TTL in a routing loop

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Locate TTL expiry in a routing loop.

Sign in and launch

First: Routing between two subnets

Network

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PC1 connects to R1; R1, R2 and R3 form a triangle with cyclic default routes.
Default routes form R1 → R2 → R3 → R1.
Open a console

1. Find the loop

R1, R2, and R3

ip -j route show

Trace the route to 12.34.45.67. Keep the loop for this experiment.

2. Compare TTL values

PC1

ping -n -c 3 -W 1 10.0.0.2
ping -n -c 3 -W 1 -t 2 12.34.45.67
ping -n -c 3 -W 1 -t 3 12.34.45.67

The gateway replies. Compare error senders for different TTL values.

PC1

timeout 120 tcpdump -U --immediate-mode -i eth0 -w /tmp/lab-ttl.pcap "icmp" &
capture_pid=$!
sleep 1

PC1

ping -n -c 3 -W 1 -t 5 12.34.45.67

PC1

sleep 2
kill -INT "$capture_pid"
wait "$capture_pid" || true
tcpdump -nn -e -r /tmp/lab-ttl.pcap "icmp[0] == 11 and icmp[1] == 0"

Find ICMP 11/0 — TTL exceeded. Identify the router that dropped the packet.

Validation

Click "Check". Once all steps pass, click "End".