[c-nsp] Difference in IP FRR Link vs Per Prefix Option

[Adam Vitkovsky]

The presentation is really good thanks Phil,

Right so the per-link LFA computation requires an LFA candidate to have
connectivity to primary next-hop node. 
So if C would not have valid upstream link to D via F it would not be
considered an LFA candidate right? 

In other hand if C happens to have a best-path link to D (since this is not
mpls-te and each node on the path makes an independent routing decision) if
C receives traffic destined for D it will forward it according to the
best-path decision to D directly avoiding the original primary next-hop F. 

One think that got my eye is the sentence that the backup next-hop must be
valid for all the prefixes using the particular primary next-hop. 
That restrains the backup path selection for per-link LFA even more.

Since I haven't found that in the presentation I'll mention it here for
further reference. 
On XR:
Line-card disjoined considers bundle interfaces as line-card disjoined
interfaces. 
Also if you start tweaking the default tie-breakers the default tie-breakers
are not considered anymore. 
So if you'd like to let's say move "lowest backup metric" before "line-card
disjoined" by changing its default index to 20 you have just deleted all the
other tiebreakers. 
That is to modify the list somehow you have to mention all the tiebreakers
you'd like to be considered with index values defining their priorities as
tiebreakers not specified are not considered. 


Per RFC 5286
Inequality 1: Loop-Free Criterion
Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)

Inequality 3: Criteria for a Node-Protecting Loop-Free Alternate
Distance_opt(N, D) < Distance_opt(N, E) + Distance_opt(E, D)

So yes for per-link LFA the inequality 1. would be enough. 
Though for per-prefix LFA with node-protection also inequality 3. has to
hold true for the LFA candidate to be considered for node protection. 


adam