VMware ESXi and vSphere Cluster Management
EIGRP Configuration on Cisco Routers
Learn how to configure basic IPv4 EIGRP on Cisco routers, form neighbors, advertise networks, use wildcard masks, and verify learned routes.
EIGRP, or Enhanced Interior Gateway Routing Protocol, is a Cisco dynamic routing protocol. It allows routers to exchange reachability information so that each router can learn paths to networks that are not directly connected.
Before EIGRP routers exchange routes, they must form an adjacency. An EIGRP neighbor is a directly connected router with which EIGRP has successfully established this relationship. EIGRP configuration is performed from router configuration mode.
Example topology and addressing plan
In this lesson, R1 and R2 share the 10.0.0.0/24 transit network. R2 also connects to a LAN in the 192.168.0.0/24 network. The goal is for R1 to learn the R2 LAN through EIGRP.
Starting the EIGRP routing process
Use the router eigrp command to start an EIGRP process:
configure terminal
router eigrp 1The number after router eigrp is the autonomous system number (ASN). In this lesson, the valid ASN range is 1 through 65535. The ASN identifies the EIGRP process and must match on directly connected routers that are expected to become neighbors.
For example, if R1 uses ASN 1 and R2 uses ASN 2, the routers will not form an EIGRP adjacency across their shared link. Both routers must use router eigrp 1 for this example.
Using EIGRP network statements
After entering router configuration mode, use a network statement to select interfaces for EIGRP:
network network-number [wildcard-mask]The command does not assign an address to an interface. Instead, IOS compares the command's address value with the IPv4 addresses configured on router interfaces.
When an interface matches a network statement, two important things happen:
- EIGRP sends and receives neighbor-discovery traffic on that interface.
- The connected subnet associated with that interface becomes available for advertisement to EIGRP neighbors.
Network statements without wildcard masks
If no wildcard mask is supplied, IOS uses the default classful interpretation of the network number. For example:
network 10.0.0.0Because 10.0.0.0 is a Class A network number, this command can match interfaces whose addresses fall within the broad 10.0.0.0/8 major network. It is not limited to the 10.0.0.0/24 transit subnet.
Similarly, network 192.168.0.0 uses the classful /24 interpretation for this Class C-range network and matches 192.168.0.0/24.
Using an explicit wildcard mask
An explicit wildcard mask gives more control over which interfaces are selected:
network 10.0.0.0 0.0.0.255This statement matches addresses in 10.0.0.0/24, so it selects the intended transit interface without broadly selecting other 10.x.x.x interfaces. A wildcard mask is particularly useful when a router has multiple interfaces and EIGRP should operate only on specific subnets.
Configuring the two-router example
R1 configuration
R1 needs EIGRP enabled on its 10.0.0.1 transit interface:
configure terminal
router eigrp 1
network 10.0.0.0 0.0.0.255R2 configuration
R2 needs EIGRP enabled on the transit interface and on the interface connected to its LAN:
configure terminal
router eigrp 1
network 10.0.0.0 0.0.0.255
network 192.168.0.0 0.0.0.255The transit statement enables EIGRP neighbor discovery between R1 and R2. The LAN statement causes R2's connected 192.168.0.0/24 subnet to become available for advertisement. The expected result is that R1 learns a route to 192.168.0.0/24 through R2.
Understanding wildcard masks
A wildcard mask is the inverse of a subnet mask for address matching in Cisco IOS commands. A subnet mask identifies network bits and host bits for forwarding and addressing. A wildcard mask controls which address bits must match and which bits may vary when IOS selects interfaces.
In a wildcard mask:
- A zero bit requires the corresponding address bit to match.
- A one bit permits the corresponding address bit to vary.
For a /24, the first three octets must match and the final octet may vary. Therefore, 10.0.0.0 0.0.0.255 matches addresses from 10.0.0.0 through 10.0.0.255.
A broader mask can select more interfaces. A more specific mask can limit selection. For example, a wildcard mask of 0.0.0.0 requires every bit to match, so a statement such as network 10.0.0.1 0.0.0.0 targets one exact interface address. The /24 form is usually clearer when the intention is to select an entire transit subnet.
Verifying EIGRP neighbors
Use the following privileged EXEC command to inspect established adjacencies:
show ip eigrp neighborsOn R1, the output should contain one neighbor whose address is R2's transit-interface address, such as 10.0.0.2. The exact output columns can vary by IOS version, but the neighbor address and interface are the important concepts.
R1# show ip eigrp neighbors
Neighbor Address Interface
10.0.0.2 <R1-transit-interface>A missing neighbor entry means that an EIGRP adjacency has not been established, so routing information will not be exchanged through that peer.
Verifying learned EIGRP routes
Use this command on R1 to display routes installed through EIGRP:
show ip route eigrpR1 should show 192.168.0.0/24 as an EIGRP-learned route with R2's transit address, 10.0.0.2, as the next hop. A simplified example is:
R1# show ip route eigrp
D 192.168.0.0/24 [90/metric] via 10.0.0.2The letter D commonly identifies an EIGRP route in the IPv4 routing table. The neighbor table verifies adjacency; the routing table verifies that a learned path has actually been installed for forwarding.
Successors and feasible successors
A successor is the selected primary next-hop path for a destination. It is the best EIGRP path and is installed in the routing table for forwarding.
A feasible successor is a qualified backup path that satisfies EIGRP's feasibility condition. Because it is considered loop-free, EIGRP can use it quickly if the successor path becomes unavailable. A feasible successor is retained by EIGRP even though it is not normally the active forwarding path.
Troubleshooting EIGRP
No entry appears in show ip eigrp neighbors
- Compare the EIGRP process numbers. Both routers must use the same ASN.
- Confirm that the transit-facing interfaces are selected by
network 10.0.0.0 0.0.0.255, or by an equivalent statement. - Check that both transit interfaces have compatible IPv4 addresses in 10.0.0.0/24.
- Confirm that the interfaces are operational and that each router can reach the other's directly connected transit address.
The neighbor exists, but R1 has no route to 192.168.0.0/24
- On R2, confirm that the LAN interface address is in 192.168.0.0/24.
- Confirm that R2 includes
network 192.168.0.0 0.0.0.255. - Confirm that the R2 LAN interface and its connected network are available.
- Run
show ip route eigrpagain on R1 and check whether another routing source has influenced route selection.
An unexpected interface participates in EIGRP
A classful network statement or broad wildcard mask may match more interfaces than intended. Compare all interface addresses with the configured statements, then replace a broad statement with a more specific network value and wildcard mask.
Exam-relevant distinction
show ip eigrp neighbors answers, “Which EIGRP routers have formed adjacencies?” show ip route eigrp answers, “Which routes has this router installed from EIGRP?” A neighbor can exist without a particular remote LAN being advertised.
Command summary
configure terminal
router eigrp 1
network 10.0.0.0 0.0.0.255
network 192.168.0.0 0.0.0.255
show ip eigrp neighbors
show ip route eigrpFor related material, see EIGRP configuration.