At 02:50 AM 1/9/02, Madisa Ramagoffu wrote: >I have this question for you pls help > >In a token ring environment a sender send a packet to a destination >with the ff in a packet
Where is FF in the packet? That's just one byte. Is it a one-byte field somewhere? In decimal, it's 255. Where might that appear? Or do you mean that the Destination Address is all FFs. The Destination Address is 48 bits. Each numeral represents one nibble (4 bits). So a Destination Address of broadcast is: FF FF FF FF FF FF You will see many books that get this wrong and don't put enough FFs. >Everything specified excerpt the dsap >DSAP ???? >da specified >sa specified >SSAP specified > The recipient wouldn't be able to figure out that the DSAP was missing. The bytes are processed serially. The first byte that follows Source Address (or Routing Info, if present) would be considered the DSAP even if the source somehow forgot to actually insert a DSAP. DSAP values could be: 00 This is a null SAP, but I think it does have meaning in SNA FF Broadcast. This means go to all processes on the recipient device. 42 BPDU 06 IP F0 NetBIOS E0 IPX AA SNAP SNA also uses 04, 05, 08, and 0C >The question . what will happen to the packet if the DASP is not >specified???? It can't not be specified from the recipient's viewpoint. The frame could be just a MAC frame used for Token Ring processing rather than to carry upper-layer data. In that case, MAC data would follow the Source Address, rather than a DSAP. The first two bits of the Frame Control byte specify whether the frame is a MAC (overhead) frame or upper-layer data carried in LLC. 00 MAC 01 LLC >Is the packet going to be copied ? If you are talking about the setting of the Frame Copied bits, this happens at the MAC layer (not the LLC layer). If a device recognizes that the frame is for it, it sets the Address Recognized bits. It also sets the Frame Copied bits unless it doesn't have enough buffer space to actually copy the frame. (You may want to look into the special case of a bridge and whether it should set the Address Recognized and Frame Copied when forwarding a frame). >Is the packet going to be dropped ? >Will it wait for the next packet or request a retransmit?? It certainly won't request a retransmit. This is a LAN data-link-layer we're talking about. >What about the A C bit ? There's no such thing. You're probably thinking of the Address Recognized or the Frame Copied bits, or maybe the Access Control byte at the beginning of the frame. >Pls send me any information or links I can read if you have . See IEEE 802.5. It's available here: http://standards.ieee.org/getieee802/ Also, there's some white paper that everyone always recommends by Rossi. It's on Token Ring. Search the archives or ask the group. You should probably wait until Cisco finally takes Token Ring off the CCIE tests. What's the point in learning all this? It's not going to help you be a better network engineer!? ;-) Instead focus on general principles. What is a Destination Address? What is its format? What is a DSAP? What is its format? What is its purpose? What is LLC? How does a recipient process a received frame? ETC. Good luck! Priscilla ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=31421&t=31358 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
