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Re: Question regarding multi-homed prefix LFA computation

Subject: Re: Question regarding multi-homed prefix LFA computation
From: mike shand
Date: Thu, 14 Feb 2008 12:17:35 +0000
Kui Zhang wrote:

Hi all,

I have a doubt about multi-homed prefix LFA computation described in draft-ietf-rtgwg-ipfrr-spec-base-10:

“If the alternate next-hop for the

   prefix p is simply inherited from the router advertising it on the

   shortest path to p, then the prefix p's alternate next-hop would be

   the link to C. This would provide link protection, but not the node

   protection that is possible via A.

                      5   +---+  4   +---+  5  +---+

                    ------| S |------| A |-----| B |

                    |     +---+      +---+     +---+

                    |       |                    |

                    |     5 |                  5 |

                    |       |                    |

                  +---+ 5 +---+   5       7    +---+

                  | C |---| E |------ p -------| F |

                  +---+   +---+                +---+


                       Figure 6: Multi-homed prefix


It seems router A is not a valid node-protecting LFA, which will cause a forwarding loop.

And the following words seem incorrect since we have to take the cost from the advertising router to the advertised prefixes into consideration.

   “If there exist multiple multi-homed prefixes that share the same

   connectivity and the difference in metrics to those routers, then a

   single node can be used to represent the set.”


For example, in figure 6, if the cost for prefix p to F decreases to 4, then router A will be a valid node-protecting LFA.


Do I miss something here? Thanks for helping me in advance.

There does seem to be something wrong here as you point out. Actually even decreasing F-p to 4 doesn't completely fix it because then A would have an ECMP path to p (both cost 14). The cost would need to reduce to 3 to fix this.

However, it would be better to fix it by (say) increasing the cost of link SA to 8. This would ensure that the later text about prefix X was also correct.

But maybe Alia had something else in mind?







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