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DebugPrivileged EXEC

debug ip eigrp

This command enables real-time debugging of EIGRP packets and events on a Cisco router, used to troubleshoot EIGRP neighbor relationships, route exchanges, and metric calculations.

Syntax·Privileged EXEC
debug ip eigrp

When to Use This Command

  • Troubleshooting why an EIGRP neighbor is flapping or not forming adjacency
  • Investigating missing routes in the routing table that should be learned via EIGRP
  • Verifying EIGRP update, query, and reply packet exchanges during convergence
  • Diagnosing EIGRP metric or K-value mismatches between neighbors

Command Examples

Basic EIGRP debug for neighbor 10.1.1.2

debug ip eigrp 10.1.1.2
EIGRP: Received HELLO on FastEthernet0/0 nbr 10.1.1.2
EIGRP: New peer 10.1.1.2
EIGRP: Sending HELLO to 224.0.0.10 on FastEthernet0/0
EIGRP: Received UPDATE on FastEthernet0/0 nbr 10.1.1.2
EIGRP: Update contains 3 routes
EIGRP: Installing route 192.168.2.0/24 via 10.1.1.2

Line 1: Received a HELLO packet from neighbor 10.1.1.2 on FastEthernet0/0. Line 2: A new EIGRP neighbor adjacency is formed. Line 3: Router sends its own HELLO to the multicast address 224.0.0.10. Line 4: Received an UPDATE packet from the neighbor. Line 5: The update contains 3 routes. Line 6: A specific route is installed in the routing table via that neighbor.

Debug EIGRP packets with access list filtering

debug ip eigrp packets
EIGRP: Sending HELLO on FastEthernet0/0
EIGRP: Received HELLO on FastEthernet0/0 nbr 10.1.1.2
EIGRP: Sending UPDATE on FastEthernet0/0 nbr 10.1.1.2
EIGRP: Received ACK on FastEthernet0/0 nbr 10.1.1.2
EIGRP: Sending QUERY on FastEthernet0/0 nbr 10.1.1.2
EIGRP: Received REPLY on FastEthernet0/0 nbr 10.1.1.2

This shows the sequence of EIGRP packet types: HELLO for neighbor discovery, UPDATE for route exchange, ACK for reliable delivery, QUERY for route query, and REPLY for response. Each line indicates the direction (sending/receiving), interface, and neighbor IP.

Understanding the Output

The debug ip eigrp command outputs real-time messages about EIGRP events. Each line starts with 'EIGRP:' followed by the action (Received/Sending), packet type (HELLO, UPDATE, ACK, QUERY, REPLY), interface, and neighbor IP. Key fields: 'nbr' indicates the neighbor router ID; 'Update contains X routes' shows how many routes are in an update; 'Installing route' means the route is added to the routing table. Good signs: consistent HELLO exchanges and successful route installations. Bad signs: 'Retransmission limit exceeded' or 'Neighbor not on common subnet' indicate problems. Watch for 'K-value mismatch' or 'AS mismatch' which prevent adjacency. High retransmission counts suggest unreliable links or MTU issues.

CCNA Exam Tips

1.

CCNA exam tip: Remember that 'debug ip eigrp' is a privileged EXEC command and can generate heavy CPU load; always use with caution and disable with 'undebug all'.

2.

CCNA exam tip: The exam may ask you to identify why EIGRP neighbors are not forming; look for K-value mismatch, AS number mismatch, or passive interface configuration.

3.

CCNA exam tip: Be able to interpret debug output to determine if a route is being learned via EIGRP and whether it is feasible successor or successor.

4.

CCNA exam tip: Know that EIGRP uses multicast 224.0.0.10 for HELLO and UPDATE packets; debug will show this address.

Common Mistakes

Mistake 1: Leaving debug enabled on a production router, causing high CPU usage and potential crashes. Always disable with 'undebug all' or 'no debug ip eigrp'.

Mistake 2: Forgetting to specify an access list to filter debug output, resulting in overwhelming amount of messages. Use 'debug ip eigrp [neighbor-ip]' to limit.

Mistake 3: Misinterpreting 'Sending QUERY' as a problem; QUERY packets are normal during convergence when a route is lost.

Related Commands

show ip eigrp neighbors

Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.

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.

show ip route

Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.

undebug all

Disables all active debug operations on the router, used to stop debugging output and reduce CPU load.

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