3Com MSR 50 Series, Руководство по настройке

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636 C
HAPTER
41: IP P
ERFORMANCE
C
ONFIGURATION
Configuring ICMP to
Send Error Packets
Sending error packets is a major function of ICMP protocol. In case of network
abnormalities, ICMP packets are usually sent by the network or transport layer
protocols to notify corresponding devices so as to facilitate control and
management.
Advantage of sending ICMP error packets
There are three kinds of ICMP error packets: redirect packets, timeout packets and
destination unreachable packets. Their sending conditions and functions are as
follows.
1
Sending ICMP redirect packets
A host may have only a default route to the default gateway in its routing table
after startup. The default gateway will send ICMP redirect packets to the source
host and notify it to reselect a correct next hop router to send the subsequent
packets, if the following conditions are satisfied:
■
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 device.
■
There is no source route option in the packet.
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.
The device will send an ICMP timeout packet under the following conditions:
■
If the device 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 device 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 device 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.
Conditions for sending this ICMP packet:
■
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.