Hi, Sergey:
ABR-1 advertised the less optimal route information to other routers(in virtual link environment, for avoiding other possible loop scenario, mentioned in G.2 of RFC 2328 as pointed out by Acee), then the other end(ABR-3) of the virtual link should also prefer this less optimal route information. This is similar to your proposal at https://mailarchive.ietf.org/arch/browse/lsr/?gbt=1 <https://mailarchive.ietf.org/arch/browse/lsr/?gbt=1&index=rPZK_euJTkK0TcRhnI-l1sehxRA> &index=rPZK_euJTkK0TcRhnI-l1sehxRA On the other hand, is it necessary to build another virtual link between ABR-2 and ABR-3 in your topology, or else the traffic between ASBR and ABR-3 will be unidirectional? (ASBR can reach the ABR-3, but ABR-3 can’t reach the ASBR?) Best Regards. Aijun Wang China Telecom 发件人: [email protected] [mailto:[email protected]] 代表 Sergey SHpenkov 发送时间: 2020年3月12日 4:31 收件人: Aijun Wang 抄送: [email protected] 主题: Re: [Lsr] Question about OSPF (transit area routing loop) Hi Aijun, ср, 11 мар. 2020 г. в 13:46, Aijun Wang <[email protected]>: Hi, Sergey: If so, ABR-3 should also receive this SumLSA-4 for the ASBR(with cost 300), and then prefer the path via ABR-2 to reach ASBR(with cost 20). Then there will be no loop then? no, because sumLSA-4 in step 16.3 will be less preferred for ABR_3 than previously calculated using the Dijkstra algorithm (with cost 3) Or, how many SumLAS-4 will be advertised by ABR-1? If it selects and advertises only one (3 or 300), then the loop will not be emerged. Currently, it seems it advertises this SumLAS-4 with the cost 300 to RT_1 and with the cost 3 to ABR-3? only one with a cost of 300 Best Regards. Aijun Wang China Telecom 发件人: [email protected] [mailto:[email protected]] 代表 Sergey SHpenkov 发送时间: 2020年2月26日 15:20 收件人: [email protected] 主题: Re: [Lsr] Question about OSPF (transit area routing loop) 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] <mailto:[email protected]> " <[email protected] <mailto:[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: 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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