Subject: Re: [OSPF] ospf-lite welcomes comments

From: mike shand <[email-masked]>
Date: Mon, 01 Oct 2007 12:30:54 +0100

[email-masked] wrote:

In einer eMail vom 27.09.2007 16:58:46 Westeuropäische
Normalzeit schreibt [email-masked]:

Are you familiar with the work being done in the rtgwg? For
example:

draft-ietf-rtgwg-ipfrr-spec-base-09.txt

Acee,

thank you for your quick response. No, I am not familiar with
this rtgwg-draft.

Having just read the first "inequality  1 loop-free criterion"
and having drawn a chess board network to visualize it, I must say: it
is just not true. "<=" would be correct at a first glance.

I think you must have misunderstood something. If dist_opt(N,D) =
dist_opt(N,S)+dist_opt(S,D) then it is absolutely clear that N is NOT a
loop free Alternate for S, since any traffic N receives from S has an
equal probability of being forwarded BACK to S because there is an ECMP
from N to D, one branch of which traverses S.

The requirement for an LFA is that it MUST NOT cause a loop under any
circumstances. Clearly in the ECMP case it won't for some traffic, but
it WILL for other traffic.

But perhaps this discussion should be taking place in RTGWG rather than
OSPF.

Mike

But that is by far not the limit. I can distinguisch between
guaranteed loop-free routes, loop-endangered routes which needs a
little bit of caution, loop-endangered routes which needs some more
intensive care, and I can identify any next hop which definitively
enters a dead end.

If you look at www.hummel-research.de and
look at my example network you will see that all links are, by means of
my algorithm, converted into arrows which LOOP-FREE direct to the red
destination node.

It takes some more labor as to safely use arrows from head to
tail. In the end the proper relevant routing information could be
assigned to each adjacent link, from "best-hop" to "dead end entrance".

But I will continue to read this rtgwg-draft.

Heiner

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---

From: [email-masked]
Date: Mon, 1 Oct 2007 08:01:08 EDT

In einer eMail vom 01.10.2007 13:31:16 Westeuropäische Normalzeit schreibt  
[email-masked]:

I think  you must have misunderstood something. If dist_opt(N,D) =  
dist_opt(N,S)+dist_opt(S,D) then it is absolutely clear that N is NOT a loop  free 
Alternate for S, since any traffic N receives from S has an equal  probability of 
being forwarded BACK to S because there is an ECMP from N to D,  one branch of 
which traverses S.

Do you agree that this possibility is excluded by not forwarding any packet  
to precisely that neighbor node from which the package has been received ?
 
Heiner