Aijun,
On 15/06/2022 12:12, Aijun Wang wrote:
Hi, Peter:
If you use LSInfinity as the indicator of the prefixes unreachable, then
how about you solve the situations that described in
https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.4
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.4>,
in which the the metric in parent TLV MUST be set to LSInfinity?
if the IP Algorithm Prefix Reachability Sub-TLV is present the metric
from that Sub-TLV is used instead. There is no problem.
Will you consider all such prefixes unreachable? This is certainly not
the aim of the IP FlexAlgo document.
In conclusion, the prefixes unreachable information should be indicated
explicitly by other means, as that introduced in the PUA draft.
the meaning of LSInfinity has been defined decades ago. No matter how
much you may not like it, but it means unreachable.
thanks,
Peter
Aijun Wang
China Telecom
On Jun 15, 2022, at 17:09, Peter Psenak
<[email protected]> wrote:
Hi Gunter,
On 15/06/2022 11:02, Van De Velde, Gunter (Nokia - BE/Antwerp) wrote:
Hi Robert,
I agree with you that the operator problem space is not limited to
multi-area/levels with IGP summarisation.
With the PUA/UPA proposals I get the feeling that LSR WG is jumping
into the deep-end and is re-vectoring the IGP to carry opaque
information not used for SPF/cSPF.
I believe we should be conservative for such and if LSR WG progresses
with such decision.
please note that UPA draft builds on existing protocol specification
defined in RFC5305 and RFC5308 that allow the metric larger then
MAX_PATH_METRIC to be used "for purposes other than building the
normal IP routing table". We are just documenting one of them.
thanks,
Peter
It could very well be that re-vectoring is the best solution, but I
guess we need to agree first on understanding the operator problem space.
G/
*From:*Robert Raszuk <[email protected]>
*Sent:* Tuesday, June 14, 2022 11:51 AM
*To:* Van De Velde, Gunter (Nokia - BE/Antwerp)
<[email protected]>
*Cc:* lsr <[email protected]>;
[email protected];
draft-wang-lsr-prefix-unreachable-annoucement
<[email protected]>
*Subject:* Re: [Lsr] Thoughts about PUAs - are we not over-engineering?
Hello Gunter,
I agree with pretty much all you said except the conclusion - do
nothing :).
To me if you need to accelerate connectivity restoration upon an
unlikely event like a complete PE failure the right vehicle to signal
this is within the service layer itself. Let's keep in mind that
links do fail a lot in the networks - routers do not (or they do it
is multiple orders of magnitude less frequent event). Especially
links on the PE-CE boundaries do fail a lot.
Removal of next hop reachability can be done with BGP and based on
BGP native recursion will have the exact same effect as presented
ideas. Moreover it will be stateful for the endpoints which again to
me is a feature not a bug.
Some suggested to define a new extension in BGP to signal it even
without using double recursion - well one of them has been proposed
in the past -
https://www.ietf.org/archive/id/draft-raszuk-aggr-withdraw-00.txt
<https://www.ietf.org/archive/id/draft-raszuk-aggr-withdraw-00.txt>
At that time the feedback received was that native BGP withdraws are
fast enough so no need to bother. Well those native withdrawals are
working today as well as some claim that specific implementations can
withdraw RD:* when PE hosting such RDs fail and RDs are allocated in
a unique per VRF fashion.
Then we have the DROID proposal which again may look like overkill
for this very problem, but if you consider the bigger picture of what
networks control plane pub-sub signalling needs, it establishes the
foundation for such.
Many thanks,
Robert
On Tue, Jun 14, 2022 at 10:59 AM Van De Velde, Gunter (Nokia -
BE/Antwerp) <[email protected]
<mailto:[email protected]>> wrote:
Hi All,
When reading both proposals about PUA's:
* draft-ppsenak-lsr-igp-ureach-prefix-announce-00
* draft-wang-lsr-prefix-unreachable-annoucement-09
The identified problem space seems a correct observation, and indeed
summaries hide remote area network instabilities. It is one of the
perceived benefits of using summaries. The place in the network
where this hiding takes the most impact upon convergence is at
service nodes (PE's for L3/L2/transport) where due to the
summarization its difficult to detect that the transport tunnel
end-point suddenly becomes unreachable. My concern however is if it
really is a problem that is worthy for LSR WG to solve.
To me the "draft draft-wang-lsr-prefix-unreachable-annoucement-09"
is not a preferred solution due to the expectation that all nodes in
an area must be upgraded to support the IGP capability. From this
operational perspective the draft
"draft-ppsenak-lsr-igp-ureach-prefix-announce-00" is more elegant,
as only the A(S)BR's and particular PEs must be upgraded to support
PUA's. I do have concerns about the number of PUA advertisements in
hierarchically summarized networks (/24 (site) -> /20 (region) ->
/16 (core)). More specific, in the /16 backbone area, how many of
these PUAs will be floating around creating LSP LSDB update churns?
How to control the potentially exponential number of observed PUAs
from floating everywhere? (will this lead to OSPF type NSSA areas
where areas will be purged from these PUAs for scaling stability?)
Long story short, should we not take a step back and re-think this
identified problem space? Is the proposed solution space not more
evil as the problem space? We do summarization because it brings
stability and reduce the number of link state updates within an
area. And now with PUA we re-introduce additional link state updates
(PUAs), we blow up the LSDB with information opaque to SPF best-path
calculation. In addition there is suggestion of new state-machinery
to track the igp reachability of 'protected' prefixes and there is
maybe desire to contain or filter updates cross inter-area
boundaries. And finally, how will we represent and track PUA in
the RTM?
What is wrong with simply not doing summaries and forget about these
PUAs to pinch holes in the summary prefixes? this worked very well
during last two decennia. Are we not over-engineering with PUAs?
G/
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