[quagga-users 9733] Re: Inter-area Routes not being correctly propagated back to ABR when Link to Backbone Broken

Brian Dickson briand at ca.afilias.info
Thu Jul 31 17:09:36 IST 2008


Hirsch, Stephen wrote:
> I am having problems getting the following topology to 
> work properly. It is based on rings looping out from 
> and back to the backbone and is shown in simplified 
> form below. 
>
> Routers R1, R3, R4 and R6 are ABR's. R2 and R5 are 
> internal routers in areas 1 and 2 respectively. All 
> links are Ethernet.
>
> Everything works OK when all links are in service and I 
> can break any one of the internal links and the routing 
> adjusts correctly. 
>
> If however I break the link between one of the ABR's and 
> the backbone, that ABR loses connectivity with both the 
> backbone and the other area. It still retains full 
> connectivity with its own area and the rest of the area 
> still has full connectivity via the other ABR.
>
> It seems that inter-area routes are not being propagated 
> correctly back from the area to the ABR that has lost 
> direct connectivity with the backbone. 
>
> Surely this is not correct Quagga behaviour? Does anyone 
> have any suggestions as to what might be wrong? Do I 
> need to make some specific configuration to make this 
> topology work?
>
>   

This is correct behavior for OSPF, and by extension, for Quagga.

What you need to do, is ensure that your Area 0 is always contiguous.
This means you need to add links R1-R3, and R4-R6.

R2 and R5, by definition, never carry, and thus can never send back,
area 0 LSAs, nor external LSAs from other areas. So, if either of the
links R1-R4, or R3-R6, fails, the routers on either end of the link won't
have any way to get the LSAs from the other area.

Adding in the extra area 0 links fixes that.

Brian

> Stephen
>
>        R2
>      /    \
>     /      \    Area 1
>     |      |
>     R1     R3
>     |      |
>     |      |
>   ------------- Area 0
>     |      |
>     |      |
>     R4     R6
>     \      /    Area 2
>      \    /
>        R5
>
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