Enable sending ICMP destination unreachable messages

A device sends ICMP destination unreachable messages by following these rules:

• The device sends the source an ICMP network unreachable message when the following conditions are met:

  • The packet does not match any route.

  • No default route exists in the routing table.

• The device sends the source an ICMP protocol unreachable message when the following conditions are met:

  • The packet is destined for the device.

  • The transport layer protocol of the packet is not supported by the device.

• The device sends the source an ICMP port unreachable message when the following conditions are met:

  • The UDP packet is destined for the device.

  • The packet's port number does not match the corresponding process.

• The device sends the source an ICMP source route failed message when the following conditions are met:

  • The source uses Strict Source Routing to send packets.

  • The intermediate device finds that the next hop specified by the source is not directly connected.

• The device sends the source an ICMP fragmentation needed and DF set message when the following conditions are met:

  • The MTU of the sending interface is smaller than the packet.

  • The packet has DF set.

If a DHCP-enabled device receives an ICMP echo reply without sending any ICMP echo requests, the device does not send any ICMP protocol unreachable messages to the source. For more information about DHCP, see Layer 3—IP Services Configuration Guide.

1. Enter system view.

   system-view

2. Enable sending ICMP destination unreachable messages.

3. ip unreachables enable

   By default, the sending of ICMP destination unreachable messages is disabled.