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Use the router eigrp and network commands to create an EIGRP routing process. Note that
EIGRP requires an autonomous system (AS) number. The AS number does not have to be
registered as is the case when routing on the Internet with the Border Gateway Protocol (BGP)
routing protocol. However, all routers within an AS must use the same AS number to exchange
routing information with each other. Figure 5-3 shows the EIGRP configuration of a simple
network.
Figure 5-3 EIGRP Configuration
The network command defines a major network number to which the router is directly connected.
The EIGRP routing process looks for interfaces that have an IP address that belongs to the
A B C
172.16.1.1
172.16.1.0 192.168.1.0
10.1.1.1 10.1.1.2 10.2.2.2 10.2.2.3 192.168.1.1
E0 S2 S2 S3 S3 E0
Autonomous System = 100
router eigrp 100
network 10.0.0.0
router eigrp 100
network 192.168.1.0
network 10.0.0.0
router eigrp 100
network 172.16.0.0
network 10.0.0.0
Implementing EIGRP 175
networks that are specified with the network command and begins the EIGRP process on these
interfaces.
Table 5-1 applies to the EIGRP configurations on Router A in the EIGRP configuration example.
EIGRP sends updates out of the interfaces in networks 10.0.0.0 and 172.16.0.0. The updates include information about
networks 10.0.0.0 and 172.16.0.0 and any other networks that EIGRP learns.
EIGRP automatically summarizes routes at the classful boundary. In some cases, you might not
want automatic summarization to occur. For example, if you have discontiguous networks, you
need to disable automatic summarization to minimize router confusion. Figure 5-4 shows an
example of how this summarization can cause advertisements for the 172.16.0.0 network to be
sent from both Router A and Router B to Router C.
Figure 5-4 Autosummarization Causing Discontinuous Subnets
To disable automatic summarization, use the no auto-summary command in the EIGRP router
configuration mode. When this command is used, both Router A and Router B will advertise the
route specific to the subnet of a given interface, as shown in Figure 5-5.
Table 5-1 EIGRP Command Example
Command Description
router eigrp 100 Enables the EIGRP routing process for AS 100
network 172.16.0.0 Associates network 172.16.0.0 with the EIGRP routing process
network 10.0.0.0 Associates network 10.0.0.0 with the EIGRP routing process
C
EIGRP will advertise
network 172.16.0.0.
B
EIGRP will advertise
network 172.16.0.0.
172.16.5.0
255.255.255.0
192.168.14.16
255.255.255.240
172.16.6.0
255.255.255.0
A
176 Chapter 5: Implementing EIGRP
Figure 5-5 Disabling Autosummarization Corrects Problem
After you enable EIGRP, various commands can be used to display information about how the
protocol is operating. The show ip route eigrp command displays the current EIGRP entries in
the routing table.
The show ip protocols command displays the parameters and current state of the active routing
protocol process. This command shows the EIGRP AS number. It also displays filtering and
redistribution numbers and neighbor and distance information. This also shows the networks that
are currently being advertised on the router by the protocol.
Use the show ip eigrp interfaces [type number] [as-number] command to determine on which
interfaces EIGRP is active, and to learn information about EIGRP that relates to those interfaces.
If you specify an interface by using the type number option, only that interface is displayed.
Otherwise, all interfaces on which EIGRP is running are displayed. If you specify an AS using the
as-number option, only the routing process for the specified AS is displayed. Otherwise, all
EIGRP processes are displayed. Example 5-1 shows the output of the show ip eigrp interfaces
command.
Example 5-1 Determining Router Interface EIGRP Status/Information
RouterX# show ip eigrp interfaces
IP EIGRP interfaces for process 109
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Di0 0 0/0 0 11/434 0 0
Et0 1 0/0 337 0/10 0 0
SE0:1.16 1 0/0 10 1/63 103 0
Tu0 1 0/0 330 0/16 0 0
C
EIGRP will advertise
network 172.16.5.0.
B
EIGRP will advertise
network 172.16.6.0.
172.16.5.0
255.255.255.0
192.168.14.16
255.255.255.240
172.16.6.0
255.255.255.0
A
Implementing EIGRP 177
Table 5-2 describes the significant fields generated by the show ip eigrp interfaces output.
Use the show ip eigrp neighbors command to display the neighbors that were discovered by
EIGRP and to determine when neighbors become active and inactive, as demonstrated in Example
5-2. This command is also useful for debugging certain types of transport problems.
Table 5-2 show ip eigrp interfaces Output
Field Description
Interface Interface over which EIGRP is configured
Peers Number of directly connected EIGRP neighbors on the interface
Xmit Queue Un/Reliable Number of packets remaining in the Unreliable and Reliable queues
Mean SRTT Average smoothed round-trip time (SRTT) interval (in milliseconds) for all
neighbors on the interface
Pacing Time Un/Reliable Number of milliseconds to wait after transmitting unreliable and reliable
packets
Multicast Flow Timer Number of milliseconds to wait for acknowledgment of a multicast packet
by all neighbors before transmitting the next multicast packet
Pending Routes Number of routes in the packets in the transmit queue waiting to be sent
Example 5-2 Displaying Discovered Active/Inactive EIGRP Neighbors
RouterX# show ip eigrp neighbors
IP-EIGRP Neighbors for process 77
Address Interface Holdtime Uptime Q Seq SRTT RTO
(secs) (h:m:s) Count Num (ms) (ms)
172.16.81.28 Ethernet1 13 0:00:41 0 11 4 20
172.16.80.28 Ethernet0 14 0:02:01 0 10 12 24
172.16.80.31 Ethernet0 12 0:02:02 0 4 5 20
178 Chapter 5: Implementing EIGRP
Table 5-3 describes the significant fields for the show ip eigrp neighbors command.
The show ip eigrp topology command displays the EIGRP topology table, the active or passive
state of routes, the number of successors, and the feasible distance to the destination, as
demonstrated in Example 5-3.
Table 5-3 show ip eigrp neighbors Output
Field Description
process 77 AS number that is specified with the router command.
Address IP address of the EIGRP peer.
Interface Interface on which the router is receiving hello packets from the peer.
Holdtime Length of time (in seconds) that Cisco IOS Software waits to hear from the peer before
declaring it down. If the peer is using the default hold time, this number is less than 15. If
the peer configures a nondefault hold time, the nondefault hold time is displayed. The hold
time would be less than 180 on a sub-T1 multipoint interface.
Uptime Elapsed time (in hours:minutes:seconds) since the local router first heard from this
neighbor.
Q Count Number of EIGRP packets (update, query, and reply) that the software is waiting to send.
Seq Num Sequence number of the last update, query, or reply packet that was received from this
neighbor.
SRTT Smooth round-trip time (SRTT). The number of milliseconds that is required for an
EIGRP packet to be sent to this neighbor and for the local router to receive an
acknowledgment of that packet.
RTO Retransmission timeout (RTO) (in milliseconds). This is the amount of time the software
waits before resending a packet from the retransmission queue to a neighbor.
Example 5-3 Displaying EIGRP Topology Information
RouterX# show ip eigrp topology
IP-EIGRP Topology Table for process 77
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status
P 172.16.90.0 255.255.255.0, 2 successors, FD is 46251776
via 172.16.80.28 (46251776/46226176), Ethernet0
via 172.16.81.28 (46251776/46226176), Ethernet1
via 172.16.80.31 (46277376/46251776), Serial0
P 172.16.81.0 255.255.255.0, 2 successors, FD is 307200
via Connected, Ethernet1
via 172.16.81.28 (307200/281600), Ethernet1
via 172.16.80.28 (307200/281600), Ethernet0
via 172.16.80.31 (332800/307200), Serial0
Implementing EIGRP 179
Table 5-4 describes the significant fields for the show ip eigrp topology command output.
Table 5-4 show ip eigrp topology Output
Field Description
Codes The state of this topology table entry. Passive and Active refer to the EIGRP state
with respect to this destination; Update, Query, and Reply refer to the type of packet
that is being sent.
P - Passive Indicates that no EIGRP computations are being performed for this destination.
A - Active Indicates that EIGRP computations are being performed for this destination.
U - Update Indicates that an update packet was sent to this destination.
Q - Query Indicates that a query packet was sent to this destination.
R - Reply Indicates that a reply packet was sent to this destination.
r - Reply status A flag that is set after the software has sent a query and is waiting for a reply.
172.16.90.0 Destination IP network number.
255.255.255.0 Destination subnet mask.
successors Number of successors. This number corresponds to the number of next hops in the IP
routing table. If “successors” is capitalized, the route or next hop is in a transition
state.
FD Feasible distance. The feasible distance is the best metric to reach the destination or
the best metric that was known when the route went active. This value is used in the
feasibility condition check. If the reported distance of the router (the metric after the
slash) is less than the feasible distance, the feasibility condition is met and that path
is a feasible successor. After the software determines it has a feasible successor, it
does not need to send a query for that destination.
replies The number of replies that are still outstanding (have not been received) with respect
to this destination. This information appears only when the destination is in active
state.
state The exact EIGRP state that this destination is in. It can be the number 0, 1, 2, or 3.
This information appears only when the destination is in the active state.
via The IP address of the peer that told the software about this destination. The first n of
these entries, where n is the number of successors, are the current successors. The
remaining entries on the list are feasible successors.
(46251776/
46226176)
The first number is the EIGRP metric that represents the cost to the destination. The
second number is the EIGRP metric that this peer advertised.
Ethernet0 The interface from which this information was learned.
Serial0 The interface from which this information was learned.
180 Chapter 5: Implementing EIGRP
The show ip eigrp traffic command displays the number of packets sent and received, as
demonstrated in Example 5-4.
Table 5-5 describes the fields that might be shown in the display.
The debug ip eigrp privileged EXEC command helps you analyze the EIGRP packets that an
interface sends and receives, as demonstrated in Example 5-5. Because the debug ip eigrp
command generates a substantial amount of output, use it only when traffic on the network is light.
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