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There are two modes from which EIGRP commands are entered: router configuration mode
and interface configuration mode. Router configuration mode enables the protocol, determines
which networks will run EIGRP, and sets global characteristics. Interface configuration
mode allows customization of summaries, metrics, timers, and bandwidth.
262 Chapter 4 Configure, verify, and troubleshoot basic router operation
To start an EIGRP session on a router, use the router eigrp command followed by the
autonomous system number of your network. You then enter the network numbers connected
to the router using the network command followed by the network number.
Let’s look at an example of enabling EIGRP for autonomous system 20 on a router connected
to two networks, with the network numbers being 10.3.1.0/24 and 172.16.10.0/24:
Router#config t
Router(config)#router eigrp 20
Router(config-router)#network 172.16.0.0
Router(config-router)#network 10.0.0.0
Remember—as with RIP, you use the classful network address, which is all subnet and host
bits turned off.
Understand that the AS number is irrelevant—that is, as long as all routers use the same
number! You can use any number from 1 to 65,535.
Verifying EIGRP
There are several commands that can be used on a router to help you troubleshoot and verify
the EIGRP configuration. Table 4.10 contains all of the most important commands that are
used in conjunction with verifying EIGRP operation and offers a brief description of what
each command does.
I’ll demonstrate how you would use the commands in Table 4.10 by using them on our
internetwork that we just configured—not including the discontiguous network example.
TABLE 4 . 1 0 EIGRP Troubleshooting Commands
Command Description/Function
show ip route Shows the entire routing table
show ip route eigrp Shows only EIGRP entries in the routing table
show ip eigrp neighbors Shows all EIGRP neighbors
show ip eigrp topology Shows entries in the EIGRP topology table
debug eigrp packet Shows Hello packets sent/received between adjacent
routers
Debug ip eigrp notification Shows EIGRP changes and updates as they occur on
your network
4.13 Configure, verify, and troubleshoot EIGRP 263
The following router output is from the Corp router in the example:
Corp#sh ip route
10.0.0.0/24 is subnetted, 12 subnets
D 10.1.11.0 [90/2172416] via 10.1.5.2, 00:01:05, Serial0/2/0
D 10.1.10.0 [90/2195456] via 10.1.5.2, 00:01:05, Serial0/2/0
D 10.1.9.0 [90/2195456] via 10.1.4.2, 00:01:05, Serial0/1/0
D 10.1.8.0 [90/2195456] via 10.1.4.2, 00:01:05, Serial0/1/0
D 10.1.12.0 [90/2172416] via 10.1.5.2, 00:01:05, Serial0/2/0
C 10.1.3.0 is directly connected, Serial0/0/1
C 10.1.2.0 is directly connected, Serial0/0/0
C 10.1.1.0 is directly connected, FastEthernet0/1
D 10.1.7.0 [90/2195456] via 10.1.2.2, 00:01:06, Serial0/0/0
D 10.1.6.0 [90/2195456] via 10.1.2.2, 00:01:06, Serial0/0/0
C 10.1.5.0 is directly connected, Serial0/2/0
C 10.1.4.0 is directly connected, Serial0/1/0
You can see that all routes are there in the routing table (10.1.3.0 shows that it’s directly
connected again), and we have only one link to networks 10.1.6.0 and 10.1.7.0! Notice that
EIGRP routes are indicated with simply a D designation (DUAL) and that the default AD of
these routes is 90. This represents internal EIGRP routes. Let’s take a look at the R1 router
table now that we’ve changed the metrics:
R1#sh ip route
10.0.0.0/24 is subnetted, 12 subnets
D 10.1.11.0 [90/2684416] via 10.1.2.1, 00:00:09, Serial0/0/0
D 10.1.10.0 [90/2707456] via 10.1.2.1, 00:00:09, Serial0/0/0
D 10.1.9.0 [90/2707456] via 10.1.2.1, 00:00:09, Serial0/0/0
D 10.1.8.0 [90/2707456] via 10.1.2.1, 00:00:09, Serial0/0/0
D 10.1.12.0 [90/2684416] via 10.1.2.1, 00:00:09, Serial0/0/0
C 10.1.3.0 is directly connected, Serial0/0/1
C 10.1.2.0 is directly connected, Serial0/0/0
D 10.1.1.0 [90/2172416] via 10.1.2.1, 00:00:09, Serial0/0/0
C 10.1.7.0 is directly connected, FastEthernet0/1
C 10.1.6.0 is directly connected, FastEthernet0/0
D 10.1.5.0 [90/2681856] via 10.1.2.1, 00:00:09, Serial0/0/0
D 10.1.4.0 [90/2681856] via 10.1.2.1, 00:00:09, Serial0/0/0
Now we have only one route to each remote network, and the 10.1.3.0 network is our
backup link. Obviously, it would be better if we could use both links at the same time, but in
my example, I made the 10.1.3.0 network a backup link.
264 Chapter 4 Configure, verify, and troubleshoot basic router operation
Let’s go back to the Corp router and see what it shows us in the neighbor table:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 10
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 10.1.3.2 Se0/0/1 14 00:35:10 1 200 0 81
3 10.1.5.2 Se0/2/0 10 02:51:22 1 200 0 31
2 10.1.4.2 Se0/1/0 13 03:17:20 1 200 0 20
0 10.1.2.2 Se0/0/0 10 03:19:37 1 200 0 80
We read the information in this output like this:
The H field indicates the order in which the neighbor was discovered.
The hold time is how long this router will wait for a Hello packet to arrive from a specific
neighbor.
The uptime indicates how long the neighborship has been established.
The SRTT field is the smooth round-trip timer—an indication of the time it takes for a
round-trip from this router to its neighbor and back. This value is used to determine how
long to wait after a multicast for a reply from this neighbor. If a reply isn’t received in time,
the router will switch to using unicasts in an attempt to complete the communication.
Now let’s see what’s in the Corp topology table by using the show ip eigrp topology
command—this should be interesting!
Corp#sh ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(10.1.5.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.11.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2172416/28160), Serial0/2/0
P 10.1.10.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2195456/281600), Serial0/2/0
P 10.1.9.0/24, 1 successors, FD is 2195456
via 10.1.4.2 (2195456/281600), Serial0/1/0
P 10.1.8.0/24, 1 successors, FD is 2195456
via 10.1.4.2 (2195456/72960), Serial0/1/0
P 10.1.12.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2172416/28160), Serial0/2/0
P 10.1.3.0/24, 1 successors, FD is 76839936
via Connected, Serial0/0/1
via 10.1.2.2 (9849856/7719936), Serial0/0/0
4.13 Configure, verify, and troubleshoot EIGRP 265
P 10.1.2.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0/0
via 10.1.2.2 (2681856/551936), Serial0/0/0
P 10.1.1.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/1
P 10.1.7.0/24, 1 successors, FD is 793600
via 10.1.2.2 (2195456/281600), Serial0/0/0
via 10.1.3.2 (77081600/281600), Serial0/0/1
P 10.1.6.0/24, 1 successors, FD is 793600
via 10.1.2.2 (2195456/281600), Serial0/0/0
via 10.1.3.2 (77081600/281600), Serial0/0/1
P 10.1.5.0/24, 1 successors, FD is 2169856
via Connected, Serial0/2/0
P 10.1.4.0/24, 1 successors, FD is 2169856
via Connected, Serial0/1/0
Notice that every route is preceded by a P. This means that the route is in the passive state,
which is a good thing because routes in the active state (A) indicate that the router has lost its
path to this network and is searching for a replacement. Each entry also indicates the feasible
distance, or FD, to each remote network plus the next-hop neighbor through which packets
will travel to their destination. Plus, each entry also has two numbers in parentheses. The first
indicates the feasible distance, and the second the advertised distance to a remote network.
Now here’s where things get interesting—notice that under the 10.1.7.0 and 10.1.6.0 outputs
there are two links to each network and that the feasible distance and advertised distance
are different. What this means is that we have one successor to the networks and one feasible
successor—a backup route! So very cool! You need to remember that even though both routes
to network 10.1.6.0 and 10.1.7.0 are in the topology table, only the successor route (the one
with the lowest metrics) will be copied and placed into the routing table.
In order for the route to be a feasible successor, its advertised distance must
be less than the feasible distance of the successor route.
EIGRP will load-balance across both links automatically when they are of equal variance (equal
cost), but EIGRP can also load-balance across unequal-cost links as well if we use the variance
command. The variance metric is set to 1 by default, meaning that only equal-cost links will loadbalance.
You can change the metric anywhere up to 128. Changing a variance value enables
EIGRP to install multiple, loop-free routes with unequal cost in a local routing table.
So basically, if the variance is set to 1, only routes with the same metric as the successor will
be installed in the local routing table. And, for example, if the variance is set to 2, any EIGRPlearned
route with a metric less than two times the successor metric will be installed in the local
routing table (if it is already a feasible successor).
266 Chapter 4 Configure, verify, and troubleshoot basic router operation
Now’s a great time for us to check out some debugging outputs. First, let’s use the debug
eigrp packet command that will show our Hello packets being sent between neighbor routers:
Corp#debug eigrp packet
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB,
SIAQUERY, SIAREPLY)
Corp#
*Mar 21 23:17:35.050: EIGRP: Sending HELLO on FastEthernet0/1
*Mar 21 23:17:35.050: AS 10, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Mar 21 23:17:35.270: EIGRP: Received HELLO on Serial0/1/0 nbr 10.1.4.2
*Mar 21 23:17:35.270: AS 10, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ
un/rely 0/0 peerQ un/rely 0/0
*Mar 21 23:17:35.294: EIGRP: Received HELLO on Serial0/0/0 nbr 10.1.2.2
*Mar 21 23:17:35.294: AS 10, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ
un/rely 0/0 peerQ un/rely 0/0
*Mar 21 23:17:38.014: EIGRP: Received HELLO on Serial0/2/0 nbr 10.1.5.2
*Mar 21 23:17:38.014: AS 10, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ
un/rely 0/0 peerQ un/rely 0/0
Exam Objectives
Know EIGRP features. EIGRP is a classless, advanced distance-vector protocol that supports
IP, IPX, AppleTalk, and now IPv6. EIGRP uses a unique algorithm, called DUAL, to
maintain route information and uses RTP to communicate with other EIGRP routers reliably.
Know how to configure EIGRP. Be able to configure basic EIGRP. This is configured the
same as IGRP with classful addresses.
Know how to verify EIGRP operation. Know all of the EIGRP show commands and be
familiar with their output and the interpretation of the main components of their output.
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