ICMP Messages
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As you can see, there are many line options that need to be understood
when troubleshooting extended access lists. Not only do you have to under-
stand the significance of the line, but you have to be familiar with the pro-
tocol you are troubleshooting. If necessary, debug options can be used in
conjunction with access lists to isolate and diagnose network failures.
ICMP Messages
I
nternet Control Message Protocol is used to provide information to
TCP/IP devices regarding packet status, errors, and overall network conges-
tion. This is the protocol used by ping and traceroute.
These messages are useful because they explain what is happening on the
network. If you try to connect to a remote host and get a Destination
Unreachable or Port Unreachable message, it is ICMP informing you that it
was unable to connect to the remote host.
ICMP uses several different packet types and codes. Table 6.3 lists some
of the types and their corresponding meanings. Some types have subsets
called codes; they are listed below their corresponding ICMP types.
T A B L E 6 . 3
ICMP Types and Codes
ICMP
Type
ICMP
Code
Description
0
Echo
Reply
3
Destination
Unreachable
0
Net
Unreachable
1
Host
Unreachable
2
Protocol
Unreachable
3
Port
Unreachable
4
Fragmentation Needed and Don't Fragment Was Set
5
Source Route Failed
6
Destination Network Unknown
7
Destination Host Unknown
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