3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring ICMP Error Packet Sending 239
Switch 8800s will send ICMP redirect packets to the source host under the
following conditions:
■ The receiving and forwarding interfaces are the same
■ The selected route has not been created or modified by ICMP redirect packet
■ The selected route is not the default route of the switch
■ There is no source route option in the packet
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When performing hardware forwarding, Switch 8800s will not forward ICMP
redirect packets even if the above conditions are satisfied.
ICMP redirect packets function simplifies host administration and enables a host to
gradually establish a sound routing table to find out the best route
2 Sending ICMP timeout packets
If the device received an IP packet with a timeout error, it drops the packet and
sends an ICMP timeout packet to the source.
Switch 8800s will send ICMP timeout packets under the following conditions:
■ If the switch finds the destination of a packet is not itself and the TTL field of
the packet is 1, it will send a "TTL timeout" ICMP error message.
■ When the switch receives the first fragment of an IP datagram whose
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 switch will send a
"reassembly timeout" ICMP error packet.
3 Sending ICMP destination unreachable packets
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.
Switch 8800s will send an ICMP destination unreachable error packet under the
following conditions:
■ If neither a route nor the default route for forwarding a packet is available, the
device will send a "network unreachable" ICMP error packet.
■ 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.
■ 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.
■ If the source uses "strict source routing" to send packets, but the intermediate
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
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.