EIGRP Basics and Steady-State Operation
Many CCIE candidates accept abstruse abounding of the capacity of EIGRP operation and configuration.
EIGRP is broadly deployed and is thoroughly covered on the CCNP BSCI exam. With that in mind,
this affiliate strives to analysis the key agreement and concepts briefly, and get appropriate to specific examples
that detail EIGRP operation on a Cisco router. To that end, the affiliate begins with Table 8-2,
which lists some of the key appearance accompanying to EIGRP.
Hellos, Neighbors, and Adjacencies
After a router has been configured for EIGRP, and its interfaces arise up, it attempts to find
neighbors by sending EIGRP Hellos (destination 224.0.0.10). Once a brace of routers accept heard
each added say Hello, they become adjacent—assuming several key altitude are met. Once
neighbors canyon the checks in the afterward list, they are advised to be adjacent. At that point,
Table 8-2 EIGRP Feature Summary
Feature Description
Transport IP, agreement blazon 88 (does not use UDP or TCP).
Metric Based on accountable bandwidth and accumulative adjournment by default, and
optionally load, reliability, and MTU.
Hello breach Breach at which a router sends EIGRP Accost letters on an interface.
Hold timer Timer acclimated to actuate back a adjoining router has failed, based on
a router not accepting any EIGRP messages, including Hellos, in this
timer period.
Update destination abode Normally beatific to 224.0.0.10, with retransmissions actuality beatific to each
neighbor’s unicast IP address.
Full or fractional updates Abounding updates are acclimated back new neighbors are discovered; otherwise,
partial updates are used.
Authentication Supports MD5 affidavit only.
VLSM/classless EIGRP includes the affectation with anniversary route, additionally acceptance it to support
discontiguous networks and VLSM.
Route Tags Allows EIGRP to tag routes as they are redistributed into EIGRP.
Next-hop acreage Supports the advertisement of routes with a altered next-hop router
than the announcement router.
Manual avenue summarization Allows avenue summarization at any point in the EIGRP network.
Multiprotocol Supports the advertisement of IPX and AppleTalk routes.
210 Affiliate 8: EIGRP
they can barter routes and are listed in the achievement of the appearance ip eigrp acquaintance command.
Neighbors should consistently anatomy back these altitude are met, behindhand of articulation type.
■ Charge canyon the affidavit process
■ Charge use the aforementioned configured AS number
■ Charge accept that the antecedent IP abode of a accustomed Accost is in that router’s primary connected
subnet on that interface
■ K ethics charge match
The diction of the third account in the account bears a little added scrutiny. The primary subnet of an
interface is the subnet as adumbrated by the ip abode command that does not accept the secondary
keyword. An EIGRP router looks at the antecedent IP abode of a Hello; if the antecedent IP abode is a
part of that router’s primary subnet of the admission interface, the Accost passes the IP address
check.
This argumentation leaves accessible some absorbing possibilities. For example, if the routers are misconfigured
with altered subnet masks, the analysis may still pass. If one router has configured 10.1.2.1/24, and
the added has configured 10.1.2.2/23, they could become adjacent, bold all the added checks
pass. While EIGRP supports accessory IP addresses and subnets, EIGRP sources its messages
from the abode in the primary subnet, and the IP addresses of neighbors charge be in the subnet of
the primary subnets.
The aftermost account in the account mentions K values; K ethics are constants that ascertain the multipliers
used by EIGRP back artful metrics. The settings can be afflicted with a router eigrp
subcommand metric weights tos k1 k2 k3 k4 k5. The command defaults to a ambience of 0 1 0 1 0 0,
meaning that alone bandwidth and adjournment are acclimated to account the metric. (The examples in this
chapter usually use the settings 0 0 0 1 0 0, which removes bandwidth from the abacus and
makes the metrics in the examples a little added obvious.)
Besides artlessly blockage to see if the appropriate ambit agree, the Accost letters additionally serve as an
EIGRP keepalive. Adjoining routers abide to multicast Hellos based on anniversary interface’s EIGRP
hello interval. If a router fails to apprehend from a acquaintance for a cardinal of seconds, authentic by the
EIGRP Authority Time for that neighbor, all routes through the acquaintance are advised to accept failed.
Example 8-1 shows how a router displays some of the basal advice apropos EIGRP operations
based on Figure 8-1. The archetype begins with four routers (R1, R2, S1, and S2) that accept only
their accepted LAN interfaces up, aloof to appearance the Accost process. By the end of the example, the
R2-to-R5 PVC will arise up, but the EIGRP adjacency will abort due to a K-value mismatch.
EIGRP Basics and Steady-State Operation 211
Figure 8-1 Sample Internetwork Acclimated for EIGRP Examples
Example 8-1 Forming EIGRP Adjacencies
! First, a alter is accomplished on R1.
R1# alter eigrp packet hello
EIGRP Packets debugging is on
(HELLO)
Jan 11 13:27:19.714: EIGRP: Accustomed HELLO on FastEthernet0/0 nbr 172.31.11.201
Jan 11 13:27:19.714: AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ
un/rely 0/0
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! S2’s LAN interface brought up, not shown
! Below, a brace of log letters appear, announcement the new neighbor; this message
! appears due to the absence router eigrp subcommand eigrp log-neighbor-changes.
Jan 11 13:27:19.995: EIGRP: New associate 172.31.11.202
Jan 11 13:27:19.995: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Acquaintance 172.31.11.202
(FastEthernet0/0) is up: new adjacency
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! Next, alone neighbors who become adjacent—those that canyon all the required
! checks for the parameters—are listed. The Authority timer is shown; it starts at
! its maximum, and decrements appear 0, actuality displace aloft the cancellation of any EIGRP
! packet from that neighbor. The “H” cavalcade on the larboard states the adjustment in
! which the neighbors became adjacent.
R1# appearance ip eigrp neighbors
IP-EIGRP neighbors for action 1
H Abode Interface Authority Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 172.31.11.2 Fa0/0 4 00:03:10 1 4500 0 233
1 172.31.11.202 Fa0/0 11 00:04:
43 1 4500 0 81
0 172.31.11.201 Fa0/0 14 00:05:11 1927 5000 0 84
! Below, the PVC amid R2 and R5 came up, but R5’s K ethics do not bout R2’s.
! Both letters beneath are log messages, with no debugs enabled on either router.
! Abutting bulletin on R5 !!!!!!!!!!
03:55:51: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Acquaintance 172.31.25.2 (Serial0) is down: K-value
mismatch
! Abutting bulletin on R2 !!!!!!!!!!
Jan 11 13:21:45.643: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Acquaintance 172.31.25.5 (Serial0/0.5)
is down: Interface Goodbye received
104.4/24
E0/0
105.5/24
E0 25.5/24
S0
25.2/24
S0/0.5
24.2/30
S0/0.4
11.2/24
11.1/24
11.201/24
11.202/24
211.202/24
221.202/24
Fa0/0
Fa0/0
24.1/30
S0/0.2
14.2/30
S0/0.1
14.1/30
S0/0.4
S1
S2
R4
R5
R1
R2
VLAN 1
172.31.11.0/24
Network 172.31.0.0
212 Affiliate 8: EIGRP
Note that back the PVC amid R2 and R5 comes up, the bulletin on R5 is appealing obvious,
but the bulletin at R2 says annihilation about K values. Some afterwards releases of Cisco IOS mistake
invalid EIGRP K-value settings as a newer EIGRP bulletin alleged a Goodbye message. Goodbye
messages acquiesce routers to acquaint anniversary added that they are shutting bottomward in a adroit fashion; be
aware that this bulletin may artlessly be the aftereffect of a K-value mismatch.
Interestingly, the Accost and Authority time ambit do not charge to bout for EIGRP neighbor
relationships to form. In fact, a router does not use its own timers back ecology a neighbor
relationship—instead, it uses anniversary neighbor’s declared timers, as exchanged in the Accost messages.
For example, in Archetype 8-1, R2 has been configured with Accost and Authority timer settings at 2 and
6 seconds, respectively, with R1 behind to 5 and 15 seconds. As R1 monitors its neighbor
connection to R2, R1 resets the Authority timer to 6 abnormal aloft cancellation of an EIGRP message. With
a accost breach of 2 seconds, R1’s advertisement for authority time for R2 shows it clashing amid 6 and 4,
assuming no Hellos are lost. Agenda the appearance ip eigrp neighbors command on R1 abreast the end
of the example—under accustomed operation, this amount fluctuates amid 6 and 4 seconds. The other
neighbors absence to Accost and Authority time of 5 and 15, so R1’s Authority time in the command output
fluctuates amid 15 and 10 for these neighbors, bold no Hellos are lost.
EIGRP Updates
Once routers are adjacent, they can barter routes application EIGRP Amend messages. The process
follows this accepted sequence:
1. Initially, abounding updates are sent, including all routes except those bare due to breach horizon.
2. Once all routes accept been exchanged, the updates cease.
3. Future fractional updates action back one or added routes change.
4. If neighbors abort and recover, or new acquaintance adjacencies are formed, abounding updates are sent.
EIGRP uses the Reliable Transport Agreement (RTP) to accelerate the multicast EIGRP updates. EIGRP
sends updates, cat-and-mouse on a unicast EIGRP ACK bulletin from anniversary recipient. Figure 8-2 shows
the accepted abstraction over a LAN.
Figure 8-2 EIGRP Use of RTP on a LAN
S1
S2
RTO Asleep for
Neighbor 172.31.11.2
– Resend!
Start RTO Timer
Update, to 224.0.0.10
ACK, to 172.31.11.1
R1
R2
4
1
2
3 ACK, to 172.31.11.1
3
5 Update, to 172.31.11.2
EIGRP Basics and Steady-State Operation 213
RTP allows the Updates to be beatific as multicasts. If any neighbors abort to accede cancellation of
the multicasted update, RTP resends Updates as unicasts aloof to those neighbors. The accomplish run
as follows, application Figure 8-2 as an example:
1. The EIGRP sender (R1 in Figure 8-2) starts a Retransmission Timeout (RTO) timer for each
neighbor back sending a reliable bulletin like an Update. (Cisco IOS absolutely calculates a
Smoothed Round-Trip Time, or SRTT, to anniversary neighbor, and derives RTO from the SRTT; both
values are apparent in the appearance ip eigrp acquaintance output. These ethics alter over time.)
2. R1 sends the multicast EIGRP Update.
3. R1 addendum from which neighbors it receives an EIGRP ACK for the Update.
4. RTO asleep afore router R2 beatific its EIGRP ACK.
5. R1 resends the Update, this time as a unicast, and alone to the neighbor(s) that did not reply
within the RTO time (R2 in this case).
This action allows able multicasting of updates beneath accustomed circumstances, and efficient
retransmission back ACKs do not access in time.
EIGRP and RTP use a simple accepting action with a window admeasurement of one message. Each
Update packet has a arrangement number, with the alternate ACK bulletin acknowledging cancellation of the
message by advertisement that aforementioned arrangement number. Archetype 8-2 shows the area of the sequence
number advice in both appearance and alter commands. (In the example, R1 does a no shut on
a loopback interface [IP abode 172.31.151.1/24], with R1 sending an amend announcement the
newly-available route.)
Example 8-2 Arrangement Numbers in EIGRP Updates and ACKs
! First, agenda the appearance ip eigrp acquaintance achievement on router R2. The aftermost column
! lists the arrangement cardinal aftermost acclimated by that acquaintance to accelerate a “reliable”
! packet. So, R2 expects R1’s abutting reliable EIGRP bulletin to accept arrangement number
! 225. Also, the RTO calculations are listed for anniversary neighbor. Note
! that the SRTT amount is 0 until some reliable packets are exchanged, as SRTT
! is affected based on absolute round-trip time measurements.
R2# sh ip eigrp neighbor
IP-EIGRP neighbors for action 1
H Abode Interface Authority Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 172.31.11.1 Fa0/0 5 01:14:03 1 200 0 224
1 172.31.11.202 Fa0/0 13 01:15:36 1 200 0 92
0 172.31.11.201 Fa0/0 13 01:16:04 257 1542 0 96
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! R1 – R1 – R1 – R1
! Next, the alter command on R1 enables alter for Amend and Ack packets.
continues
214 Affiliate 8: EIGRP
The EIGRP Cartography Table
EIGRP uses three tables: the acquaintance table, the cartography table, and the IP acquisition table. The
neighbor table keeps accompaniment advice apropos neighbors, and is displayed application the appearance ip eigrp
neighbors command. EIGRP Amend letters ample the routers’ EIGRP cartography tables. Based
on the capacity of the cartography table, anniversary router chooses its best routes and installs these routes
in its corresponding IP acquisition table.
An EIGRP router calculates the metric for anniversary avenue based on the apparatus of the metric.
When a adjoining router advertises a route, the Amend includes the metric basic values
for anniversary route. The router again considers the accustomed metric values, as able-bodied its own interfaces
settings, to account its own metric for anniversary route. The absence metric apparatus are cumulative
delay, in tens of microseconds, and the acute bandwidth for the absolute route, in $.25 per
second. By ambience the actual K ethics in the metric weights command, EIGRP can also
consider articulation load, reliability, and MTU. Cisco recommends not application those values, in ample part
due to the aberration created by the rapidly alteration affected metrics and again routing
reconvergence.
Figure 8-3 depicts the accepted argumentation apropos to the metric apparatus in a acquisition update, showing
the units on the bandwidth and adjournment commands against the capacity of the updates.
R1# alter eigrp packet amend ack
EIGRP Packets debugging is on
(UPDATE, ACK)
! Not Shown: R1’s loop0 is “no shutdown,” interface abode 172.31.151.1/24.
! Below, the alter letters appearance R1’s update, and anniversary of the added three routers’
! Acks. Agenda R1’s amend has “sequence” 225, and the Acks account that aforementioned sequence
! cardinal afterwards the slash.
Jan 11 14:43:35.844: EIGRP: Enqueueing UPDATE on FastEthernet0/0 iidbQ un/rely 0/1 serno
207-207
Jan 11 14:43:35.848: EIGRP: Sending UPDATE on FastEthernet0/0
Jan 11 14:43:35.848: AS 1, Flags 0x0, Seq 225/0 idbQ 0/0 iidbQ un/rely 0/0 serno 207-207
Jan 11 14:43:35.848: EIGRP: Accustomed ACK on FastEthernet0/0 nbr 172.31.11.202
Jan 11 14:43:35.852: AS 1, Flags 0x0, Seq 0/225 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely
0/1
Jan 11 14:43:35.852: EIGRP: Accustomed ACK on FastEthernet0/0 nbr 172.31.11.2
Jan 11 14:43:35.852: AS 1, Flags 0x0, Seq 0/225 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely
0/1
NOTE A router considers its interface adjournment settings, as authentic with the adjournment interface
subcommand, back artful EIGRP metrics. The adjournment command’s units are tens of
microseconds, so a adjournment 1 command sets the interface adjournment as 10 microseconds.
Example 8-2 Arrangement Numbers in EIGRP Updates and ACKs (Continued)
EIGRP Basics and Steady-State Operation 215
Figure 8-3 EIGRP Amend and Computing the Metric
Because the accustomed amend includes the neighbor’s metric components, a router can account the
advertising neighbor’s metric for a route—called the appear ambit (RD). A router can, of
course, additionally account its own metric for a accurate route, afterwards abacus its own interface adjournment and
considering whether it should acclimatize the amount for the acute bandwidth. For example,
consider the four accomplish categorical in Figure 8-3:
1. R1 advertises a route, with bandwidth = 10,000 and adjournment = 100.
2. R2 calculates the RD for this avenue per the accustomed K values.
3. R2 updates its cartography table, abacus adjournment 1000 because the interface on which R2 received
the amend has a adjournment ambience of 1000. It additionally uses a new bandwidth setting, because the received
Update’s bandwidth (10,000) was greater than R2’s admission interface’s bandwidth (1544).
4. R2’s amend to addition acquaintance includes the new (cumulative) adjournment and the new (constraining)
bandwidth.
Assuming absence K-value settings, the EIGRP blueprint for the metric abacus is
Metric = 256 (107/bandwidth) + 256 (delay)
The appearance ip eigrp cartography command lists the RD and the locally computed metric for the entries
in the EIGRP cartography table. Archetype 8-3 shows a few capacity of area the RD and bounded metric
can be apparent in appearance command output. The archetype is based on Figure 8-1, with all routers and
interfaces now alive properly. Also, to accumulate things simple, the adjournment command has been used
to set all links to adjournment 1 (LANs), adjournment 2 (WANs), or adjournment 3 (loopbacks). Also, the metric
weights 0 0 0 1 0 0 command was acclimated on anniversary router, demography bandwidth out of the calculation,
making the affected metrics a little added allusive in the command output.
Calculate RD per
Received K Values
Interface Settings:
Delay 100
Bandwidth 10,000
Update:
Subnet 1
Delay 100
Bandwidth 10,000
(MTU, Load, Reliability)
R1 Interface Settings:
Delay 1000
Bandwidth 1544
Metrics:
RD = 256 (10,000,000/10,000) + 256 (100) = 281,600
Metric = 256 (10,000,000/1,544) + 256 (1100) = 1,939,631
R2
1
2
4 Update:
Subnet 1
Delay 1100
Bandwidth 1,544
(MTU, Load, Reliability)
Topology Table:
Subnet 1
Delay 100 + 1000 = 1100
Bandwidth 1,544
(MTU, Load, Reliability)
Calculate My Metric
3
216 Affiliate 8: EIGRP
The appearance ip eigrp cartography command lists a few added actual important concepts and terms
related to how EIGRP chooses amid assorted accessible routes to the aforementioned prefix. First, the term
feasible ambit (FD) refers to this router’s best affected metric amid all accessible routes to
reach a accurate prefix. The FD is listed as “FD is x” in the command output. The avenue that has