Subject: LFAs and OSPF applicability & problem reporting

From: Alia Atlas <[email-masked]>
Date: Thu, 28 Apr 2005 18:01:34 -0400

As the current draft mentions, the loop-free alternates can only be safely 
used within certain network scenarios for OSPF.  The current version of the 
draft discusses the concept of strict downstream alternates;  I am planning 
on removing those from the draft & instead discussing the abstract concept 
(in an appendix) of being able to guarantee an alternate provides 
protection based upon the correct distance checks.

One concern/suggestion raised by Dave Ward this last IETF was that it would 
be good to have an automatic method to detect that a potential problem 
existed with the network topology for OSPF.

This is challenging for the router computing the LFA to do.  However, it 
seems that an ABR could detect, for a given route when an problematic 
decision-point is observed.  For instance, for an ASE route, the ABR could 
detect that it has the option of intra-area routes from multiple 
areas.   For such detection to work, support would be required on the 
ABR.  Otherwise, LFAs can be incrementally deployed - as their computation 
and use can be a local decision.

If such detection by an ABR is desirable, I think it makes sense to put it 
in an OSPF specific MIB.  Presumably, one could get a count of the number 
of such routes as well as which they were.  One could also have a trap for 
the trigger from 0 to non-zero for that count & vice-versa.

Thoughts?  Does this seem like a reasonable, operator-friendly way of 
handling the applicability requirements for LFAs in OSPF?

I'll be sending out text with the proposed section on those applicability 
requirements relatively shortly.

Alia

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