Re: [Lsr] https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05
Peter Psenak <ppsenak@cisco.com> Tue, 09 March 2021 11:12 UTC
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To: Robert Raszuk <robert@raszuk.net>
Cc: Tony Li <tony.li@tony.li>, Gyan Mishra <hayabusagsm@gmail.com>, Aijun Wang <wangaijun@tsinghua.org.cn>, Aijun Wang <wangaj3@chinatelecom.cn>, lsr <lsr@ietf.org>, "Acee Lindem (acee)" <acee@cisco.com>, draft-wang-lsr-prefix-unreachable-annoucement <draft-wang-lsr-prefix-unreachable-annoucement@ietf.org>
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Tue, 09 Mar 2021 12:12:55 +0100
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Subject: Re: [Lsr] https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05
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Hi Robert, On 09/03/2021 12:02, Robert Raszuk wrote: > Hey Peter, > > Well ok so let's forget about LDP - cool ! > > So IGP sends summary around and that is all what is needed. > > So the question why not propage information that PE went down in service > signalling - today mainly BGP. because BGP signalling is prefix based and as a result slow. > > > And forget BFD, does not scale with 10k PEs. > > You missed the point. No one is proposing full mesh of BFD sessions > between all PEs. I hope so at least. > > PE is connected to RRs so you need as many BFD sessions as RR to PE BGP > sessions. that can be still too many. In addition you may have a hierarchical RR, which would still involve BGP signalling. Once that session is brought down RR has all it needs to > trigger a message (withdraw or implicit withdraw) to remove the > broken service routes in a scalable way. that is the whole point, you need something that is prefix independent. thanks, Peter > > Thx, > R. > > PS. Yes we still need to start support signalling of unreachability in > BGP itself when BGP is used for underlay but this is a bit different use > case and outside of scope of LSR > > > On Tue, Mar 9, 2021 at 11:55 AM Peter Psenak <ppsenak@cisco.com > <mailto:ppsenak@cisco.com>> wrote: > > Robert, > > On 09/03/2021 11:47, Robert Raszuk wrote: > > > You’re trying to fix a problem in the overlay by morphing the > > underlay. How can that seem like a good idea? > > > > I think this really nails this discussion. > > > > We have discussed this before and while the concept of signalling > > unreachability does seem useful such signalling should be done > where it > > belongs. > > > > Here clearly we are talking about faster connectivity restoration > for > > overlay services so it naturally belongs in overlay. > > > > It could be a bit misleading as this is today underlay which > propagates > > reachability of PEs and overlay relies on it. And to scale, > > summarization is used hence in the underlay, failing remote PEs > remain > > reachable. That however in spite of many efforts in lots of > networks are > > really not the practical problem as those networks still relay on > exact > > match of IGP to LDP FEC when MPLS is used. So removal of /32 can and > > does happen. > > think SRv6, forget /32 or /128 removal. Think summarization. > > I'm not necessary advocating the solution proposed in this particular > draft, but the problem is valid. We need fast detection of the PE loss. > > And forget BFD, does not scale with 10k PEs. > > thanks, > Peter > > > > > > > In the same time BGP can pretty quickly (milliseconds) > remove affected > > service routes (or rather paths) hence connectivity can be > restored to > > redundantly connected endpoints in sub second. Such removal can > be in a > > form of atomic withdraw (or readvertisement), removal of recursive > > routes (next hop going down) or withdraw of few RD/64 prefixes. > > > > I am not convinced and I have not seen any evidence that if we > put this > > into IGP it will be any faster across areas or domains (case of > > redistribution over ASBRs to and from IGP to BGP). One thing for > sure - > > it will be much more complex to troubleshoot. > > > > Thx, > > R. > > > > On Tue, Mar 9, 2021 at 5:39 AM Tony Li <tony.li@tony.li > <mailto:tony.li@tony.li> > > <mailto:tony.li@tony.li <mailto:tony.li@tony.li>>> wrote: > > > > > > Hi Gyan, > > > > > Gyan> In previous threads BFD multi hop has been > mentioned to > > track IGP liveliness but that gets way overly complicated > especially > > with large domains and not viable. > > > > > > This is not tracking IGP liveness, this is to track BGP endpoint > > liveness. > > > > Here in 2021, we seem to have (finally) discovered that we can > > automate our management plane. This ameliorates a great deal of > > complexity. > > > > > > > Gyan> As we are trying to signal the IGP to trigger the > > control plane convergence, the flooding machinery in the IGP > already > > exists well as the prefix originator sub TLV from the link or > node > > failure. IGP seems to be the perfect mechanism for the control > > plane signaling switchover. > > > > > > You’re trying to fix a problem in the overlay by morphing the > > underlay. How can that seem like a good idea? > > > > > > > Gyan>As I mentioned advertising flooding of the longer > > prefix defeats the purpose of summarization. > > > > > > PUA also defeats summarization. If you really insist on faster > > convergence and not building a sufficiently redundant > topology, then > > yes, your area will partition and you will have to pay the > price of > > additional state for your longer prefixes. > > > > > > > In order to do what you are stating you have to remove the > > summarization and go back to domain wide flooding > > > > > > No, I’m suggesting you maintain the summary and ALSO > advertise the > > longer prefix that you feel is essential to reroute immediately. > > > > > > > which completely defeats the goal of the draft which is to > make > > host route summarization viable for operators. We know the > prefix > > that went down and that is why with the PUA negative > advertisement > > we can easily flood a null0 to block the control plane from > > installing the route. > > > > > > So you can also advertise the more specific from the > connected ABR… > > > > > > > We don’t have any prior knowledge of the alternate for the > egress > > PE bgp next hop attribute for the customer VPN overlay. So > the only > > way to accomplish what you are asking is not do any summarization > > and flood al host routes. Of course as I stated defeats the > > purpose of the draft. > > > > > > Please read again. > > > > Tony > > > > _______________________________________________ > > Lsr mailing list > > Lsr@ietf.org <mailto:Lsr@ietf.org> <mailto:Lsr@ietf.org > <mailto:Lsr@ietf.org>> > > https://www.ietf.org/mailman/listinfo/lsr > > >
- [Lsr] https://tools.ietf.org/html/draft-wang-lsr-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Li
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Li
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Acee Lindem (acee)
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Li
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Li
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Li
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Przygienda
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Przygienda
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Robert Raszuk
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Tony Przygienda
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Acee Lindem (acee)
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Shraddha Hegde
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Acee Lindem (acee)
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Aijun Wang
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Gyan Mishra
- Re: [Lsr] https://tools.ietf.org/html/draft-wang-… Peter Psenak