RE: [ethmac] how about VLAN?

[jcwh@sina.com]

if we need to insert a vlan tag in a received frame.we should:
1 check the non-vlan packet is good or bad by calculate crc for it.
2 intert a VLAN tag.
3 reculate crc for the new packet.

so, we shall implement 2 CRC modules in this design. and the circuits is 
very complicated.

if we desing a special arithmetic, maybe only one crc module is needed. 
and circuits became simple?

i mean when recalculate crc, we just calculate the crc for VLAN tag, 
and use this crc value to modify the original CRC.
i don't know the details, but i think it is possible.


----- Original Message ----- 
From: "Illan Glasner" <iglasner@z... > 
To: <ethmac@o... > 
Date: Thu, 4 Apr 2002 08:38:00 -0800 
Subject: RE: [ethmac] how about VLAN? 

> 
> 
> 
> 
>    Hi, 
> 
>          I'm not sure I follow what you try to say when you write 
> special aritmetic, but basicly when you get a packet and want to 
> remove/change the vlan while you remove the vlan your crc checker 
> will check the original packet before the remove/change and NOT 
> after. 
> 
> and when sending packet if you insert/change a vlan you recalculate 
> the crc on the fly while sending it away and just override the old 
> crc with the new one. 
> 
> Notice that 64-67 byte packet with vlan that you decide to remove 
> the vlan you need to pad to prevent the packet to become less than 
> 64. 
> 
> have a nice day 
> 
>    Illan 
> 
> 
> -----Original Message----- 
> From: jcwh@s...  
> [/cgi-bin/post.cgi?cmd=new&to=jcwh%20at%20sina%20dot%
20com&msg=/ml-archive/ethmac/msg00007.shtml] 
> Sent: Thursday, April 04, 2002 1:52 AM 
> To: ethmac@o...  
> Subject: [ethmac] how about VLAN? 
> 
> 
> Is someone going to implement VLAN tag insertion and extraction in 
> this 
> mac controller? 
> 
> when inserting /extracting the VLAN tag, CRC should be calculated 
> once 
> more. 
> 
> Can we just change the old crc on some kind of spcial arithmetic 
> ,while 
> not "crc through" the whole frame? 
> 
> does someone here know this kind of arithmetic? 
> 
> thanks 
> 
--
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