On 30/05/2025 11:24, Aijun Wang wrote:
Hi, Peter:
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Peter Psenak
*Sent:* Friday, May 30, 2025 4:08 PM
*To:* Aijun Wang <[email protected]>; 'Robert Raszuk'
<[email protected]>
*Cc:* 'Gunter van de Velde' <[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
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.
[WAJ]: Please see RFC 2328. “If the cost specified by the LSA is
LSInfinity, or if the LSA's LS age is equal to MaxAge, then examine
the the next LSA.”
That does not mean the LSA is ignored during the processing. It means
the prefix in the LSA becomes reachable. If it was reachable before, it
must me made unreachable and removed from the forwarding. If we follow
the logic that you trying to impose you would never be able to make a
reachable prefix unreachbale, because you would ignore the LSA that is
trying to make it unreachable.
Same applies when the LSA was advertised with the LSInfinity and that
LSA is removed (e.g. MaxAged) - that means that the unreachability that
was advertised previously is not announced anymore.
I'm done with this thread now.
Peter
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.
[WAJ]: Please see the example that described by Gunter and my
responses:
https://mailarchive.ietf.org/arch/msg/lsr/HnRhGkekX7aDRLxIAZ0Qim1dufI/
In the example, when ABR stops advertising UPA after the
configured time(to let R1 finish the BGP PIC FRR process),
20.20.20.2/32 is still unreachable.
The summary address 20.20.20.0/24 from ABR gives still the
wrong information.
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]
<mailto:[email protected]>] *On Behalf Of *Robert Raszuk
*Sent:* Friday, May 30, 2025 7:18 AM
*To:* Aijun Wang <[email protected]>
<mailto:[email protected]>
*Cc:* Gunter van de Velde <[email protected]>
<mailto:[email protected]>; Peter Psenak
<[email protected]> <mailto:[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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