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| VLAN, PPPoE and DM9003 |
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| Aug 4, 2008 |
A PA1688 IP phone user wrote 802.1Q and 802.1p VLAN support for RTL8019AS a few years ago. We copied the implementation to AR1688 software in early 2006. In the software, the Z80 controller in AR1688 is responsible to remove VLAN tag in incoming packet and add VLAN tag in outgoing packet.
DM9003 switch support VLAN in a better way, the hardware knows how to remove VLAN tag in incoming packet, and if necessary, automatically add VLAN tag in outgoing packet. It is another good feature DM9003 has to speed up network performance when 802.1Q VLAN is used.
When PPPoE is also used, it is interesting to consider which to be put right after those MAC addresses, the VLAN tag, or the PPPoE header? In other words, VLAN inside PPPoE packet, or PPPoE inside VLAN packet, or both possible?
The answer is clear by looking up into those standard documents. And it is also quite clear just by reading DM9003 datasheet, since VLAN tag is added or removed by hardware after we processed PPPoE packet by Z80 controller, the only possible way is PPPoE inside VLAN packet. Which means, PPPoE can be used in VLAN environment, but you can not build a VLAN by PPPoE dial up. Looking back on the PA1688 RTL8019AS 802.1Q VLAN software, it does just the right thing! We can not help to wonder how many network genius were playing with our little devices all those years.
When 802.1Q VLAN is used, the max 802.3 data length is increased from 1514 to 1518 bytes (without CRC), with four extra bytes for the VLAN tag. Again, PPPoE does not has this length "previlege", it will only cause 8 bytes short of data, for the PPPoE header information it is carrying.
Good good study, day day up!
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