Re: Type-4 LSA and NSSA

Narbik,
         You are the MAN

Thanks alot

On Sat, May 9, 2009 at 5:54 PM, Narbik Kocharians <narbikk_at_gmail.com> wrote:

> guys, you can't totally depend on RFCs, you have to see how Cisco
> implemented the RFC.
>
> Here:
>
> Lets say you have the following topology:
>
> R2----R3-----R4-----R5
> The links are all Frame-relay P2P
>
> R2's connection to R3 and R3's connection to R4 are in area 0
> R4's connection to R5 and R5's connection to R4 are in area 1
>
> If R5 redistributes a route. Area 1 routers will NOT see an LSA type 4. But
> the ABR of area 1 will generate an LSA type 4 and it will flood it into
> area
> 0, as a result of that if a "Show ip ospf da asbr-summ" command is entered
> on R2 or R3, you should see the LSA type 4 being injected by R4, the output
> of the display will show R5 as the ASBR (Link state ID) and the advertising
> router will be the ABR which is R4.
> Now...
> If area 1 is changed into an NSSA area, R4 translates LSA type 7s into LSA
> type 5s, correct?
>
> Therefore, area 0 routers will NOT see an LSA type 4, because now R4
> behaves
> like an ASBR, (Please dont twist words around). i said it BEHAVES like one.
> Therefore area 0 routers will NOT see an LSA type 4.
>
> Now, let's take this one step deeper and add R1 to R2 and let's run that
> connection in OSPF area 2.
>
> Now R2 is an ABR, it sees LSA type 5s being generated in its own area, so
> what does it do? it generates LSA type 4s, and as a result of that the
> routers in area 2 will see LSA type 4s.
> On Sat, May 9, 2009 at 5:35 AM, Scott Morris <
> smorris_at_internetworkexpert.com
> > wrote:
>
> > Ahhhh... The RFC's! Definitely the best answer to the question! :)
> >
> >
> >
> >
> > *Scott Morris*, CCIE*x4* (R&S/ISP-Dial/Security/Service Provider) #4713,
> >
> > JNCIE-M #153, JNCIS-ER, CISSP, et al.
> >
> > JNCI-M, JNCI-ER
> >
> > smorris_at_internetworkexpert.com
> >
> >
> > Internetwork Expert, Inc.
> >
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> http://www.internetworkexpert.com/>
> >
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> >
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> >
> >
> >
> > Mike Leske wrote:
> >
> > Thanks Scott and Narbik for confirming my thoughts.
> >
> > I think I finally found an "official" confirmation for that.
> >
> > RFC 1587 The OSPF NSSA Option states on page 5:
> > "Type-7 LSAs are only flooded within the NSSA. The flooding of type-7
> LSAs
> > follow the same rules as the flooding of type 1-4 LSAs."
> > But it describes at no point, that Type-4 LSAs will not be flooded into
> > NSSAs.
> >
> >
> > However, RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option, which
> obsoletes
> > RFC 1587, clearly states on page 5:
> > "Since Type-5 AS-external-LSAs are not flooded into NSSAs, NSSA border
> > routers should not originate Type-4 summary-LSAs into their NSSAs."
> >
> >
> > Cheers
> > Mike
> >
> >
> > 2009/5/9 Scott Morris <smorris_at_internetworkexpert.com> <
> smorris_at_internetworkexpert.com>
> >
> > Type 4's are used for next hop reachability to get to the Type 5 LSAs
> > (ASBR). If you have no Type 5's, you have zero need for a Type 4 LSA.
> :)
> >
> > So I'd suggest that someone messed up in the book?
> >
> > *Scott Morris*, CCIE*x4* (R&S/ISP-Dial/Security/Service Provider) #4713
> >
> >
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> >
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> >
> >
>
>
> --
> Narbik Kocharians
> CCSI#30832, CCIE# 12410 (R&S, SP, Security)
> www.MicronicsTraining.com <http://www.micronicstraining.com/>
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Received on Sat May 09 2009 - 20:14:53 ART