Hi Priscilla, My reference to cisco's apparent tweak of the "Port ID" field in 802.1d comes from the book "Cisco Lan Switching" by K. Clark, page 230.
Based on your comments and further review of the text, I think I'm in agreement that this new bit slicing approach should not cause any inconsistent behavior. Much like you suggest, the Port ID field is similar in use to the Root BID field. Even with more than 256 ports, the 16 bit "Port ID" field will be unique for a device. So, as long as the Port ID field is always tested as a 16 bit quantity, and there's no expectations of uniqueness of the "port number" octet, than all should work as expected. Thanks Priscilla Oppenheimer wrote: > > Hello Bergenpeak, > > Please see some comments below...... > > At 11:11 AM 1/21/02, bergenpeak wrote: > >In 802.1d spanning tree, the BPDU contains a number of fields > >including the "Port ID". THis is a two byte value where one > >octet contains a priority value and the second byte contains > >a value assigned to each port. > > > >For some of the higher density switches, (55xx, 65xx), one can > >have more then 256 ports on a switch. > > > >It looks like cisco has "extended" the 802.1d standard to allow > >for these higher density port counts by using 10 of this field's > >16 bits for port identification and 6 bits for priority. > > Where did you get this info? Do you have a link? Thanks. > > >What impact, if any, does this have on 802.1d operation in a > >multi-vendor environment? > > It might not have any impact. I would guess that the encoding of the Port > ID (and the priority component of the Port ID) isn't relevant to > inter-switch communication. IEEE says this: "The more significant octet of > a Port Identifier is a settable priority component that permits the > relative priority of Ports on the same Bridge to be managed." So, it sounds > like IEEE thinks it's just used internally, even though it is transmitted > in Configuration BPDUs. > > Think about when the Port ID actually gets used on Cisco switches. The only > time I've ever had to set the priority was when using two redundant trunk > links between switches. The priority gets used to determine which VLANs by > default are associated with each trunk on a single switch. > > With ordinary STP, the Bridge ID is much more relevant. It also has a > priority component. Messing with the encoding of that would affect > multi-vendor interoperability. I don't think messing with the Port ID would > cause a problem, however. > > Feel free to correct me on any of this. I didn't have time to review my STP > knowledge and STP is rather convoluted. > > Priscilla > > >Is the 802.1d standard being updated to address the limitation > >in the current 802.1d standard? > > > >Thanks > ________________________ > > Priscilla Oppenheimer > http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=32774&t=32697 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
