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## Re: ordered SPFs

 Subject: Re: ordered SPFs Alia Atlas 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 _______________________________________________ Rtgwg mailing list [email protected] https://www1.ietf.org/mailman/listinfo/rtgwg ```
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