Mike, yes the IEEE802.1Q does insert the VLAN tag into the frame (ISL
encapsulates the frame.) I am going to try and get this part right. The max
frame size for an ethernet frame is 1518, however if you are going to insert
the 801.1Q Vlan tag you would require an additional 4 bytes.
2 bytes for Length/Type = 802.1Q Tag Type
2 bytes for Tag Control Information)
Bringing the grand total to 1522. I belive that the older chip sets
supported a max of 1518 (later the standard was changed to support 1522.)
So, I belive that the age on the hardware has a bit to do with this as the
change to the frame size (or the IEEE standard change) did not occur until
sometime in '98.
Now Cisco's ISL came along and decided to take it a bit further.
Cisco's implementation encapsulates the frame (the original 1518 byte frame)
and by doing so adds 30 more bytes. This 1548 byte max. frame again presents
a problem to older Ethernet chip sets.
So you may find certine Ethernet HW that will support the 1522 byte
size and not the 1548 byte size (I am pretty sure of this but will not swear
to it) and hence some hardware will support 802.1Q and not ISL.
I hope I am not wrong about this.
Oleg Oz...
http://www.techfest.com/networking/lan/ethernet2.htm
http://www.cisco.com/warp/public/473/741_4.html
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=35404&t=35245
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