Re: Type-4 LSA and NSSA

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
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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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-- 
Narbik Kocharians
CCSI#30832, CCIE# 12410 (R&S, SP, Security)
www.MicronicsTraining.com
www.Net-Workbooks.com
Sr. Technical Instructor
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