No problem. I don't have that reference but let me try
to explain. I've had a long day and am beat - this is
a quick response.
The book description you describe and quote sounds
correct. The lowest cost route will be prefered when
inter-area routes are used. If the same route exists
but is an intra-area route then that will be used over
the inter-area route. For intra-area routes lowest
cost route is prefered as well.
In the example you gave, the route you have listed is
an External type 2 route which is neither intra-area
or inter-area so it has different rules.
I think you may be getting yourself confused. Set up a
lab with no ASBR's/redistribution like below.
e0---R1---e1
| |
R2---e2----R3
R1 e0 and e1 are in area 0.
R2 e0 and R3 e1 is in area 0.
R2 and R3 e2 is in area 1.
R1 and R2 link, set cost to 10.
R1 and R3 link, set cost to 10.
R2 and R3 link, set cost to 300.
Put some loopbacks on R2 and R3 and put them in area 1
and put some in area 1.
Look at your OSPF database after everything has come
up, etc. You should see that to get R2's area 1
loopback from a Area 1 R3 address it will go over e2
instead of going through area 0. This is because
intra-area routes are prefered. Now, if you were to
ping from a network in area 0 on R3 then it will go
through R1 then to R2 because it is inter-area.
--- Jaeheon Yoo wrote:
> Hi, Erick
>
> Thanks for your kind reply again.
> But it is a different story.
> I guess I know what you're trying to say.
> Have you really read the page 175 yourself.
> If you read it, you would know what I'm curious
> about.
>
> This is about multiple intra-area paths to ASBR.
>
> Please refer to "OSPF Complete Implementation",
> p.223
>
> It says that:
> "... , paths through nonbackbone areas area
> preferred over paths
> through the backbone area 0.0.0.0. When multiple
> preferred paths are
> available, the one with the smallest cost is used."
>
> But in Cisco routers with IOS 12.0, we have
> different result.
> it seems to conform to rfc 1583.
>
> Hope somebody can clarify this for me.
>
> Regards,
> Jaeheon
>
>
> On 22 May 2001 08:55:46 -0400, [EMAIL PROTECTED]
> ("Erick B.") wrote:
>
> >Did you get my response to your original post?
> >
> >The route on r2 is a E2 route so it is not
> intra-area,
> >but a externally learned route so different rules
> come
> >into play. With external routes, there is a
> forwarding
> >address (next-hop value) of the lowest cost route
> to
> >that destination. Look at the RFC again - pages
> 23-25
> >or so, the section dealing with external routes.
> They
> >explain it there well and have an example.
> >
> >--- Jaeheon Yoo wrote:
> >> Hi, all
> >> According to rfc2328. p.175, when we have
> multiple
> >> intra-area paths to
> >> ASBR,a intra-area path using non-backbone area
> >> should be chosen over
> >> one using backbone area.
> >> But I always have opposite result. doesn't
> Cisco's
> >> implementation yet
> >> conform to rfc2328 in this respect?
> >>
> >> Here's from my cisco 2501 router.
> >>
> >> r2#sh ip ospf data
> >>
>
>-------------------------------------------------omitted
> >> Type-5 AS External Link States
> >>
> >> Link ID ADV Router Age Seq#
>
> >> Checksum Tag
> >> 131.108.0.0 192.168.40.3 324
> >> 0x80000003 0xE930 0
> >> r2#
> >> r2#sh ip ospf border
> >>
> >> OSPF Process 10 internal Routing Table
> >>
> >> Codes: i - Intra-area route, I - Inter-area route
> >>
> >> i 192.168.40.3 [74] via 172.16.2.2, Serial0,
> >> ABR/ASBR, Area 0, SPF 7
> >> i 192.168.40.3 [1562] via 172.16.30.33, Serial1,
> >> ABR/ASBR, Area 6, SPF
> >> 5
> >>
> >> r2#sh ip route
> >> -----------------omitted
> >> O E2 131.108.0.0/16 [110/20] via 172.16.2.2,
> >> 00:03:38, Serial0
> >> r2#
> >>
> >> Could somebody explain this to me?
> >> Thanks in advance.
> >
> >
> >
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