Configuration Guide Configuring IP Addresses and Services
1.3.3 Sending ICMP Packets
Working Principle
ICMP Protocol Unreachable Message
A device receives non-broadcast packets destined for itself, and he packets contain the IP protocol that cannot be processed
by the device. The device sends an ICMP protocol unreachable message to the source host. Besides, if the device does not
know a route to forward packets, it also sends an ICMP host unreachable message.
ICMP Redirection Message
Sometimes, a route may be less than optimal, which makes a device send packets from the interface that receives packets. If
a device sends packets from an interface on which it receives the packets, the device sends an ICMP redirection message to
the source, informing the source that the gateway is another device on the same subnet. In this way, the source sends
subsequent packets according to the optimal path.
ICMP Mask Response Message
Sometimes, a network device sends an ICMP mask request message to obtain the mask of a subnet.. The network device
that receives the ICMP mask request message sends a mask response message.
Related Configuration
Enabling ICMP Protocol Unreachable Message
By default, the ICMP Protocol unreachable message function is enabled on an interface.
You can run the [no] ip unreachables command to disable or enable the function.
Enabling ICMP Redirection Message
By default, the ICMP redirection message function is enabled on an interface.
You can run the [no] ip redirects command to disable or enable the function.
Enabling ICMP Mask Response Message
By default, the ICMP mask response message function is enabled on an interface.
You can run the [no] ip mask-reply command to disable or enable the function.
1.3.4 Limiting Transmission Rate of ICMP Error Packets
Working Principle
This function limits the transmission rate of ICMP error packets to prevent DoS attacks by using the token bucket algorithm.
If an IP packet needs to be fragmented but the Don’t Fragment (DF) bit in the header is set to 1, the device sends an ICMP
destination unreachable packet (code 4) to the source host. This ICMP error packet is used to discover the path MTU. When
there are too many other ICMP error packets, the ICMP destination unreachable packet (code 4) may not be sent. As a result,
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