Great answer. One minor thing. ISL came first. It was designed by Kalpana years ago. Cisco bought Kalpana in the mid-1990s.
Priscilla At 10:53 AM 2/14/02, Oleg Oz wrote: >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 ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=35449&t=35245 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
