Subject: "Basic Specification for IP Fast-Reroute: Loop-free Alternates"

From: [email-masked]
Date: Tue, 2 Oct 2007 06:50:13 EDT

In figure 2 everyone can see that not only S but also N has two choices to  
get to D. However
according to the rules of the game N has only one choice.
 
It just takes a change of the rules of the game, i.e. to come up with  better 
rules as  to enable two choices for N as well and also for detours  which are 
big enough to avoid a loop.  
It will take more effort but still of the same order of complexity as   is 
needed for IPFRR according to this draft.

Heiner

_______________________________________________
rtgwg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/rtgwg

---

From: mike shand <[email-masked]>
Date: Tue, 02 Oct 2007 12:13:30 +0100

[email-masked] wrote:

In figure 2 everyone can see that not only S but also N has two
choices to get to D. However

I'm sorry, but I haven't a clue what you are talking about. Figure 2 is
illustrating the problem of using link protecting LFAs which can cause
loops when a node failure occurs.

If we are talking about LFAs, then BOTH N and S have two choices (the
primary path, and the LFA). That is the whole point of the illustration.

If we want to AVOID the possibility of looping, then we can use
downstream paths. In this case it is true that S has N as a downstream
path, but N doesn't have S as a downstream path. That again is the
whole point. If they both had downstream paths there would still be a
loop.

Of course there are other ways to solve this problem, like using node
protecting LFAs as described in the draft. And in the case where there
are NO LFAs, then other methods such as not-via can be applied.

It just takes a change of the rules of the game, i.e. to come up
with better rules as  to enable two choices for N as well and also for
detours which are big enough to avoid a loop.

It will take more effort but still of the same order of
complexity as  is needed for IPFRR according to this draft.

Heiner