Administrator Guide

EXEC Privilege mode
show arp retries
ICMP
For diagnostics, the internet control message protocol (ICMP) provides routing information to end stations by choosing the best
route (ICMP redirect messages) or determining if a router is reachable (ICMP Echo or Echo Reply).
ICMP error messages inform the router of problems in a particular packet. These messages are sent only on unicast traffic.
Configuration Tasks for ICMP
The following lists the configuration tasks for ICMP.
● Enabling ICMP Unreachable Messages
For a complete listing of all commands related to ICMP, refer to the Dell Networking OS Command Line Reference Guide.
Enabling ICMP Unreachable Messages
By default, ICMP unreachable messages are disabled.
When enabled, ICMP unreachable messages are created and sent out all interfaces.
To disable and re-enable ICMP unreachable messages, use the following commands.
● To disable ICMP unreachable messages.
INTERFACE mode
no ip unreachable
● Set the system to create and send ICMP unreachable messages on the interface.
INTERFACE mode
ip unreachable
To view if ICMP unreachable messages are sent on the interface, use the show config command in INTERFACE mode. If it is
not listed in the show config command output, it is enabled. Only non-default information is displayed in the show config
command output.
ICMP Redirects
When a host sends a packet to a destination, it sends the packet to the configured default gateway. If the gateway router finds
that a better route is available through a different router in the same network, that is, the same data link, the gateway router
sends the source host an ICMP redirect message with the better route. The gateway router routes the packet to its destination
and the host sends subsequent packets to that particular destination through the correct router.
Dell EMC Networking OS supports both ICMP and ICMP6 redirect messages. The following diagram depicts a topology in which
ICMP redirect messages are useful.
IPv4 Routing
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