Hi, Robert:
Yes, in link state protocols, when the LSP/LSA is updated, the SPF will run again, the node will recalculate the RIB, and exclude the missing prefixes. But for UPA, the situation is different: 1) The LSP/LSA that include UPA doesn’t participate the SPF calculation. 2) There are at least two reasons that can lead UPA disappearing [1], which is to say, the missing of UPA doesn’t represent the specific prefix is reachable again. Then, for UPA, the explicit withdraw procedure, which indicates the specific prefix is back again, is necessary. [1]: https://mailarchive.ietf.org/arch/msg/lsr/s1I2Fj7kcYm85CwwYBURYL8RPQE/ Best Regards Aijun Wang China Telecom From: forwardingalgori...@ietf.org [mailto:forwardingalgori...@ietf.org] On Behalf Of Robert Raszuk Sent: Friday, May 30, 2025 7:18 AM To: Aijun Wang <wangai...@tsinghua.org.cn> Cc: Gunter van de Velde <gunter.van_de_ve...@nokia.com>; Peter Psenak <ppse...@cisco.com>; lsr@ietf.org Subject: [Lsr] Re: 答复: 答复: 答复: Re: 答复: I-D Action: draft-ietf-lsr-igp-ureach-prefix-announce-06.txt Hi Aijun, > How to revoke the UPA explicitly when the prefix is reachable again? In link state protocols when LSP/LSA is readvertised without UPA that is equivalent to withdrawing it - but I think Peter already indicated that at least twice here. > Please note “stopping sending UPA” is not equal to “revoking the UPA”. > For example, in BGP, when you want to revoke some prefixes, you will > advertise explicitly “withdrawn” prefixes , not just stopping sending the > related BGP Updates. Yes it is very different in distance vector protocols ... I don't think LSR can't help with that :( Thx, R. On Fri, May 30, 2025 at 1:09 AM Aijun Wang <wangai...@tsinghua.org.cn <mailto:wangai...@tsinghua.org.cn> > wrote: Hi, Robert: We are discussing how and when to back to the normal state before UPA is triggered, not how to configure BGP active/backup via Local_Pref Attribute. Or, let’s change the statement in more general manner: How to revoke the UPA explicitly when the prefix is reachable again? Please note “stopping sending UPA” is not equal to “revoking the UPA”. For example, in BGP, when you want to revoke some prefixes, you will advertise explicitly “withdrawn” prefixes , not just stopping sending the related BGP Updates. Aijun Wang China Telecom On May 29, 2025, at 18:33, Robert Raszuk <rob...@raszuk.net <mailto:rob...@raszuk.net> > wrote: Once that’s done, the ABR can safely withdraw the UPA, and the network remains stable (i.e. from R1 perspective the backup egress router became the new primary egress router once BGP converged because session with R2 failed). [WAJ] Then, R1 will keep using the backup egress router forever? When, how and what trigger the R1 switchback to the original egress router? Even without UPA at all in the picture if operators chooses active/backup scheme (as opposed to active/active model) for multihomed sites or networks typically BGP paths carry properly set LOCAL_PREF attribute. UPA does not have anything to do with it. Thx, R.
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