Hi Chris,
On 3/8/20, 7:26 AM, "Lsr on behalf of Christian Hopps" <[email protected]
on behalf of [email protected]> 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
<[email protected]> 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) <[email protected]>:
> Hi Sergey,
>
> I don’t see why RT_1 wouldn’t go through ABR_1 to get to the ASBR.
>
> Thanks,
>
> Acee
>
>
>
> From: Lsr <[email protected]> on behalf of Sergey SHpenkov
<[email protected]>
> Date: Tuesday, February 25, 2020 at 2:38 PM
> To: "[email protected]" <[email protected]>
> 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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