Telnet to Loopback Interface IP Address Not Working? How to Fix in Packet Tracer.

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In this lab, we’ll configure three Cisco 2911 routers with Enhanced Interior Gateway Routing Protocol and verify end-to-end connectivity.

Everything will appear correct at first glance.

But there’s a hidden configuration issue somewhere in the network that prevents the routers from forming proper neighbor adjacencies.

Your challenge is to find it.

By the end of this lab, you should be able to:

  • Configure basic EIGRP routing
  • Configure loopback interfaces
  • Verify routing tables
  • Use troubleshooting commands to diagnose EIGRP issues
  • Identify why routes are not being exchanged

Network Topology

  • Router 1 connected to Router 2 using G0/0
  • Router 2 connected to Router 3 using G0/1
  • Router 1 contains a loopback interface:
    • 1.1.1.1/32

The goal is for all routers to learn the loopback route dynamically through EIGRP.

Step 1 – Build the Topology

  1. Drag three Cisco 2911 routers into the logical workspace.
  2. Label them:
    • Router 1
    • Router 2
    • Router 3
  3. Connect:
    • Router 1 G0/0 → Router 2 G0/0
    • Router 2 G0/1 → Router 3 G0/1

Step 2 – Configure Interfaces

Router 1

enable
configure terminal

interface g0/0
ip address 10.0.0.1 255.255.255.0
no shutdown

interface loopback1
ip address 1.1.1.1 255.255.255.255

Router 2

enable
configure terminal

interface g0/0
ip address 10.0.0.2 255.255.255.0
no shutdown

interface g0/1
ip address 10.0.1.1 255.255.255.0
no shutdown

Router 3

enable
configure terminal

interface g0/1
ip address 10.0.1.2 255.255.255.0
no shutdown

Step 3 – Configure EIGRP

Configure EIGRP on all three routers.

For router 1 and router 3:

router eigrp 1
network 0.0.0.0

For router 2:

router eigrp 2
network 0.0.0.0

Step 4 – Configure Passwords on Router 1

line vty 0 4
password testlogin
login

enable password test

Save the configuration:

copy running-config startup-config

Step 5 – Verify Routing

Run:

show ip route

on all three routers.

If everything is working correctly, Router 2 and Router 3 should learn the loopback route:

1.1.1.1/32

through EIGRP.

You should also be able to telnet into Router 1 from Router 2 and Router 3.

The Problem

Here’s where things get interesting.

A network engineer made a configuration mistake somewhere in this lab.

The routers are no longer exchanging routes correctly.

Users are reporting connectivity problems.

Your task is to troubleshoot the network and identify the issue.

Symptoms

You may notice:

  • Missing EIGRP routes
  • Failed telnet attempts
  • Missing neighbor adjacencies
  • Incomplete routing tables

Troubleshooting Challenge

Without looking ahead:

  1. Verify interface connectivity
  2. Check IP addressing
  3. Verify interfaces are up
  4. Check EIGRP neighbors
  5. Compare EIGRP configurations between routers
  6. Determine why routes are not being exchanged

Helpful commands:

show ip eigrp neighbors
show ip protocols
show running-config
show ip route

Think Before You Scroll

Try to solve the issue yourself before revealing the answer.

Real networking jobs often involve tiny mistakes that cause major outages.

This lab simulates exactly that scenario.

Solution

The issue is on Router 2.

Instead of:

router eigrp 1

the router was configured with:

router eigrp 2

Because EIGRP autonomous system numbers must match between neighbors, Router 2 fails to form adjacencies with the other routers.

As a result:

  • Routes are not exchanged
  • Neighbor relationships fail
  • Router 3 never learns the loopback route
  • Telnet connectivity breaks

Correcting the AS number restores routing immediately.

Why This Is Important

This is one of the most common routing protocol mistakes beginners make.

The configurations can look almost identical, yet one incorrect number completely breaks dynamic routing.

Learning how to systematically troubleshoot issues like this is what separates someone who memorizes commands from someone who truly understands networking.