H3C Technologies H3C S7500E Series Switches - Manual (Page 114)

11-5
If the device receives an IP packet with the destination unreachable, it will drop the packet and send an
ICMP destination unreachable error packet to the source.
Conditions for sending this ICMP packet:
z If neither a route nor the default route for forwarding a packet is available, the device will send a
“network unreachable” ICMP error packet.
z If the destination of a packet is local while the transport layer protocol of the packet is not
supported by the local device, the device sends a “protocol unreachable” ICMP error packet to the
source.
z When receiving a packet with the destination being local and transport layer protocol being UDP, if
the packet’s port number does not match the running process, the device will send the source a
“port unreachable” ICMP error packet.
z 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 will send the source a
“source routing failure” ICMP error packet.
z When forwarding a packet, if the MTU of the sending interface is smaller than the packet but the
packet has been set “Don’t Fragment”, the device will send the source a “fragmentation needed
and Don’t Fragment (DF)-set” ICMP error packet.
Disadvantages of sending ICMP error packets
Although sending ICMP error packets facilitates network control and management, it still has the
following disadvantages:
z Sending a lot of ICMP packets will increase network traffic.
z If a device receives a lot of malicious packets that cause it to send ICMP error packets, its
performance will be reduced.
z As the redirection function increases the routing table size of a host, the host’s performance will be
reduced if its routing table becomes very large.
z If a host sends malicious ICMP destination unreachable packets, end users may be affected.
To prevent such problems, you can disable the device from sending ICMP error packets.
Follow these steps to enable sending of ICMP error packets:
To do… Use the command… Remarks
Enter system view system-view —
Enable sending of ICMP redirect packets ip redirects enable
Required
Disabled by default.
Enable sending of ICMP timeout packets ip ttl-expires enable
Required
Disabled by default.
Enable sending of ICMP destination
unreachable packets
ip unreachables enable
Required
Disabled by default.