Administrator 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.
Figure 56. ICMP Redirect
Host H is connected to the same Ethernet segment as SW1 and SW2. SW1 and SW2 are multi-layer switches which can route packets.
The default gateway of Host H is configured as SW1. Although the best route to the remote branch office host may be through SW2,
Host H sends a packet destined for Host R to its default gateway — SW1. After SW1 finds that the route to Host R is through SW2, and
that SW1 must forward the packet out the same Ethernet interface on which packet was received, it forwards the packet to SW2 and
sends an ICMP redirect message to Host H. Host H learns of the new route using the ICMP redirect message and sends all future packets
to the remote branch office host to SW2.
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IPv4 Routing