Cisco Systems CCNA 2 manual Echo messages, Destination unreachable message

8.1.7 Echo messages

ICMP message formats have three fields:

•Type

•Code

•Checksum

The type field indicates the type of ICMP message that is sent. The code field includes additional information that is specific to the message type. The checksum field is used to verify the integrity of the data. Create an example to help students understand this format. This is an important concept to help the student understand causes of ICMP "destination unreachable" messages.

8.1.8 Destination unreachable message

Hardware failures, improper protocol configuration, disabled interfaces, and incorrect routing information are some of the reasons for unsuccessful delivery of data. Give the students examples similar to the figures in the curriculum. Specify values and indicate the reason for each failure. Explain that the students must understand the various causes of ICMP "destination unreachable" messages to effectively troubleshoot an IP network.

8.1.9 Miscellaneous error reporting

Devices that process datagrams may not be able to forward a datagram due to some type of error in the header. The error does not relate to the state of the destination host or network, but it will still prevent the datagram from being processed and delivered.

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