metric is 32 bits long
 EIGRP metric is 256 times greater for the
same route
 EIGRP allows for finer comparison of
potential routes
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IGRP vs EIGRP Metric Calculation
IGRP
 bandwidth = (10,000,000 ÷ bandwidth)
 delay
= delay ÷ 10
EIGRP
 bandwidth = (10,000,000 ÷ bandwidth) * 256
 delay
= (delay ÷ 10) * 256
Note: The reference-bandwidth
For both IGRP and EIGRP: 107,
(10,000,000/bandwidth), whereas with OSPF it
was 108 (100,000,000/bandwidth)
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Metrics – HOP COUNT
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EIGRP also imposes a maximum hop count
of 224
IGRP maximum hop count of 255
RIP maximum hop count of 15
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EIGRP Configuration
RTA(config)# router eigrp AS
RTA(config-router)# network network
RTA(config-router)# eigrp log-neighbor-changes
RTA(config-if)#bandwidth kilobits
AS must be the same on
all routers in the AS
Enables logging of neighbor adjacency
changes to monitor stability & help
detect problems
If AS’s are identical on a
router running both IGRP
and EIGRP, routing
tables are redistributed
automatically.
CISCO recommends this command.
EIGRP assumes default if not set.
If the link is slower, router might not
converge, routing updates may be lost,
or suboptimal path selection may result.
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EIGRP tables
 Neighbor
 Topology
 Routing
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EIGRP Neighbor tables
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Lists adjacent routers
One table for each routed protocol that
EIGRP supports
RTX#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address
Interface Hold Uptime SRTT RTO Q
(sec)
(ms)
Cnt
1 10.2.0.2
Se1
12 00:27:39
333 1998 0
0 10.1.0.1
Se0
14 01:17:14
40
240
0
Seq
Num
10
27
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EIGRP Neighbor tables
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Fields in neighbor table:
 Neighbor address (Address)
 Hold time (Hold Uptime)
 Smooth round-trip timer (SRTT)
 Queue count (Q Cnt)
 Sequence number (Seq No)
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DUAL – calculating best paths
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DUAL (Diffusing Update Algorithm) –
allows routers involved in a topology
change to synchronize at the same
time, while not involving routers that
are unaffected by the change
(distance-vector algorithm)
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EIGRP Topology tables
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Contains all of the EIGRP routing tables in the
autonomous system
 DUAL combines information supplied by
neighbor and topology tables and calculates
the lowest cost routes to each destination.
 EIGRP routers track changes and switch to
alternate routes when necessary
 DUAL places this primary route, or successor
route, information in the routing table
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EIGRP Topology tables
Topology tables includes:
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Feasible distance (FD) – lowest calculated metric to
each destination.
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Route source (via xxx.xxx.xxx.xxx) – ID of router that
originally advertised route
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Reported distance (RD) – distance that an adjacent
neighbor reports to specific destination
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Interface information – interface through which the
destination is reachable
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Route status –
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passive (P) – route is stable and ready for use
Active (A) – route is in the process of being recomputed by
DUAL
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EIGRP Topology tables
RTX#show ip eigrp topology
IP-EIGRP Topology Table for process 100
Codes: P - Passive, A - Active, U - Update, Q - Query, R Reply, r - Reply status
P 200.10.3.0/24, 1 successors, FD is 2169856
via Connected, Serial1
via Redistributed (2169856/0)
P 200.10.4.0/24, 1 successors, FD is 2169856
via Connected, Serial0
RD
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EIGRP Topology tables
Successor – route that is selected as the primary
route to use to reach a destination.
 DUAL identifies and places route in routing table
 There can be up to four successor routes for any
particular route.
 Copy of successor is also placed in topology table
EIGRP sorts the topology table so that the
successor routes are at the top, followed by the
feasible successors. Bottom of list may include
routes that DUAL believes to be loops.
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EIGRP Topology tables
Feasible Successor – backup route
 Identified at same time as successor but kept only
in the topology table
 May have multiple feasible successors but not
mandatory to have any
 They are neighbors that are downstream (closer
to the destination)
 Must have lower advertised cost than existing
successor
 If feasible successor not identified, active status
placed on route and query packets sent to all
neighbors to recompute topology.
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Successor & Feasible Successor
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Recomputation of Routes
If route becomes unavailable and DUAL
cannot find feasible successor.
Neighbor routers are compelled to answer
query for a loop-free path.
If neighbor has no route, it responds so.
Excess computation = network instability
To prevent convergence problems, DUAL
always tries to find a feasible successor before
resorting to recomputation
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Recomputation of Routes
Stuck in Active routers –
one or more routers don’t respond to query
(180 seconds) then routes placed in stuck in
active state
EIGRP then clears its table of neighbors
that did not respond
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Route Tagging
Routing Table –
Internal routes
routes originating within the EIGRP AS
External routes
routes originating outside the EIGRP AS
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Routing Table & Tagging
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M mobile, B – BGP, D - EIGRP, EX - EIGRP external, O - OSPF, IA
- OSPF inter area, E1 - OSPF external type 1, E2 - OSPF
external type 2, E – EGP, i - IS-IS, L1 - IS-IS level-1, L2 IS-IS level-2, * - candidate default U - per-user static route
Gateway of last resort is not set
C
10.1.1.0 is directly connected, Serial0
D
172.16.0.0 [90/2681856] via 10.1.1.0, Serial0
D EX 192.168.1.0 [170/2681856] via 10.1.1.1, 00:00:04, Serial0
External
EIGRP routes are redistributed from IGRP networks
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Hellos & RTM
Hello
Packets
Sent every 5 seconds (high-bandwidth links)
and every 60 seconds by default (lowbandwidth links)
As long as routers receive hello packets –
route in passive state
Reliable Transport Protocol (RTM)
Layer 4 protocol
Guarantees delivery of packets
Allows EIGRP to multicast and unicast to
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different peers simultaneously
DUAL FSM
DUAL
Finite-State Machine (DUAL FSM)
Full name of DUAL technology
Tracks routes advertised by neighbors and
uses composite metric to compare them
Guarantees each path is loop-free
Inserts lowest-cost paths (successor routes)
into routing table
Neighbor & Topology tables supply DUAL with
route information to make decisions quickly
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DUAL FSM
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EIGRP Packets
•Hello Packets
•5 second updates (T1/Point-to-Point link)
•Multicast on IP address 224.0.0.10
•Hold Time = 3 times the Hello interval
(15 seconds)
•Neighbor routers hello and dead intervals
do not need to match with EIGRP (unlike
OSPF)
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EIGRP Packets
•Acknowledgement
•Used by RTP to signal reliable exchange of information
•Unicast to specific host
•Update
•Discovery of new neighbor
•EIGRP router sends unicast to neighbor so that it can
add to its topology table
•EIGRP router send multicast to all neighbors if topology
change occurs
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EIGRP Packets
•Query & Reply
•Query - Used to obtain specific information from one or
all of its neighbors (multicast or unicast)
•Reply – response to query (unicast)
•Used, for example, if DUAL places a route in active
state and multicasts neighbors, searching for successor
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EIGRP Route Summarization
EIGRP automatically summarizes routes at the
classful boundary
This may not be preferable, especially if using
VLSM
To disable auto-summarization:
Router(config-router)#no auto-summary
Manual Summarization command:
Router(config-if)#ip summary-address
eigrp AS ip-address subnet-mask
administrative distance
(configured on a per-interface basis)
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EIGRP Route Summarization
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Verifying EIGRP
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Show ip eigrp neighbors
Show ip eigrp interfaces
Show ip eigrp topology
Show ip eigrp topology all-links
Show ip eigrp traffic AS
Debug eigrp fsm
Debug eigrp packets
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RIP, IGRP, & OSPF
Troubleshooting
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Show & debug commands
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Debug ip rip
Debug ip igrp events
Debug ip igrp transactions
Debug ip ospf events
Show ip route
Show running-config
Show ip protocols
show ip ospf neighbor
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