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

Hi Chris, 

On 3/8/20, 7:26 AM, "Lsr on behalf of Christian Hopps" <lsr-bounces@ietf.org on behalf of chopps@chopps.org> wrote:

    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?

Based on RFC 2328, section 16.4.1, intra-area paths through non-backbone areas are preferred over backbone paths. 
This counter intuitive preference was added way back in the late 90s to resolve a different routing loop with virtual links (refer to RFC 2328, Appendix G.2). Sigh - if we ever did OSPFv4, I'd remove virtual links. 

Thanks,
Acee

    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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