Re: [Lsr] Question about OSPF (transit area routing loop)

Why does ABR_1 create a summary LSA to ASBR using the more expensive non-backbone path? This would seem to violate 12.4.3:

    Else, if the destination of this route is an AS boundary
    router, a summary-LSA should be originated if and only
    if the routing table entry describes the preferred path
    to the AS boundary router (see Step 3 of Section 16.4).
    If so, a Type 4 summary-LSA is originated for the
    destination, with Link State ID equal to the AS boundary
    router's Router ID and metric equal to the routing table
    entry's cost. Note: these LSAs should not be generated
    if Area A has been configured as a stub area.

But even so, why is ABR_1 even looking at a cost 300 path to summarize, why would it not be looking at the cost 1 path it has through the backbone? That's the path that ABR_3 has chosen when it forwarded towards ABR_1 through RT_1, right?

Thanks,
Chris.

> On Feb 26, 2020, at 2:19 AM, Sergey SHpenkov <sergey.v.shpenkov@gmail.com> wrote:
> 
> Acee,
> 
> Because ABR_1 creates SumLSA-4 for the ASBR not from the backbone area. The cost of SumLSA-4 for ASBR is 300.
> 
> Thanks,
> Sergey
> 
> вт, 25 февр. 2020 г. в 22:44, Acee Lindem (acee) <acee@cisco.com>:
> Hi Sergey,
> 
> I don’t see why RT_1 wouldn’t go through ABR_1 to get to the ASBR.
> 
> Thanks,
> 
> Acee
> 
>  
> 
> From: Lsr <lsr-bounces@ietf.org> on behalf of Sergey SHpenkov <sergey.v.shpenkov@gmail.com>
> Date: Tuesday, February 25, 2020 at 2:38 PM
> To: "lsr@ietf..org" <lsr@ietf.org>
> Subject: [Lsr] Question about OSPF (transit area routing loop)
> 
>  
> 
> Hi,
> 
> In section 16.3 of the OSPF RFC 2328 standard, it is stated that all ABR routers
> 
> connected to a transit area are required to check the sumLSA contained within
> 
> this area in order to possibly improve the intra-area and inter-area backbone routes
> 
> for themselves.
> 
> 
> See the picture:
> <image001.png>
> The RT_1 and ABR_3 routers will use different paths to the ASBR router:
> 
> ABR_3 -> RT_1 -> ABR_1 -> ASBR = cost 3
> RT_1 -> ABR_3 -> ABR_2 -> ASBR = cost 21
> 
> route loop between RT_1 and ABR_3
> 
> Please explain this situation
> 
> Thanks,
> Sergey
> 
>  
> 
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