Ccna routing and Switching Complete Study Guide

Destination unreachable    If a router can’t send an IP datagram any further, it uses ICMP 

to send a message back to the sender, advising it of the situation. For example, take a look 

at Figure 3.17, which shows that interface e0 of the Lab_B router is down.

f I g u r e   3 .17     ICMP error message is sent to the sending host from the remote router.

Lab_A

Host A

ICMP packet

e0

Lab_B

Host B

e0

When Host A sends a packet destined for Host B, the Lab_B router will send an ICMP des-

tination unreachable message back to the sending device, which is Host A in this example.