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Three-site chain 37 — EIGRP repair
Enterprise Intermediate 5 devices 6 checked tasks 74 points Plan lab
Scenario — Trichy reaches Lucknow only through the Ludhiana router, a chain of three sites. Everything was configured last week and the network worked. Today the end offices cannot reach each other: one router is running a different autonomous-system number. Find it, fix it and prove the end-to-end paths again. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Trichy reaches Lucknow only through the Ludhiana router, a chain of three sites. Everything was configured last week and the network worked. Today the end offices cannot reach each other: one router is running a different autonomous-system number. Find it, fix it and prove the end-to-end paths again.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 6 goals show as passed (74 points).
Network status
Devices: 3 routers, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. R-Trichy starts with addresses (G0/0 10.56.1.1/24, G0/1 172.20.36.1/30) and EIGRP 136 already configured. PC-Trichy is a host with 10.56.1.10/24, gateway 10.56.1.1. R-Ludhiana starts with addresses (G0/0 172.20.36.2/30, G0/1 172.20.36.5/30) and EIGRP 143 already configured. R-Lucknow starts with addresses (G0/0 10.56.2.1/24, G0/1 172.20.36.6/30) and EIGRP 136 already configured. PC-Lucknow is a host with 10.56.2.10/24, gateway 10.56.2.1. Known problems
R-Ludhiana has fewer EIGRP neighbours than it has cables to routers, so routes are missing beyond it. No cable or interface is faulty: the fault is one line of configuration on one router. Everything else is correct and must be left alone. Goals
R-Trichy has 1 EIGRP neighbour in AS 136 (8 pts) R-Ludhiana has 2 EIGRP neighbours in AS 136 (8 pts) R-Lucknow has 1 EIGRP neighbour in AS 136 (8 pts) R-Trichy learns 10.56.2.0/24 from the protocol (10 pts) 10.56.1.10 (Trichy) reaches 10.56.2.10 (Lucknow) (20 pts) 10.56.2.10 (Lucknow) reaches 10.56.1.10 (Trichy) (20 pts) The activity, in order
Confirm the symptom: ping the far PC from the near PC and note where it fails. Why: EIGRP shares routes only with neighbours; a mismatched AS number simply never forms one. Verify with: show ip eigrp neighbors Follow the path hop by hop. On each router read its neighbours and its routes, starting with the one nearest the failure. Why: EIGRP shares routes only with neighbours; a mismatched AS number simply never forms one. Verify with: show ip eigrp neighbors Compare the suspect router with its neighbour: the same protocol, the same numbers, every link covered. Why: EIGRP shares routes only with neighbours; a mismatched AS number simply never forms one. Verify with: show ip eigrp neighbors Fix the single wrong line and nothing else. Why: If the prefix is not in the routing table, the router drops packets for it, whatever else is configured. Verify with: show ip route Retest: neighbours, routes, then pings in both directions. Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.56.2.10 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Different AS numbers on the two ends — no neighbour and no error Common mistake: Leaving auto-summary on Common mistake: Forgetting that a passive interface forms no neighbours Why this matters
EIGRP is Cisco’s advanced distance-vector protocol: quick convergence and little configuration. Routers become neighbours only when the AS number matches, and automatic summarisation must be off for discontiguous networks.
What you will be able to do
Enable EIGRP with an AS number and wildcard networks
Disable auto-summary
Find an AS mismatch from a missing neighbour
The network
ISR 2911, PC — 5 devices in all, already cabled.
Fa0 Gi0/0 10.56.1.1/24 Fa0 Gi0/0 10.56.2.1/24 Gi0/1 172.20.36.1/30 Gi0/0 172.20.36.2/30 Gi0/1 172.20.36.5/30 Gi0/1 172.20.36.6/30 R-Trichy PC-Trichy 10.56.1.10/24 · gw 10.56.1.1 R-Ludhiana R-Lucknow PC-Lucknow 10.56.2.10/24 · gw 10.56.2.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R-Trichy ISR 2911 configure it PC-Trichy PC 10.56.1.10 / 255.255.255.0 gw 10.56.1.1 R-Ludhiana ISR 2911 configure it R-Lucknow ISR 2911 configure it PC-Lucknow PC 10.56.2.10 / 255.255.255.0 gw 10.56.2.1
Cabling
PC-Trichy Fa0 ↔ R-Trichy Gi0/0 PC-Lucknow Fa0 ↔ R-Lucknow Gi0/0 R-Trichy Gi0/1 ↔ R-Ludhiana Gi0/0 R-Ludhiana Gi0/1 ↔ R-Lucknow Gi0/1
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
R-Trichy interface g0/0
ip address 10.56.1.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 172.20.36.1 255.255.255.252
no shutdown
exit
router eigrp 136
network 10.56.1.0 0.0.0.255
network 172.20.36.0 0.0.0.3
no auto-summary
passive-interface GigabitEthernet0/0
exit R-Ludhiana interface g0/0
ip address 172.20.36.2 255.255.255.252
no shutdown
exit
interface g0/1
ip address 172.20.36.5 255.255.255.252
no shutdown
exit
router eigrp 143
network 172.20.36.0 0.0.0.3
network 172.20.36.4 0.0.0.3
no auto-summary
exit R-Lucknow interface g0/0
ip address 10.56.2.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 172.20.36.6 255.255.255.252
no shutdown
exit
router eigrp 136
network 10.56.2.0 0.0.0.255
network 172.20.36.4 0.0.0.3
no auto-summary
passive-interface GigabitEthernet0/0
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 R-Trichy has 1 EIGRP neighbour in AS 136 8 On R-Trichy: show ip eigrp neighbors shows at least 1 neighbour(s). 2 R-Ludhiana has 2 EIGRP neighbours in AS 136 8 On R-Ludhiana: show ip eigrp neighbors shows at least 2 neighbour(s). 3 R-Lucknow has 1 EIGRP neighbour in AS 136 8 On R-Lucknow: show ip eigrp neighbors shows at least 1 neighbour(s). 4 R-Trichy learns 10.56.2.0/24 from the protocol 10 On R-Trichy: show ip route lists 10.56.2.0/24. 5 10.56.1.10 (Trichy) reaches 10.56.2.10 (Lucknow) 20 On PC-Trichy: ping 10.56.2.10 must succeed. 6 10.56.2.10 (Lucknow) reaches 10.56.1.10 (Trichy) 20 On PC-Lucknow: ping 10.56.1.10 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Different AS numbers on the two ends — no neighbour and no error
Leaving auto-summary on
Forgetting that a passive interface forms no neighbours
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
R-Ludhiana no router eigrp 143
router eigrp 136
network 172.20.36.0 0.0.0.3
network 172.20.36.4 0.0.0.3
no auto-summary
exit Where this is examined
CCNA 3.0 EIGRP concepts; CCNP ENCOR/ENARSI EIGRP.
This lab practises these topics in the zNetLaB certification courses:
Related labs
Where to start
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