Re: [RE] Grand master identifier
David,
I am not an STP expert but my understanding is that spanning tree is
used by bridges to find out which of their ports are connected to other
bridges. The protocol then determines a root bridge and shuts down
redundant links to make sure there are no network loops. As I understand
it the MAC address is for the bridge rather than the end station. I
don't think the end stations are involved in STP.
I am not sure whether you are inviting general feedback on your
working paper but I have some concerns. It assumes that there will be
access control, bandwidth allocation and time slots for transmission.
Is bandwidth allocation really necessary to meet RE requirements?
Over-provisioning and best-effort (with class of service) may be
adequate. You can get a lot of data through a conventional gigabit
switch with very low latencies. The RE traffic can be given a higher
priority and so not be held up by less urgent traffic.
With access control what happens if access is denied? My assumption
is that a user connecting to a RE network would prefer best-effort
service to no service at all if there is no spare bandwidth to be
allocated. If you decide you need to support best-effort as a fallback
then you need buffers in your end stations and the reason for using time
slots goes away.
Arthur.
-----Original Message-----
From: owner-stds-802-3-re@IEEE.ORG [mailto:owner-stds-802-3-re@IEEE.ORG]
On Behalf Of David V James
Sent: 29 April 2005 22:20
To: STDS-802-3-RE@LISTSERV.IEEE.ORG
Subject: [RE] Grand master identifier
All,
I was starting to update a group-contribution working paper,
when a couple of questions arose. For reference, these questions
are with respect to:
http://www.ieee802.org/3/re_study/material/index.html
Subclause 5.2.
We assume that the grand master is selected by picking one
of the clock-master capable stations. To do this, IDs need
to be distributed externally (between bridges and stations)
as well as internally (between bridge ports).
To avoid invention, we assume the existing STP identifier
format should be used (why be different?). The format, not
the actual values; an grand master could be different from
the STP root.
My original assumption was that the precedence value,
transmitted between stations, consists of:
16 bits -- system
48 bits -- MAC address
16 bits -- port
Having started to write things in more detail, it seems
like the port information need only be used within a
bridge, and need never appear on Ethernet.
Pardon my asking, but the appropriate 802.1 documents are
not that easy for me to read. Am I correct in my recent
thoughts, that the port portion of this identifier is
only used within the bridge, and never appears on the outside?
DVJ