From: ccie2be (ccie2be@nyc.rr.com)
Date: Sun Jul 06 2003 - 20:17:45 GMT-3
Hi,
You're question is ambiguous. If you're asking about the situation where
there are 2 or more ABR's connecting the same 2 areas, for example, rtrA and
rtrB are both ABR's between Area 0 and Area 1 and you configure
summarization on rtrA then summarization must also be configured on rtrB
because otherwise specific routes will leak out of area 1 into the ospf
backbone with the result that nothing is accomplished by the summarization
done on rtr A.
If, on the other hand, you're asking about the situation where there are
multiple non backbone areas and you configure summarizaton between one non
backbone area and the backbone and now you want to know if summarization is
required between another non-backbone and the backbone. The answer is that
it is NOT required.
As far as black holes are concerned, as I recall - ( always double check my
recollection as it's subject to major inaccuracies) there are 2 rules to
keep in mind:
a) a static route to the summarized address pointing to the null
interface must always be configured and
b) the summarized address should never include specific addresses
that don't exist in the non-backbone area
OSPF includes several mechanisms for controlling where routes are advertised
within the ospf routing domain. The Area range command allows you to
control which subnets from within the non-backbone area are allowed to be
known outside that specific non-backbone area. If the subnet addresses in
the non-backbone were well thought out, using the area range command can
greatly reduce the number of routes advertise from the non backbone area
into the rest of the ospf domain. If the addressing wasn't done wisely,
summarizing may not accomplish much if anything.
In constrast to the Area range command which reduces the number of routes
advertised OUT from the non backbone area into the backbone area, ospf also
allows you to control what subnets are permitted to be advertised INTO the
non backbone from the backbone. This is done be making the non backbone area
into a stub, a totally stubby, or a NSSA (Not So Stubby Area).
Since you didn't ask about Stub areas, I won't talk about them. I just
mention them because it helps hightlight which direction routes are being
advertised - from the backbone into the non backbone or from the
non-backbone into the backbone. Jim
----- Original Message -----
From: "Phuong" <kieu@hn.vnn.vn>
To: "'SHARMA,MOHIT (HP-Germany,ex1)'" <mohit.sharma@hp.com>;
<ccielab@groupstudy.com>
Sent: Saturday, July 05, 2003 7:48 PM
Subject: RE: OSPF- Area range!!!!!!
> It is unnecessary in some cases and nacessary in the other cases ;-)
> Summurization always important when you design a large Internetwork, it
> will save your resource (CPU, memory).
>
> But, in the other hand, it will creat blackhole, that is a more specific
> route that included in summary route may down or not exist. So in a
> small network with not too much route, it is not necessary to summary.
>
> Phuong
>
> -----Original Message-----
> From: nobody@groupstudy.Itm [mailto:nobody@groupstudy.com] On Behalf Of
> SHARMA,MOHIT (HP-Germany,ex1)
> Sent: Sunday, July 06, 2003 6:32 AM
> To: ccielab@groupstudy.com
> Subject: OSPF- Area range!!!!!!
>
> HI all,
>
> When summarizing a network from an ospf area to another, is it
> necessary/important to specify this range on all the ABR's??
> Would it not suffice just to put the command on one ABR???
>
> What difference does it make??
>
> M Really confused??
>
> Thanx for helping.
>
> MFG,
>
> Mohit.
>
>
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