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3com S7906E Manuel De Montage

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1-5 
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. 
 
 
The device stops sending “TTL timeout” ICMP error packets after sending ICMP timeout packets is 
disabled. However, “reassembly timeout” error packets will be sent normally. 
 
Displaying and Maintaining IP Performance Optimization 
To do… 
Use the command… 
Remarks 
Display current TCP connection 
state 
display tcp status 
Available in any view 
Display TCP connection statistics 
display tcp statistics 
Available in any view