3Com Switch 4800G 24-Port Configuration Manual page 131

When the device receives the first fragment of an IP datagram whose
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destination is the device itself, it will start a timer. If the timer times out before
all the fragments of the datagram are received, the device will send a
"reassembly timeout" ICMP error packet.
Sending ICMP destination unreachable packets
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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:
If neither a route nor the default route for forwarding a packet is available, the
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device will send a "network unreachable" ICMP error packet.
If the destination of a packet is local while the transport layer protocol of the
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packet is not supported by the local device, the device sends a "protocol
unreachable" ICMP error packet to the source.
When receiving a packet with the destination being local and transport layer
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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.
If the source uses "strict source routing" to send packets, but the intermediate
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device finds the next hop specified by the source is not directly connected, the
device will send the source a "source routing failure" ICMP error packet.
When forwarding a packet, if the MTU of the sending interface is smaller than
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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.
Disadvantage of sending ICMP error packets
Although sending ICMP error packets facilitate network control and management,
it still has the following disadvantages:
Sending a lot of ICMP packets will increase network traffic.
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If receiving a lot of malicious packets that cause it to send ICMP error packets,
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the device's performance will be reduced.
As the redirection function increases the routing table size of a host, the host's
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performance will be reduced if its routing table becomes very large.
If a host sends malicious ICMP destination unreachable packets, end users may
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be affected.
To prevent such problems, you can disable the device from sending ICMP error
packets.
Follow these steps to disable sending ICMP error packets:
To do...
Enter system view
Disable sending ICMP redirection packets
Configuring ICMP to Send Error Packets
Use the command...
system-view
undo ip redirects
131
Remarks
-
Required
Enabled by default.