# Algorithms for computing Maximally Redundant Trees for IP/LDP Fast-Reroute draft-enyedi-rtgwg-mrt-frr-algorithm-01 Alia Atlas (Juniper Networks) Gabor.

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Algorithms for computing Maximally Redundant Trees for IP/LDP Fast-Reroute draft-enyedi-rtgwg-mrt-frr-algorithm-01 Alia Atlas (Juniper Networks) Gabor Enyedi, Andras Csaszar (Ericsson) IETF 83, Paris, France

Agenda Briefly about the algorithm Problem Avoid using a node
Non-2-connected networks

Agenda Briefly about the algorithm Problem Avoid using a node
Non-2-connected networks

ADAG and partial order E G D Root C S B A H

A<<B if there is a path from A to B Root is both the shortest and the greatest E G D Root C S B A H

ADAG and partial order S<<E Blue path: increasing [S, E]
Red path: decreasing [Root, S] and [E, Root] E G D Root C S B A H

Blue path: increasing [S,Root] and [Root, A] Red path: decreasing [A, S] E G D Root C S B A H

ADAG and partial order S and C are not ordered
Blue path: [S, E] and [C, E] Red path: [A, S] and [A, C] E G D Root C S B A H

Agenda Briefly about the algorithm Problem Avoid using a node
Non-2-connected networks

Three trees We have tree trees
SPT Two MRTs There is no connection between SPT and MRTs Impossible to find a redundant pair for SPT Example: Shortest path 1 C D 10 1 Dest S No redundant pair for that! 1 10 1 B A 1

Agenda Briefly about the algorithm Problem Avoid using a node
Non-2-connected networks

Total order Partial order can compare any X only with S
We need to compare any two nodes Make a total order as well If A<<B, let A<B If A and B are not ordered select either A<B or B<A This can be done with a topological oder after converting the ADAG into a DAG Results: If A<B, either A<<B or A and B are not ordered

A possible total order Numbers are written next to nodes 8 7 E G 6 D
Root C S 4 5 3 B A H 1 2

Possible cases If dst>>src, failed node F
If S<<F<E, it may be on the BLUE path 8 7 E G 6 D Root C S 4 5 3 B Otherwise: it may be on the RED path A H 1 2

Possible cases If dst<<src, failed node F
Otherwise: it may be on the BLUE path 8 7 E G 6 D Root C S 4 5 3 If A<F<<S, it may be on the RED path B A H 1 2

Possible cases If dst and src are not ordered There are four sub-paths
If F>>src, it may be on the first part of the RED path If F and src are not ordered, and F>dest, it may be on the second part of the RED path 7 8 E G 6 D Root 4 C S 5 3 F and src are not ordered, and F<dest, it may be on the second part of BLUE path B If F<<src, it may be on the first part of the BLUE path 1 A H 2

Agenda Briefly about the algorithm Problem Avoid using a node
Non-2-connected networks

Non-2-connected problem
In this case we don’t have a single order Neither a partial order Nor a total order Convert the GADAG into an ADAG! C A Non-root block X Local root block D B

Non-2-connected problem
In this case we don’t have a single order Neither a partial order Nor a total order Convert the GADAG into an ADAG! C A X1 Non-root block Local root block X2 D B

Thank you!

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