Source 172.16.4.2
4
G0/0/1
R1
R2
Figure 18-13
TTL=2 Message Sent by traceroute
The figure shows these four steps:
1.
The traceroute command sends a packet from the second set with TTL=2.
2.
Router R1 processes the packet and decrements TTL to 1. R1 forwards the packet.
3.
Router R2 processes the packet and decrements TTL to 0. R2 discards the packet.
4.
R2 notifies the sending host of the discarded packet by sending a TTL Exceeded
ICMP message. The source IP address of that message is 172.16.4.2.
Finally, the choice of source IP address to use on the time-exceeded message returned by
routers has a big impact on the output of the traceroute command. Most routers use simpler
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