All messages begin with a percent sign, and are displayed in the following
format:
%FACILITY-SEVERITY-MNEMONIC:
Message-text
FACILITY is a code, consisting of
two to five uppercase letters, indicating the facility to which the message
refers. A facility may be a hardware device, a protocol, or a module of the
system software.
The IOS has over
500 service identifiers.
SEVERITY is a single-digit code from 0
to 7 that reflects the severity of the condition. The lower the number, the
more serious the situation.

MNEMONIC is a code, consisting of uppercase letters that uniquely
identify the message.
Message-text is a text string describing the condition. This portion
of the message sometimes contains detailed information about the event being
reported, including terminal port numbers, network addresses, or addresses that
correspond to locations in the system memory address space. Because the
information in these variable fields changes from message to message (see
below), it is represented here by short strings enclosed in square brackets ([
]). For example, a decimal number is represented as [dec].

Some
example error messages could be as follows:
Error
message: %HELLO-2-NORDB: Redistributed IGRP without rdb
In this message, HELLO is the facility, 2 is the severity, and
NORDB is the MNEMONIC. This message indicates that an internal software
error has occurred and technical support should be contacted for assistance.
Error message: %IP-4-DUPADDR Duplicate address [inet]
on [chars], sourced by [enet]
This error message
indicates that another system on the network segment is using this IP address
and that the IP address on one of the two systems should be changed.