EIGRP Neighbor Down: AS Mismatch
A healthy link, an empty EIGRP neighbor table. Diagnose the mismatched AS number and fix it yourself, live, right below.
Symptom
Healthy link, no neighbor
Your mission
Prove the cause, then fix it
Format
Guided CLI investigation
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify adjacency and reachability.
Console access: R1 · R2
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
The diagnosis is free to reveal at any point. Try the investigation first, then use this as your escape hatch, not a dead end.
Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
Two routers on the same subnet can ping each other directly, but their EIGRP adjacency never forms. Neither shows up in the other's neighbor table, and nothing about the interface or IP configuration looks wrong.
What you'll practice
- Bring up a point-to-point EIGRP adjacency between two routers
- Read show ip eigrp neighbors and verify neighbor state
- Understand the EIGRP Autonomous System requirement
- Correct a mismatched AS number on an EIGRP router instance
- Verify adjacency and EIGRP-learned loopback reachability
1. Bring up a point-to-point EIGRP adjacency between two routers
EIGRP adjacency requires both routers to run the same routing process (AS number). Two routers on the same subnet with EIGRP configured won't exchange hellos unless their AS numbers match exactly.
2. Read show ip eigrp neighbors and verify neighbor state
Unlike OSPF's complex state machine (Init, 2-Way, ExStart, Exchange, Loading, Full), EIGRP neighbors are either listed as FULL or not listed at all. An empty neighbor table on an up/up link points to a configuration problem, not a transient state.
3. Understand the EIGRP Autonomous System requirement
Each EIGRP router process is tied to a single Autonomous System number (1-65535). Two routers in different AS numbers will not exchange EIGRP hellos, even if they're directly connected and configured for EIGRP on the same interface.
4. Correct a mismatched AS number on an EIGRP router instance
Changing a router's AS number requires removing the old router eigrp X instance and creating a new one with the correct AS number. Unlike OSPF area changes, the entire EIGRP process must be re-created.
5. Verify adjacency and EIGRP-learned loopback reachability
Once AS numbers match, check show ip eigrp neighbors to confirm FULL adjacency, then verify the remote loopback is learned via EIGRP (show ip route eigrp) and reachable via ping.
The topology
R1 and R2 connect over a single point-to-point /30 link and each advertise a loopback into EIGRP, the minimum topology needed to isolate an adjacency problem to the routing protocol itself.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show ip interface briefping 10.0.12.2show ip eigrp neighborsshow run | include router eigrpshow logging | include EIGRPshow ip route eigrpFrequently asked
Is this real Cisco IOS or a simulator?
Real Cisco IOS on virtualized router platforms, full CLI behavior including console log messages on protocol rejections, not a simulated subset like Packet Tracer.
Does this cover the CCNA 200-301 EIGRP topics?
Yes. Forming a basic EIGRP adjacency and troubleshooting why one won't come up are explicit CCNA 200-301 routing blueprint items, and this lab is scoped directly to that.
How is this different from the OSPF lab?
Both cover neighbor adjacency problems, but the root cause is different: OSPF requires matching areas, EIGRP requires matching AS numbers. Seeing both patterns teaches you to debug routing protocol state independently.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.