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• 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.
Restrictions and guidelines
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.
Procedure
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.
Configuring rate limit for ICMP error messages
About the token bucket algorithm
To avoid sending excessive ICMP error messages within a short period that might cause network
congestion, you can limit the rate at which ICMP error messages are sent. A token bucket algorithm
is used with one token representing one ICMP error message.
A token is placed in the bucket at intervals until the maximum number of tokens that the bucket can
hold is reached.
A token is removed from the bucket when an ICMP error message is sent. When the bucket is empty,
ICMP error messages are not sent until a new token is placed in the bucket.
Procedure
1. Enter system view.
system-view
2. Set the bucket size and the interval for tokens to arrive in the bucket for ICMP error messages.
ip icmp error-interval interval [ bucketsize ]
By default, the bucket allows a maximum of 10 tokens. A token is placed in the bucket at an
interval of 100 milliseconds.
To disable the ICMP rate limit, set the interval to 0 milliseconds.
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