Hitachi GR2000 Series - Manual (Page 214)

Hitachi Gigabit Router GR2000 Series Enhanced Version Operations - Device Management Overview
7-48 GR2K-GA-0015
Ver. 07-02
7.5.3.6 Setting Value for mru (MTU)
It is possible that a particular HP will be inaccessible even though the configuration
definition (mru value) of PPPoE was not changed. The solution for this is to change
the mru (MTU) size. Shown below are examples of communication impediments in
Flets ADSL and B Flets service, as well as the procedure for changing the mru (MTU)
size.
Example: In the PPPoE including the Flets ADSL and B Flets service, the MTU uses
1454 bytes. The MTU used in general Ethernet uses 1500 bytes.
An error may occur if this packet size does not remain constant. This could cause
a blockage in data flow even if a connection was already established with the
server. Try an operation in which you adjust this device’s mru (MTU) value.
Procedure for changing the mru (MTU)
Confirm the setting value of mru for the PPPoE information by using the
configuration definition
show pppoe command.
[Result of the execution]
7.5.3.7 Setting Value of mss Information
It is possible that a particular HP will be inaccessible even though the configuration
definition (MSS value) of PPPoE was not changed. This occurs when the data size
exceeds the upper limit of the size of a receivable signal. The MSS size change and
the changing procedure are given below.
Changing the MSS size
The MTU for Ethernet is normally 1500 bytes, whereas MSS must be 1460 bytes
to allow for the communication (TCP/IP) header size (40 bytes) plus data size
(MSS).
[MSS (1460) = MTU (1500) - (40 TCP/IP header size)
However, since the PPPoE header (8 bytes) is added to the PPPoE line on the WAN
side, the header data size generates an overflow causing a fragment. As a result,
performance decreases a packet may be abolished by the black hole router
present in the communication path, rendering a particular home page
un-viewable. This problem cannot be resolved in this device unless MSS size is
changed on both the up and down packets.
(config)# show pppoe OsakaISP1
pppoe Department2 OsakaISPÇP {
user_name "user2@osakaisp2";
password "osakaisp2password";
authentication_protcol chap;
echo_interval 3600;
mru 1492;
mss 1414;
};
(config)# pppoe OsakaISPÇP -mru 1454
(config)# quit
# close OsakaISP1
# free OsakaISP1
#
Set a value smaller than the current
setting value.
Input the next command to make the setting
information effective.
current mru value.