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Re: ordered SPFs

Subject: Re: ordered SPFs
From: Alia Atlas
Date: Thu, 21 Oct 2004 12:56:11 -0400
At 11:33 AM 10/21/2004, mike shand wrote:
At 11:25 21/10/2004 -0400, Alia Atlas wrote:
I'm thinking of the case where there's a broadcast link that fails. Otherwise, there's a race condition that depends on when the LSPs are received.
Could you give an example?
I'm working on it. Basically, I'm assuming that arbitrary failures of
connectivity can happen inside a broadcast link due to layer-2 equipment.
Could you give an example where the correct behavior is to increase the
delay in the event of a failure?
I believe that the correct behavior would be the other way (for a failure
case), because of the need to remove dependencies; also, all the nodes
upstream of the one whose hopCount was increased would also have increased
hopCounts, so reducing the delay is appropriate.
The example that I have so far is rather contrived and assumes arbitrary
(including 1-way) failure at layer-2. It is as follows:
    50
   ----[ I ]
   |     |
   |  10 |
   |     |    3       3
   |   [ S ]----[ H ]---[ G ]
   |     |                | 3
   |  12 |              [ F ]
   |     |                | 3
   |     |                |
   |   [ A ]            [ B ]
   |     |              5 |
   |   5 |----------------|
   |             |        |
   |             | 5      | 5
   |           [ C ]    [ E ]
   |             |
   |----[ D ]----| 10


Consider the above topology where D is the destination for the traffic
under consideration.  A partial failure occurs on the broadcast link
between A, B and C, as could occur due to a layer-2 switch.  After the
failure, the following connections exist:
        A to C, A to E,
        B to A, B to E,
        C to E,
        E to A, E to B, E to C

Before the failure, S had equal-cost paths to D via A and via H.

As a result of this failure, 3 LSPs would be sent out:
     LSP 1 from A: reports connection to B is down
     LSP 2 from B: reports connection to C is down
     LSP 3 from C: reports connections to A and B are down

The following are the hopCounts from doing RSPTs on A, B and C based
on the topology before the failure.

Node  |  hopCount | hopCount | max from | hopCount |
      |   from A  | from B   |  A & B   |  from C  |
------+-----------+----------+----------+----------+
  A   |     0     |   1      |   1      |   1
  B   |     1     |   0      |   1      |   1
  C   |     1     |   1      |   1      |   0
  D   |     2     |   2      |   2      |   1
  E   |     1     |   1      |   1      |   1
  F   |     2     |   1      |   2      |   2
  G   |     3     |   2      |   3      |   3
  H   |     4     |   3      |   4      |   4
  S   |     1     |   4      |   4      |   5
  I   |     2     |   5      |   5      |   6

Assume that LSP 1 arrives before LSP 2 or LSP 3.

In that case, H and S would use the same delay and thereby cause a micro-loop.

Does this match with what you are saying?

I realize that the Ordered SPFs works most of the time; I'm trying to understand the corner cases (however improbable) better.
Thanks,
Alia


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