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Cisco IOS in a Nutshell, 2nd Edition by James Boney

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Name

ip unreachables — interface

Synopsis

ip unreachables
no ip unreachables

Configures

Sending of ICMP unreachable messages for an interface

Default

Enabled

Description

ICMP unreachable messages are generated when something about an incoming packet is unknown to the router. For example, an “ICMP host unreachable” message is generated if the router cannot deliver a package to its final destination. There are many different types of ICMP unreachable messages, and they all mean that the packet can’t be delivered for some reason. Disabling these messages can improve security because the messages can be used to discover information about your network.

Example

The following commands prevent the router from sending IP unreachable messages through the serial0 interface:

interface serial0
  no ip unreachables

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