H3C Technologies H3C S5560 Series Switches - Manual (Page 145)

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{
The selected route is not destined for 0.0.0.0.
{
There is no source route option in the received packet.
ICMP redirect messages simplify host management and enable hosts to gradually optimize their routing
table.
• ICMP time-exceeded messages
A device sends ICMP time-exceeded messages by following these rules:
{
If a received packet is not destined for the device and the TTL field of the packet is 1, the device
sends an ICMP TTL Expired in Transit message to the source.
{
When the device receives the first fragment of an IP datagram destined for it, it starts a timer. If
the timer expires before all the fragments of the datagram are received, the device sends an
ICMP Fragment Reassembly Timeout message to the source.
• ICMP destination unreachable messages
A device sends ICMP destination unreachable messages by following these rules:
{
If a packet does not match any route and there is no default route in the routing table, the device
sends a Network Unreachable ICMP error message to the source.
{
If a packet is destined for the device but the transport layer protocol of the packet is not
supported by the device, the device sends a Protocol Unreachable ICMP error message to the
source.
{
If a UDP packet is destined for the device but the packet's port number does not match the
corresponding process, the device sends the source a Port Unreachable ICMP error message.
{
If the source uses Strict Source Routing to send packets, but the intermediate device finds that the
next hop specified by the source is not directly connected, the device sends the source a Source
Routing Failure ICMP error message.
{
If the MTU of the sending interface is smaller than the packet and the packet has DF set, the
device sends the source a Fragmentation Needed and DF-set ICMP error message.
To enable sending ICMP error messages:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable sending ICMP
error messages.
• Enable sending ICMP redirect messages:
ip redirects enable
• Enable sending ICMP time-exceeded
messages:
ip ttl-expires enable
• Enable sending ICMP destination
unreachable messages:
ip unreachables enable
The default settings are
disabled.
Sending ICMP error messages facilitates network management, but sending excessive ICMP messages
increases network traffic. A device's performance degrades if it receives a lot of malicious ICMP
messages that cause it to respond with ICMP error messages.
To prevent such problems, you can disable the device from sending ICMP error messages. A device that
is disabled from sending ICMP time-exceeded messages does not send ICMP TTL Expired messages.
However, it can still send ICMP Fragment Reassembly Timeout messages.