D-Link DI-1750 Reference Manual page 193

(02)beigrp Enhanced Interior Gateway Routing Protocol
(03)directed-broadcast Enable forwarding of directed broadcasts
(04)fast-switch Fast-Switch interface commands
(05)helper-address Specify a destination address for UDP broadcasts
(06)igmp IGMP interface command
(07)irdp ICMP Router Discovery Protocol
(08)mask-reply Enable sending ICMP Mask Reply messages
......
Please Input the code of command to be excute(0-18): 8
input 8，Select mask-reply option .
4. Support Path MTU Discovery
D-Link routers support the IP Path MTU Discovery mechanism, as defined in RFC 1191. IP Path MTU
Discovery allows a host to dynamically discover and cope with differences in the maximum allowable
maximum transmission unit (MTU) size of the various links along the path. Sometimes the router finds
out that the length of an IP message is larger than MTU configured on the redirection interface, then the
message is needed to be segmented. But if the bit "Don't Segment" of this IP message is set, the
message can only be discarded. Here the router will send out an ICMP message to inform source host
about the failing reason and the MTU on redirection interface. Source host will reduce the length of
messages to accommodate the least MTU of this route.
If one of the links in route is cut, the message will use another route and maybe its least MTU is
different with former route's. Here the router will inform the source host about MTU of the new route.
The least MTU in route should be adopted as much as possible to encapsulate IP message. In that it
will avoid segment and send out messages as few as possible. And that can improve the
communicating efficiency.
The corresponding host must support IP Route MTU Discovery, thus it can adapt the length of IP
messages and avoid the segment in redirection course according the MTU informed by router.
5. Set the IP MTU Packet Size
All interfaces have a default MTU packet size. You can adjust the IP MTU size so that if an IP packet
exceeds the MTU set for a router's interface, the router will fragment it.
Changing the MTU value can affect the CONFIG-IP MTU value. If the current IP MTU value is the same
as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to
match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the
value for the mtu interface configuration command. The IP MTU must be less than MTU on current
interface configuration. Also, all devices on a physical medium must have the same protocol MTU in
order to operate.
To set the IP MTU, perform the following command in interface configuration mode:
ip mtu bytes
In the prompt Select ip option ，it will list all arguments.
(00)access-group
......
(08)mtu
......
Please Input the code of command to be excute(0-18): 8
input 8，Selectmtu option ，prompt is as below:
(00)<68-1500>
Please Input the code of command to be excute(0-0): 0
Select0，prompt is as below:
Please input a string:
input the specified IP MTU.
Command
Specify access control for packets
Maximum Transmission Unit
bytes
Model Name
Function
Set the IP MTU packet size for an interface.
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