Courseiva
EIGRPGlobal Config

router eigrp [as-number]

Enters EIGRP router configuration mode for a specific autonomous system number, allowing you to configure EIGRP routing protocol parameters.

Definition: router eigrp [as-number] is a Cisco IOS global config command. Enters EIGRP router configuration mode for a specific autonomous system number, allowing you to configure EIGRP routing protocol parameters.

Overview

The `router eigrp [as-number]` command is the gateway to configuring Enhanced Interior Gateway Routing Protocol (EIGRP) on Cisco IOS devices. EIGRP is a Cisco-proprietary advanced distance-vector routing protocol that combines the best features of link-state and distance-vector protocols. It uses Diffusing Update Algorithm (DUAL) to guarantee loop-free paths and fast convergence. This command enters EIGRP router configuration mode for a specific autonomous system (AS) number, which is a 16-bit integer (1-65535) that identifies the EIGRP routing domain. All routers within the same AS must share the same AS number to exchange routes. The AS number serves as a process ID; multiple EIGRP processes can run on a single router, each with a unique AS number, though this is rare in practice.

Why does this matter? EIGRP is widely used in enterprise networks due to its ease of configuration, support for unequal-cost load balancing (variance), and efficient use of bandwidth with partial updates and reliable transport. The `router eigrp` command is the first step in enabling EIGRP; without it, no EIGRP configuration can be applied. After entering this mode, you typically use `network` statements to advertise interfaces, configure timers, set metrics, or enable authentication.

When would you reach for this command vs alternatives? If you need a simple, scalable IGP for a medium-to-large enterprise with Cisco gear, EIGRP is a strong choice. Compared to OSPF, EIGRP is easier to configure (no areas) and converges faster in many topologies. However, OSPF is an open standard, making it preferable in multi-vendor environments. For small networks, static routing or RIP might suffice, but EIGRP offers better convergence and scalability. In a Cisco-only shop, EIGRP is often the go-to IGP.

This command fits into the broader workflow: after initial interface IP configuration, you enable a routing protocol. For EIGRP, you first create the EIGRP process with `router eigrp [as]`, then advertise directly connected networks with `network` commands under router configuration mode. You may also configure passive interfaces, distribute lists, or adjust metrics. Verification uses `show ip eigrp neighbors`, `show ip route eigrp`, and `show ip eigrp topology`.

Important IOS behaviour: The command requires global configuration mode (privilege level 15). Entering it creates an EIGRP process in the running configuration; removing it with `no router eigrp [as]` deletes all EIGRP configuration for that AS. The AS number must match on all routers in the same EIGRP domain. EIGRP uses multicast address 224.0.0.10 for hello packets. The command does not produce immediate output; it changes the prompt to `Router(config-router)#`. There is no buffered output; configuration changes take effect immediately. The command is available in all IOS versions (12.x, 15.x, 16.x) and IOS-XE. On NX-OS, the equivalent is `router eigrp [as]` but with different subcommands. On ASA, EIGRP is not supported. Understanding this command is fundamental for CCNA and CCNP candidates as it underpins EIGRP configuration and troubleshooting.

Syntax·Global Config
router eigrp [as-number]

When to Use This Command

  • Enable EIGRP on a router and advertise directly connected networks to neighbors.
  • Configure EIGRP router ID, timers, or stub routing for a specific AS.
  • Set EIGRP metric weights or redistribute routes from other routing protocols.
  • Modify EIGRP hello and hold timers to tune convergence.

Parameters

ParameterSyntaxDescription
as-number<1-65535>The autonomous system number that identifies the EIGRP routing process. This must be the same on all routers in the EIGRP domain. Valid values are from 1 to 65535. A common mistake is using a number that conflicts with another EIGRP process on the same router or using a number that does not match neighboring routers, which prevents neighbor adjacency.

Command Examples

Basic EIGRP Configuration with Network Statements

router eigrp 100 network 192.168.1.0 network 10.0.0.0
Router(config)# router eigrp 100
Router(config-router)# network 192.168.1.0
Router(config-router)# network 10.0.0.0
Router(config-router)#

The command 'router eigrp 100' enters EIGRP configuration mode for AS 100. The 'network' commands advertise the specified networks to EIGRP neighbors. No output is shown unless there is an error.

EIGRP with Router ID and Stub Configuration

router eigrp 200 eigrp router-id 1.1.1.1 eigrp stub connected summary
Router(config)# router eigrp 200
Router(config-router)# eigrp router-id 1.1.1.1
Router(config-router)# eigrp stub connected summary
%DUAL-5-NBRCHANGE: EIGRP-IPv4 200: Neighbor 10.0.0.2 (GigabitEthernet0/0) is down: stub configuration change
Router(config-router)#

The 'eigrp router-id' sets the router ID manually. The 'eigrp stub' command configures the router as a stub, advertising only connected and summary routes. The log message indicates a neighbor adjacency reset due to the stub configuration change.

Understanding the Output

The 'router eigrp [as-number]' command itself does not produce output; it changes the CLI prompt to 'config-router'. Subsequent configuration commands may generate log messages (e.g., neighbor changes) or error messages if syntax is incorrect. In a real network, after configuring EIGRP, you would use 'show ip eigrp neighbors' to verify neighbor adjacencies and 'show ip eigrp topology' to view the topology table.

Key fields in neighbor output include 'Hold' (time before neighbor is declared dead), 'Uptime' (how long adjacency has been up), and 'Q Count' (queue count, should be 0). A high Q count or frequent neighbor flapping indicates issues. In topology output, 'P' (passive) means route is stable, 'A' (active) means DUAL is recalculating—active routes should be transient.

Good values: neighbors in 'Init' or 'Up' state, routes passive. Bad values: neighbors stuck in 'Init' or 'Down', routes stuck in active (SIA).

Configuration Scenarios

Basic EIGRP Configuration between Two Routers

You have two Cisco routers (R1 and R2) connected via a serial link. You need to enable EIGRP AS 100 on both routers to exchange routes for the directly connected networks.

Topology

R1(Gi0/0)---10.0.12.0/30---(Gi0/0)R2 R1 Loopback0: 192.168.1.1/24 R2 Loopback0: 192.168.2.1/24

Steps

  1. 1.Step 1: Enter privileged EXEC mode: enable
  2. 2.Step 2: Enter global configuration mode: configure terminal
  3. 3.Step 3: On R1, enter EIGRP router configuration mode for AS 100: router eigrp 100
  4. 4.Step 4: Advertise the directly connected networks: network 10.0.12.0 0.0.0.3 and network 192.168.1.0 0.0.0.255
  5. 5.Step 5: Exit router configuration mode: exit
  6. 6.Step 6: Repeat steps 1-5 on R2 with its networks: network 10.0.12.0 0.0.0.3 and network 192.168.2.0 0.0.0.255
Configuration
! R1 configuration
interface GigabitEthernet0/0
 ip address 10.0.12.1 255.255.255.252
!
interface Loopback0
 ip address 192.168.1.1 255.255.255.0
!
router eigrp 100
 network 10.0.12.0 0.0.0.3
 network 192.168.1.0 0.0.0.255
!
! R2 configuration
interface GigabitEthernet0/0
 ip address 10.0.12.2 255.255.255.252
!
interface Loopback0
 ip address 192.168.2.1 255.255.255.0
!
router eigrp 100
 network 10.0.12.0 0.0.0.3
 network 192.168.2.0 0.0.0.255

Verify: On R1: show ip eigrp neighbors should list R2 with state 'Init' or 'Established'. show ip route eigrp should show 192.168.2.0/24 via 10.0.12.2.

Watch out: Forgetting to include the subnet mask in the network statement (using classful network) can cause the router to advertise the wrong prefix. Always use wildcard mask to match the exact subnet.

EIGRP with Passive Interface and Authentication

You have a router (R3) connected to a LAN segment (VLAN 10) where no other EIGRP routers exist. You want to prevent EIGRP hellos from being sent on that interface to reduce unnecessary traffic, and you need to enable MD5 authentication on the WAN link to R4 for security.

Topology

R3(Gi0/0)---10.0.34.0/30---(Gi0/0)R4 R3(Gi0/1)---192.168.10.0/24 (LAN) R4 Loopback0: 172.16.4.1/24

Steps

  1. 1.Step 1: Enter global config on R3: configure terminal
  2. 2.Step 2: Enter EIGRP router config: router eigrp 200
  3. 3.Step 3: Advertise networks: network 10.0.34.0 0.0.0.3 and network 192.168.10.0 0.0.0.255
  4. 4.Step 4: Set Gi0/1 as passive: passive-interface GigabitEthernet0/1
  5. 5.Step 5: Configure authentication key chain: key chain EIGRP_KEY, key 1, key-string cisco123
  6. 6.Step 6: Exit key chain config, then under router eigrp 200: authentication mode md5, authentication key-chain EIGRP_KEY
  7. 7.Step 7: Apply authentication to the WAN interface: interface GigabitEthernet0/0, ip authentication mode eigrp 200 md5, ip authentication key-chain eigrp 200 EIGRP_KEY
  8. 8.Step 8: Repeat similar steps on R4 with matching key.
Configuration
! R3 configuration
key chain EIGRP_KEY
 key 1
  key-string cisco123
!
interface GigabitEthernet0/0
 ip address 10.0.34.1 255.255.255.252
 ip authentication mode eigrp 200 md5
 ip authentication key-chain eigrp 200 EIGRP_KEY
!
interface GigabitEthernet0/1
 ip address 192.168.10.1 255.255.255.0
!
router eigrp 200
 network 10.0.34.0 0.0.0.3
 network 192.168.10.0 0.0.0.255
 passive-interface GigabitEthernet0/1
 authentication mode md5
 authentication key-chain EIGRP_KEY
!
! R4 configuration (partial)
key chain EIGRP_KEY
 key 1
  key-string cisco123
!
interface GigabitEthernet0/0
 ip address 10.0.34.2 255.255.255.252
 ip authentication mode eigrp 200 md5
 ip authentication key-chain eigrp 200 EIGRP_KEY
!
router eigrp 200
 network 10.0.34.0 0.0.0.3
 network 172.16.4.0 0.0.0.255

Verify: On R3: show ip eigrp neighbors should show R4. show ip eigrp interfaces should list Gi0/0 but not Gi0/1 (passive). show ip route eigrp should show 172.16.4.0/24. If authentication fails, neighbors will not form; use debug eigrp packets to see authentication errors.

Watch out: Authentication must be configured on the interface level (ip authentication mode/key-chain) and also under the router eigrp process (authentication mode/key-chain). Missing either will cause neighbor adjacency to fail. Also, the key chain name must match exactly.

Troubleshooting with This Command

When troubleshooting EIGRP, the `router eigrp` command itself does not provide output; it is the entry point for configuration. However, the configuration applied under it directly impacts neighbor relationships and route propagation. A common issue is that EIGRP neighbors fail to form. The first step is to verify that the EIGRP process is configured with the correct AS number on all routers. Use `show ip eigrp neighbors` to see if any neighbors are listed. If the output is empty, check the following:

1. **Interface IP and subnet**: Ensure interfaces are up/up and have IP addresses in the same subnet. EIGRP will only form adjacency on interfaces that are included in a `network` statement. Use `show ip eigrp interfaces` to see which interfaces are participating. If an interface is missing, the `network` statement may be incorrect (wrong wildcard mask or missing network).

2. **Passive interfaces**: If an interface is set as passive, it will not send or receive hellos, preventing neighbor formation. Check with `show ip eigrp interfaces`; passive interfaces are marked as 'Passive'. Use `no passive-interface` to enable hellos if needed.

3. **Authentication**: If MD5 authentication is configured, mismatched keys or missing configuration will cause neighbors to stay in 'Init' state. Use `debug eigrp packets` to see authentication failures. The output will show 'MD5 mismatch' or 'authentication failed'. Verify key chain names and key strings match exactly.

4. **ACLs or filters**: Access lists applied to interfaces can block EIGRP multicast (224.0.0.10). Check with `show access-lists` and ensure EIGRP traffic is permitted.

5. **K-value mismatches**: EIGRP uses composite metric based on bandwidth, delay, reliability, load, and MTU (K-values). If K-values differ between routers, neighbors will not form. Use `show ip eigrp topology` to see if routes are in active/passive state. By default, only bandwidth and delay are used (K1=1, K3=1, others 0).

6. **Stuck in Active (SIA) routes**: If a route is stuck in active state, it indicates a query/reply issue. Use `show ip eigrp topology active` to see which routes are active. This often points to a network partition or a router that is not replying. Check connectivity and ensure all routers have a path to the destination.

7. **Route summarization**: Automatic summarization at classful boundaries (enabled by default in older IOS) can cause suboptimal routing or missing routes. Use `no auto-summary` under router eigrp to disable it.

Correlate with `show ip route eigrp` to see which routes are installed. If a route is missing, check the EIGRP topology table with `show ip eigrp topology` to see if the route is known. If it is in the topology but not in the routing table, the metric may be higher than the current best route, or there may be a routing loop. Use `show ip eigrp traffic` to see packet counts; if hellos are not being sent/received, the interface may be down or passive.

A systematic approach: start with `show ip eigrp neighbors` to confirm adjacency. If none, check interfaces with `show ip eigrp interfaces`. Then verify network statements with `show running-config | section router eigrp`. Use `debug eigrp packets` to see hello exchange. For authentication issues, `debug eigrp packets` will show errors. For route propagation, use `debug eigrp fsm` to see DUAL events. Always ensure that the AS number matches on all routers. The `router eigrp` command is the foundation; misconfiguration here cascades into all EIGRP operations.

CCNA Exam Tips

1.

CCNA exam tip 1: EIGRP uses autonomous system numbers (1-65535); both routers must use the same AS number to form adjacency.

2.

CCNA exam tip 2: The 'network' command is classful by default; use 'network x.x.x.x 0.0.0.255' for wildcard mask to be precise.

3.

CCNA exam tip 3: EIGRP stub routing is a common exam topic—remember 'eigrp stub' limits queries and prevents transit traffic.

4.

CCNA exam tip 4: The 'eigrp router-id' command is used to set a stable router ID; if not configured, the highest loopback IP is used.

Common Mistakes

Mistake 1: Forgetting to use the same AS number on all routers in the EIGRP domain, preventing neighbor adjacency.

Mistake 2: Using a classful network statement without a wildcard mask, causing incorrect interface activation.

Mistake 3: Configuring 'eigrp stub' without specifying 'connected' or 'summary', which may not advertise all desired routes.

router eigrp [as-number] vs show ip eigrp topology

Both 'router eigrp [as-number]' and 'show ip eigrp topology' are essential for EIGRP operation, but they serve different purposes: one configures the protocol, the other verifies the learned topology. They are often confused because both are used in EIGRP troubleshooting workflows, but understanding their distinct roles is critical.

Aspectrouter eigrp [as-number]show ip eigrp topology
ScopeConfiguration of EIGRP routing processDisplay of EIGRP topology table
Configuration modeGlobal configuration modePrivileged EXEC mode
PersistencePersistent across rebootsTransient, shows current state
PrecedenceSets parameters before routes are learnedShows results after convergence
Typical useDefine network statements, passive interfacesCheck feasible successors, verify routes

Use router eigrp [as-number] when you need to configure EIGRP routing protocol parameters such as network statements or distribute lists.

Use show ip eigrp topology when you need to verify EIGRP convergence, view feasible successors, or troubleshoot routing issues.

Platform Notes

In IOS-XE (e.g., Catalyst 9000 switches, ISR 4000 series), the `router eigrp [as-number]` command syntax is identical to classic IOS. However, IOS-XE uses the same CLI as IOS 15.x/16.x, so no syntax changes. The output of `show ip eigrp neighbors` may include additional fields like 'Hold' timer in milliseconds.

On NX-OS (e.g., Nexus 9000), the equivalent command is `router eigrp [as-number]` but the configuration mode is different. Under NX-OS, you must also enable the EIGRP feature with `feature eigrp` before entering router configuration mode. The `network` command uses a prefix-length instead of wildcard mask (e.g., `network 10.0.12.0/30`).

NX-OS also supports virtual routing and forwarding (VRF) with EIGRP using `vrf` context. On ASA firewalls, EIGRP is not supported; ASAs use OSPF or RIP for dynamic routing. In IOS-XR (e.g., ASR 9000), EIGRP is not supported; IOS-XR uses OSPF or ISIS.

For older IOS versions (12.x), the `router eigrp` command behaves the same, but automatic summarization is enabled by default (use `no auto-summary` to disable). In 15.x and later, automatic summarization is disabled by default. Also, in 12.x, the `network` command without a wildcard mask assumes a classful network; in 15.x+, it is recommended to always specify the wildcard mask.

There are no significant differences in the `router eigrp` command itself across IOS versions; the changes are in subcommands and defaults. Always verify the IOS version with `show version` to know which defaults apply.

Related Commands

show ip eigrp neighbors

Displays all EIGRP neighbours with their addresses, interfaces, hold time, uptime, SRTT (smooth round-trip time), RTO, queue count, and sequence numbers.

show ip eigrp topology

Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.

Practice for the CCNA 200-301

Test your knowledge with practice questions covering all CCNA 200-301 exam domains.

Practice CCNA 200-301 Questions