From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Thu Jan 17 2008 - 18:40:35 ARST
Having a discontiguous area 0 is not actually always a problem, but it
is if you have multiple area 0 routers in each of the parts. Best
practice is to avoid it either way.
The OSPF rules for selecting the best inter-area routes are defined in
RFC 2328 (OSPFv2). The relevant section begins with:-
16.2. Calculating the inter-area routes
The inter-area routes are calculated by examining summary-LSAs.
If the router has active attachments to multiple areas, only
backbone summary-LSAs are examined.
Scenario
--------
Four routes: A, B, C, D are connected in area 0.
Router A also connects to area 1 with subnet 10.1.1.1/24,
Routers B & C also connect to area 2.
So B & C are connected both via area 0, and also by area 2.
Router A sends a summary LSA about the 10.1.1.1/24 network link into
area 0. That summary is then sent by routers B & C into area 2.
Router C receives the summary with A sent into area 0, and also the
summary which B sent into area 2. Because it has active links
(neighbors) in each area it ignores the summary from the non-backbone
area and installs the summary from A in the routing table.
But then the area 0 link between B & C fails.
- Router C looses the LSA from Router A.
- C still receives the 10.1.1.0/24 summary LSA from B.
- However router C retains an active link in area 0, router D still
being a neighbor.
- Router C ignores the summary LSA from B, which it learnt from area 2.
- C can no longer connect to 10.1.1.0/24 even though physical
connectivity is still available via area 2.
If router D did not exist and the same A-C link fails, router C would
not have an active link in area 0 and would install the route learned
via area 2. Note that a loopback in area 0 is not an active attachment,
the router must have a neighbor.
Paul.
vignesh sethuraman wrote:
> Hi Paul,
>
> I know discontiguous areas cannot communicate but i could not find the explanations anywhere.
>
> Could you please help me.
>
> Paul Cosgrove <paul.cosgrove@heanet.ie> wrote:
> Sounds a little like you may be asking a question you already know the
> answer to here. I would expect the summary route to be accepted since
> R1 has no active links in area 0. But if you are looking for a definite
> answer why not just fire up dynamips and try it out?
>
> Paul.
>
> vignesh sethuraman wrote:
>> Hello Experts,
>>
>> Router R1's S1/0 interface is in Area 0 and is connected to R2 interface s1/0 which is in area 0 and R1's S 1/1 interface is Area 1 which is connected to R3 s 1/1 Area 1and R2 s 1/2 interface is connected to R3 s1/2 which is in Area 1.
>>
>> R2 has a loopback 2.2.2.0/24 which is in area 0.
>>
>> R1 (S1/0)-----Area 0-------- (S1/0) R2
>> (s1/1) (S1/2)
>> ! !
>> ! !
>> Area 1 Area1
>> !-------------(s1/1) R3 (S12)------ -----!
>>
>> In R1 the network 2.2.2.0 is under Router link states(Area 0) and also under summary link states(Area 1) in the ospf database.
>> Normally the path is taken via Area 0 ie direct link between R1 and R2. If the path to that network via Area 0 is down, that is area 0 became discontiguous.
>>
>> R1 (S1/0)--- Area 0 Down Area 0 -------- (S1/0) R2
>> (s1/1) (S1/2)
>> ! !
>> ! !
>> Area 1 Area1
>> !-------------------------(s1/1) R3 (S12)-----------------------------!
>>
>> Now the link between R1 and R2 is via R3 (ie) the path for 2.2.2.0/24 is via R3 ie Area 1.will it include the path via Area 1.If not kindly explain me why.
>>
>> Thanks,
>> Vignesh
>>
>>
>>
>>
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>
>
-- Paul Cosgrove HEAnet Limited, Ireland's Education and Research Network 1st Floor, 5 George's Dock, IFSC, Dublin 1 Registered in Ireland, no 275301 tel: +353-1-660 9040 fax: +353-1-660 3666 web: http://www.heanet.ie/
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