VLAN

thank you jwj for pointing out that i had missed the frames question, my apologies

so about frames being tagged or untagged:

first try reading this : www.ieee802.org/1/files/public/docs1996/d96n010.pdf

then read on :
you do remember we talked about Virtual lans, that can be acessed by hosts irrespective of their geographical location. imagine how that can be achieved. all their outgoing information has to be tagged so that it remains in the virtual lan circle. its like in a crowed you will put a label on all those people whom you know, so that while standing at one point you can easily recognize them. all you do then is recognize the tag, and pass the information on to the next part of the network, or host.

Regarding IEEE 802.1Q standard, Tag-based VLAN uses an extra tag in the MAC header to identify the VLAN membership of a frame across bridges. This tag is used for VLAN and QoS (Quality of Service) priority identification. The VLANs can be created statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a specific VLAN and provides the information that switches need to process the frame across the network. A tagged frame is four bytes longer than an untagged frame and contains two bytes of TPID (Tag Protocol Identifier, residing within the type/length field of the Ethernet frame) and two bytes of TCI (Tag Control Information, starts after the source address field of the Ethernet frame).
study the figure below :
standard frame : |<--6 bytes-->|<--6 bytes-->|<-2 bytes->|<-var->|
read then in this order : destination---source---length---data
tagged frame :
|<-6 bytes->|<-6bytes->|<-TPID->|<-TCI->|<-len->|<-var->|
expanding the TCI :
|<-priority->|<-CFI->|<-VID->|
with the following size : 3 bits---1 bit---12 bits

explanation of the terms
TPID : TPID has a defined value of 8100 in hex. When a frame has the EtherType equal to 8100, this frame carries the tag IEEE 802.1Q / 802.1P.

Priority : The first three bits of the TCI define user priority, giving eight (2^3) priority levels. IEEE 802.1P defines the operation for these 3 user priority bits.

CFI : Canonical Format Indicator is a single-bit flag, always set to zero for Ethernet switches. CFI is used for compatibility reason between Ethernet type network and Token Ring type network. If a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to an untagged port.

VID : VLAN ID is the identification of the VLAN, which is basically used by the standard 802.1Q. It has 12 bits and allow the identification of 4096 (2^12) VLANs. Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN configurations are 4,094.

Note that user priority and VLAN ID are independent of each other. A frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that only the priority level is significant and the default VID of the ingress port is given as the VID of the frame.

Grr I hate it when people are right.

a few books for good reference :
THE SWITCH BOOK: THE COMPLETE GUIDE TO LAN SWITCHING TECHNOLOGY - RICH SEIFERT

this link will help: www.techfest.com/networking/prot.htm

this link will give you more books on vlans : www.cse.wustl.edu/~jain/refs/lsw_book.htm

That's an interesting tidbit about the CFI portion of the header, Arani. I didn't know that.

don't forget to check out the good VLAN writeup on this very site under the Networking section.

www.firewall.cx/vlans-intro.php