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

"Acee Lindem (acee)" <acee@cisco.com> Wed, 11 March 2020 19:29 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Aijun Wang <wangaijun@tsinghua.org.cn>, 'Sergey SHpenkov' <sergey.v.shpenkov@gmail.com>, "lsr@ietf.org" <lsr@ietf.org>
Thread-Topic: [Lsr] 答复: Question about OSPF (transit area routing loop)
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Date: Wed, 11 Mar 2020 19:29:34 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/lsr/AgrXgWUxY3d2RP6x5fzdB_Yn5F4>
Subject: Re: [Lsr] 答复: Question about OSPF (transit area routing loop)
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Hi Aijun,
Yes – it seems ASBR-2 should be removed from the diagram.
Thanks,
Acee

From: Lsr <lsr-bounces@ietf.org> on behalf of Aijun Wang <wangaijun@tsinghua.org.cn>
Date: Wednesday, March 11, 2020 at 6:48 AM
To: 'Sergey SHpenkov' <sergey.v.shpenkov@gmail.com>, "lsr@ietf.org" <lsr@ietf.org>
Subject: [Lsr] 答复: Question about OSPF (transit area routing loop)

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?

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?


Best Regards.

Aijun Wang
China Telecom

发件人: lsr-bounces@ietf.org [mailto:lsr-bounces@ietf.org] 代表 Sergey SHpenkov
发送时间: 2020年2月26日 15:20
收件人: lsr@ietf.org
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) <acee@cisco.com<mailto: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<mailto:lsr-bounces@ietf.org>> on behalf of Sergey SHpenkov <sergey.v.shpenkov@gmail.com<mailto:sergey.v.shpenkov@gmail.com>>
Date: Tuesday, February 25, 2020 at 2:38 PM
To: "lsr@ietf..org<mailto:lsr@ietf.org>" <lsr@ietf.org<mailto: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:
[cid:image001.png@01D5F7B9.DD7F1000]
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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