Figure 14-3: Sample EIGRP Network
- All routers in an EIGRP domain must be configured in the same AS number.
- EIGRP configuration on RT1:
- EIGRP configuration on RT2:
- EIGRP configuration on RT3:
RT1(config)#router eigrp ?
<1-65535> Autonomous system number
RT1(config)#router eigrp 10
RT1(config-router)#network 192.168.1.0 RT1(config-router)#network 192.168.2.0 RT1(config-router)#^Z
RT1#
RT2(config)#router eigrp 10
RT2(config-router)#network 192.168.2.0 RT2(config-router)#network 192.168.3.0 RT2(config-router)#network 192.168.4.0 RT2(config-router)#^Z
RT2#
RT3(config)#router eigrp 10
RT3(config-router)#network 192.168.4.0 RT3(config-router)#network 192.168.5.0 RT3(config-router)#^Z
RT3#
Fa1/0 192.168.1.1
192.168.1.2 192.168.3.2 192.168.3.3 192.168.5.2
RT1 RT2 RT3
PC1 PC2 PC3 PC4
Fa1/0 192.168.3.1
Fa1/0 192.168.5.1 S0/0
192.168.2.1
192.168.2.2 S0/0
192.168.4.2 S0/1 S0/0
192.168.4.1
- Verify the EIGRP configuration on RT1:
- Verify the EIGRP configuration on RT2:
- Verify the EIGRP configuration on RT3:
- The show ip eigrp interfaces EXEC command displays the information about EIGRP-enabled interfaces.
RT1#sh ip route
Gateway of last resort is not set
C 192.168.1.0/24 is directly connected, FastEthernet1/0 C 192.168.2.0/24 is directly connected, Serial0/0
D 192.168.3.0/24 [90/2172416] via 192.168.2.2, 00:04:41, Serial0/0 D 192.168.4.0/24 [90/2681856] via 192.168.2.2, 00:04:41, Serial0/0 D 192.168.5.0/24 [90/2684416] via 192.168.2.2, 00:04:41, Serial0/0 RT1#
RT2#sh ip route
Gateway of last resort is not set
D 192.168.1.0/24 [90/2172416] via 192.168.2.1, 00:05:25, Serial0/0 C 192.168.2.0/24 is directly connected, Serial0/0
C 192.168.3.0/24 is directly connected, FastEthernet1/0 C 192.168.4.0/24 is directly connected, Serial0/1
D 192.168.5.0/24 [90/2172416] via 192.168.4.2, 00:05:25, Serial0/1 RT2#
RT3#sh ip route
Gateway of last resort is not set
D 192.168.1.0/24 [90/2684416] via 192.168.4.1, 00:05:57, Serial0/0 D 192.168.2.0/24 [90/2681856] via 192.168.4.1, 00:05:58, Serial0/0 D 192.168.3.0/24 [90/2172416] via 192.168.4.1, 00:05:58, Serial0/0 C 192.168.4.0/24 is directly connected, Serial0/0
C 192.168.5.0/24 is directly connected, FastEthernet1/0 RT3#
RT2#sh ip eigrp interfaces
IP-EIGRP interfaces for process 10
Xmit Queue Mean Pacing Time Multicast Pending Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes Se0/0 1 0/0 207 5/190 250 0 Se0/1 1 0/0 217 5/190 250 0 Fa1/0 1 0/0 1 0/10 50 0 RT2#
- The show ip eigrp neighbors EXEC command displays the information about EIGRP adjacencies and discovered neighbors.
Below explains the fields in the output of the command:
H (handle) Indicates the order by time a particular neighbor was discovered.
Hold-time How long this router will wait for a Hello packet from a particular neighbor before declaring the neighbor is down.
Uptime Indicates how long the neighbor relationship has been established.
SRTT (in ms)
Smooth Round-Trip Time. Indicates how long it takes for an EIGRP packet to be sent from this router to the neighbor, and receiving an acknowledgment for the multicast packet sent to the neighbor. If an ACK packet isn’t received within this interval, EIGRP will switch to unicasts to attempt to complete the communication.
The interval between the multicast-to-unicast attempt is specified by the RTO.
RTO (in ms)
Retransmission Time Out. Indicates how long EIGRP should wait for an ACK packet before retransmitting a packet from the retransmission queue to a neighbor.
Queue Count
Indicates the numbers of outstanding Update, Query, or Reply packets waiting for retransmission. Consistently large values often indicate a problem.
Seq Num Indicates the sequence number of the last update from that neighbor. Used to maintain synchronization, avoid duplicate and out-of-sequence messages.
- The show ip eigrp topology EXEC command displays all EIGRP successor and feasible successor routes in the EIGRP topology table. Routes preceded by a P are passive state routes.
Note: Successor routes are stored in both routing and topology tables, while feasible successor routes are stored only in the topology table.
Passive The router has path to this network and is not performing route computation for it.
Active The router has lost the path to this network and is performing route computation to search for a replacement route. An active route exists when the router has no feasible successor to a destination network. A route never needs to enter into active state when the router always has feasible successors for the particular destination network.
RT2#sh ip eigrp neighbors
P 192.168.1.0/24, 1 successors, FD is 2172416
via 192.168.2.1 (2172416/28160), Serial0/0 P 192.168.2.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
P 192.168.3.0/24, 1 successors, FD is 28160 via Connected, FastEthernet1/0
P 192.168.4.0/24, 1 successors, FD is 2169856 via Connected, Serial0/1
P 192.168.5.0/24, 1 successors, FD is 2172416
via 192.168.4.2 (2172416/28160), Serial0/1
- When a link to a neighbor fails, all successor and feasible successor routes through that neighbor enter into active state and the router is required to perform route computation.
- The xxx and yyy in the via A.B.C.D (xxx/yyy), interface entry in the show ip eigrp topology
EXEC command represent feasible distance and advertised distance respectively.
- EIGRP uses the following 5 types of packets:
Hellos Used for neighbor discovery. Do not require acknowledgement.
Updates Used to tell a newly discovered neighbor the reachability of destination networks.
Requires acknowledgment.
Queries Used to ask for a replacement route when there is no feasible successor to a network.
Requires acknowledgment.
Replies Used to response to query packets to tell the originator not to recompute the route as there is feasible successor. Requires acknowledgment.
ACKs Used to acknowledge updates, queries, and replies.
- The EIGRP passive-interface {intf-type intf-num} router subcommand prevents an interface from sending and receiving Hello packets, which eventually stop it from forming adjacencies and exchanging routing updates with other routers which reside on the passive interface subnet.
- RIP passive interfaces prevent the sending but allow the receiving of routing updates. Hence the router can still learn about the networks advertised by other routers.
EIGRP passive interfaces neither send nor receive routing updates. Additionally no neighbor relationship will be formed, as EIGRP passive interfaces suppress the exchange of Hello packets.
- EIGRP performs autosummarization in discontiguous networks by default, and some networks might not work due to this feature. The best practice is disable autosummarization and performs manual summarization with ip summary-address interface subcommand when necessary.
EIGRP autosummarization can be disabled with the no auto-summary router subcommand.
Note: classful network addresses are used in EIGRP (and IGRP) configuration.
- The maximum-paths {num} router subcommand defines the total number of links allowed for equal-cost path load balancing. Its value is ranging from 1 to 6, with the default of 4. The variance router subcommand can be used to achieve unequal cost path load balancing.
- EIGRP (and IGRP) has a default maximum hop count of 100, but it can be changed to 255. Normally it is unnecessary to change the value, as hop count isn’t used in the path metric calculation. It is only being used to limit the scope of an autonomous system.
Router(config)#router eigrp 10
Router(config-router)#metric maximum-hops ? <1-255> Hop count
Router(config)#router eigrp 10
Router(config-router)#network 10.0.0.0 Router(config-router)#network 172.16.0.0 Router(config-router)#no auto-summary