On 30/05/2025 02:46, Aijun Wang wrote:
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.
that is not correct. They are processed during the SPF and they have a
special meaning defined by the protocol specification - e.g. they
represent unreachability.
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.
UPA explicitly signals unreachability of the prefix that is covered by
the summary prefix reachability advertisement.
UPA withdrawal removes the explicitly signaled unreachability of the
prefix, making it reachable by the summary prefix reachability
advertisement.
Peter
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:*[email protected]
[mailto:[email protected]] *On Behalf Of *Robert Raszuk
*Sent:* Friday, May 30, 2025 7:18 AM
*To:* Aijun Wang <[email protected]>
*Cc:* Gunter van de Velde <[email protected]>; Peter
Psenak <[email protected]>; [email protected]
*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 advertise explicitly “withdrawn” prefixes , not just stopping sending the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 <[email protected]>
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 <[email protected]>
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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