User's Manual

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automatically opened.
To enable the function, run the following command in VLAN interface configuration mode to forward ICMP re-
directional packets:
Command Purpose
ip redirects
Permit sending the ICMP redirection message.
(3) Sending ICMP Mask Response Message
Sometimes the host must know the network mask. To get the information, the host can send the ICMP mask
request message. If the routing SWITCH can confirm the mask of the host, it will respond with the ICMP mask
response message. By default, the routing SWITCH can send the ICMP mask response message.
To send the ICMP mask request message, run the following command in VLAN interface configuration mode:
Command Purpose
ip mask-reply Send the ICMP mask reply message.
(4) Supporting Route MTU Detection
The system supports the IP route MTU detection mechanism defined by RFC 1191. The IP route MTU detection
mechanism enables the host to dynamically find and adjust to the maximum transmission unit (MTU) of
different routes. Sometimes the routing SWITCH detects that the received IP message length is larger than
the MTU set on the message forwarding interface. The IP message needs to be segmented, but the
“unsegmented” bit of the IP message is reset. The message, therefore, cannot be segmented. The message
has to be dropped. In this case, the routing SWITCH sends the ICMP message to notify the source host of the
reason of failed forwarding, and the MTU on the forwarding interface. The source host then reduces the length
of the message sent to the destination to adjust to the minimum MTU of the route.
If a link in the route is disconnected, the message is to take other routes. Its minimum MTU may be different
from the original route. The routing SWITCH then notifies the source host of the MTU of the new route. The IP
message should be packaged with the minimum MTU of the route as much as possible. In this way, the
segmentation is avoided and fewer message is sent, improving the communication efficiency.
Relevant hosts must support the IP route MTU detection. They then can adjust the length of IP message
according to the MTU value notified by the routing SWITCH, preventing segmentation during the forwarding
process.
(5) Setting IP Maximum Transmission Unit (MTU)
All interfaces have a default IP maximum transmission unit (MTU), that is, the transmissible maximum IP
message length. If the IP message length exceeds MTU, the routing SWITCH segments the message.
Changing the MTU value of the interface is to affect the IP MTU value. If IP MTU equals to MTU, IP MTU will
automatically adjust itself to be the same as new MTU as MTU changes. The change of IP MTU, however,
does not affect MTU. IP MTU cannot be bigger than MTU configured on the current interface. Only when all
devices connecting the same physical media must have the same MTU protocol can normal communication
be created.
To set IP MTU on special interface, run the following command in interface configuration mode: