Subject: [rbridge] Distribution Trees determination and announcement

From: "Donald Eastlake" <[email-masked]>
Date: Mon, 12 Jan 2009 21:43:01 -0500

I have re-read the mailing list discussion at the end of October and
the presentation and minutes from the Minneapolis meeting on the topic
of distribution tree root determination and announcement in TRILL.
There seemed to be a clear desire for change although not complete
agreement on what change to make. I think a change to that described
below was the most preferred.

Thanks,
Donald

Each RBridge announces in its LSP (a) a priority, (b) a number, and
(c) a list of RBridges in the campus.

For the RBridge with the highest priority (ties in the announced
priority being broken by system ID):
      The number announced by this RBridge is the number of
distribution trees all RBridges in the campus must also compute minus
1. Assume that number is n-1. If the list being announced is empty,
the every RBridge computes a tree rooted at the n highest priority
RBridges. If the list has k entries where k is between 1 and n-1,
every RBridge computes a tree rooted at the k listed RBridges and
enough of the highest priority RBridges not on the list to compute n
trees. If k is greater than or equal to n, then every RBridge computes
a distribution tree rooted at the first n entries in the announced
list. The set of RBridges used as tree roots is considered to be
ordered starting with those in the advertised list, if any, followed
by those determined by priority with the highest priority first.

When an ingress RBridge chooses a distribution tree root for a
multi-destination frame, and that RBridge is announcing that j trees
should be calculated, it must choose from among the first j (or n if j
> n) RBridges in the ordering of roots specified in the above
paragraph. If j is zero, it can choose any of the RBridges specified
above as distribution tree roots.

=============================
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---

From: Donald Eastlake <[email-masked]>
Date: Mon, 26 Jan 2009 23:34:15 -0500

On actually changing the draft text as described below, it seems to me
that it would be better to have each RBridge announce two different
numbers in addition to a priority and list of nicknames: one number
which is the number of trees all RBridges must calculate if the
announcing RBridge is highest priority and a second number which is
the number of different trees the announcing RBridge can use. If this
second number is zero, then the announcing RBridge can use any of the
trees being computed.

Possible text is pasted at the far bottom of this message.

Thanks,
Donald

   In connection with distribution tree determination and selection,
   each RBridge RBn advertises in the core IS-IS link state database its
   priority to be chosen as a tree root, a second number k (related to
   the number of trees to be calculated), a third number j (related to
   the number of trees to be used by RBn), and an ordered list of
   RBridge nicknames.  The uses of these data items are described below.

   The priority is a 16-bit unsigned integer that defaults, for a zero
   configuration RBridge to 0x8000. All RBridge in a campus can be
   ordered by priority with ties broken by system ID. Numerically lower
   priority numbers and system IDs are considered higher priority to be
   a tree root.

   The highest priority RBridge in the campus dictates the number of
   distribution trees every RBridges MUST calculate as one plus the
   second number advertised by that highest priority RBridge. The
   specific root RBridges for which trees are calculated are determined
   as follows:
   1. Assume that the highest priority RBridge has dictated, through the
      value of the second number k that it advertises, that distribution
      trees be calculated for k roots. k is a 16-bit configurable
      unsigned integer that defaults, for a zero configuration RBridge,
      to 1.
   2. Is the ordered list of nicknames advertised by the highest
      priority RBridge empty? If so, do 2.a below, if not, do 2.b.
   2.a Take the highest priority k+1 nicknames from the ordered list of
      all campus RBridges and calculate trees rooted at them (or just at
      all the RBridges in the campus if there are k or fewer of them).
   2.b Ignore any nicknames in the ordered list advertised by the
      highest priority bridge that do not appear in the campus. Take the
      remaining nicknames in that list and re-order the priority order
      list of all RBridges by placing these nicknames in the order they
      are advertised as if they were the highest priority RBridges. Then
      do 2.a immediately above but using this re-rdered list of all
      RBridges in the campus.

   Except during transient conditions, the above method will cause all
   RBridges to calculate distribution trees for the same set of roots.

   In order to determine Reverse Path Forwarding Check filters (see
   Section 4.3.1) for multi-destination frames, each RBridge needs to
   know what trees every other RBridge might use as root when it is
   constructing TRILL encapsulated frames. The tree roots that a
   particular RBridge RBm might use can be determined as follows:
   1. Assume that the number j of different roots that RBm would like to
      choose from in indicated through the value of the third number j
      that RBm advertises. j is a 16-bit configurable unsigned integer
      that defaults, for a zero configuration RBridge, to 0.
   2. Is the ordered list of nicknames advertised by RBm empty? If so,
      do 2.a below, if not, do 2.b.
   2.a RBm can uses as tree roots only the first j RBridges in the
      ordered list resulting from the procedure given earlier in this
      section for determining what trees need be calculated except that,
      if j is zero or larger than the number of tree roots being
      calculated, RBm may choose from all trees being caculated for the
      campus.
   2.b Ignore any nicknames in the ordered list advertised by RBm that
      are not the root of a tree being calculated by all RBridges in the
      campus. Take the remaining nicknames in that list and re-order the
      list being used in 2.a immediately above by placing these
      nicknames first in the order they are advertised by RBm. Then do
      2.a immediately above but using this re-ordered list of tree roots.

   It is RECOMMENDED that RBridges prefer, when constructing TRILL
   encapsulated multi-destination frames, trees whose roots are low cost
   from the encapsulating RBridge and, among those with equal cost, to
   prefer those with higher priority to be a tree root.
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