Specifications

Chapter 9 - Miscellaneous Configuration Administration Manual (SIP/MGCP Version)
AT-VP504E FXS 65
IEEE 802.1q The 802.1q standard recommends the use of the 802.1q VLAN tags for
Ethernet frames traffic prioritization. VLAN tags are 4-byte headers in
which three bits are reserved for priority indication. The values of the
priority bits must be provisioned.
To enable the IEEE 802.1q user priority configuration:
1. Locate the following attributes under the interfaceQoSGroup
of the Provisioning MIB:
• interfaceQoSSignalingIeee8021QFilterEnable
• interfaceQoSMediaIeee8021QFilterEnable
Note on the Implementation of the IEEE Standard 802.1q
This note applies only for previous hardware revisions that do not use revision D of the main CPU, the
Motorola PowerPC 860.
The implementation of the IEEE Standard 802.1q in the AT-VP504E FXS has a minor problem due to
hardware limitations.
802.1q increases the Ethernet frame header by 4 bytes, adding a Virtual LAN ID and a user_priority.
This is useful to limit broadcasts that cross bridges, and it may also prioritize frames in the queuing
algorithm of switches. However, it also increases the maximum possible size of Ethernet frames from
1518 to 1522 bytes, and this might not be handled adequately by every hardware.
A workaround is available for PCs running Windows to avoid sending 1522 bytes packets (note that this
happens only in special and rare cases). The workaround is to reduce the MTU of the interface (the one
that sends packets with 802.1q framing) by 4 bytes.
• Use the registry editor (regedt32 on Windows 2000) and go to the key:
\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<ethernet
adapter>\Parameters\Tcpip
where <Ethernet adapter> can be found by using the command “ipconfig /all”.
• Add (or modify) a value named MTU of type REG_DWORD. Set it to 1496 (instead of 1500),
in decimal. Reboot the computer to have those changes in effect.
• To validate that your changes are correct, try to ping the AT-VP504E FXS with large packets
once rebooted:
ping -l 2000
This will cause IP fragmentation, the first fragment being as large as the interface allows it. With the
MTU reduced, you should now receive an answer. You can even ping with packets of 65500 bytes!
Note: It is the network administrator's responsibility to provision the
AT-VP504E FXS with standard and correct values.