From: Peter van Oene (pvo@xxxxxxxxxxxx)
Date: Tue Mar 26 2002 - 11:47:34 GMT-3
Yes, this is the normal application per the original nssa spec
(1587). However, my previous point was that most implementations do not
use the bit. I've yet to confirm this (need to email dave katz I
suppose). It has been my observation that most implementations translate
by default and use pre prefix filtering to constrain conversion vs
examination of the P bit.
Pat Murphy has suggested that in some cases it may be relevant to flood
type 5's into nssa areas (most notably default). However, doing so creates
the potential for routing loops to occur. Much of this potential is due to
a potential variance in the setting of P bit values by default. Pat
suggests some standardization in the default setting of P bits and the use
of the P bit much like the up/down ISIS bit in TLV 128/130's as a loop
mitigation mechanism. I've attached links to the more relevant drafts.
The CCIE lab is far more about how then why however. The how in cisco
involves the use of summary addresses on nssa attached abr's to invoke per
prefix filtration or summarization and afaik, this doesn't involve a P bit
check.
http://search.ietf.org/internet-drafts/draft-ietf-ospf-nssa-update-11.txt
http://search.ietf.org/internet-drafts/draft-ietf-ospf-5to7-01.txt
At 08:32 AM 3/26/2002 -0600, Wright, Jeremy wrote:
>i may be wrong with this..but isnt the P bit used to control if type 7 is
>converted into type 5 to go into the backbone by the abr not vica versa?
>
>-----Original Message-----
>From: Peter van Oene [mailto:pvo@usermail.com]
>Sent: Tuesday, March 26, 2002 7:55 AM
>To: 'ccielab@groupstudy.com'
>Subject: Re: OSPF P bit
>
>
>more research indicates that the p bit can be used much like the IS-IS
>up/down bit to control the leaking of backbone information into an NSSA
>area. Specifically, assuming all nssa routers set the P bit to clear by
>default, should one with to leak 5's into an NSSA area as 7's, setting the
>P bit will ensure that they do not loop back into the backbone as 5's
>again. Pat Murphy has some rather recent drafts in this regard, however it
>is likely beyond the scope of the ccie lab.
>
>At 03:01 PM 3/25/2002 -0500, Peter van Oene wrote:
> >You'd have to poll some coders, but I don't believe the P bit is currently
> >being used. I believe that Cisco and others simply translate by default
> >unless told via configuration to do otherwise and that they do not
> >actively check the setting of the P bit.
> >
> >I'm ready and open to be proven wrong here :)
> >
> >Pete
> >
> >
> >At 12:54 PM 3/25/2002 -0600, Wright, Jeremy wrote:
> >>how do you determine if the asbr has set the P bit to 1 on the type 7 lsa
> >>header in order for the abr to translate the type 7 lsa to type 5 lsa into
> >>the other areas?
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>************************
> >> Jeremy Wright
> >> Network Analyst
> >> Archer Daniels Midland
> >> ja_wright@admworld.com
> >> (217)451-4063
> >>
> >>************************
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