> Slow convergence is obviously not a good thing Could you please kindly elaborate why ?
With tons of ECMP in DCs or with number of mechanism for very fast data plane repairs in WAN (well beyond FRR) IMHO any protocol *fast convergence* is no longer a necessity. Yet many folks still talk about it like the only possible rescue ... On Tue, Mar 5, 2019 at 5:42 PM Tony Przygienda <[email protected]> wrote: > in practical terms +1 to Peter's take here ... Unless we're talking tons > of failures simultaneously (which AFAI talked to folks are not that common > but can sometimes happen in DCs BTW due to weird things) smaller scale > failures with few links would cause potentially diffused "chaining" of > convergence behavior rather than IGP-style fast healing (and on top of that > I didn't see a lot of interest in formalizing a rigorous distributed > algorithm which IMO would be necessary to ensure ultimate convergence when > only one/subset of links is used). Slow convergence is obviously not a good > thing unless we assume people will run FRR with its complexity in DC and/or > no more than one link every fails which seems to me bending assumptions to > whatever solution is available/preferred. To Tony's point though, on large > scale failures enabling all links would cause heavy flood load, yes, but in > a sense it's the "initial bootup" case anyway (especially in centralized > case) since nodes need all topology to make informed correct decisions > about what the FT should be if they don't rely on whatever the centralized > instance thinks (which they won't be able to do given the FT from > centralized instance will indicate lots links that are "gone" due to > failure). As to p2p, I suggest to agree whether you use dense mesh (DC) > case or sparse mesh (WAN) case or "every topology imaginable" since that > drives lots design trade-offs. > > my 2.71828182 cents ;-) > > --- tony > > On Tue, Mar 5, 2019 at 8:27 AM Peter Psenak <[email protected]> wrote: > >> Hi Tony, >> >> On 05/03/2019 17:16 , [email protected] wrote: >> > >> > Peter, >> > >> >>> (a) Temporarily add all of the links that would appear to remedy >> the partition. This has the advantage that it is very likely to heal the >> partition and will do so in the minimal amount of convergence time. >> >> >> >> I prefer (a) because of the faster convergence. >> >> Adding all links on a single node to the flooding topology is not >> going to cause issues to flooding IMHO. >> > >> > >> > Could you (or John) please explain your rationale behind that? It seems >> counter-intuitive. >> >> it's limited to the links on a single node. From all the practical >> purposes I don't expect single node to have thousands of adjacencies, at >> least not in the DC topologies for which the dynamic flooding is being >> primary invented. >> >> In the environments with large number of adjacencies (e.g. >> hub-and-spoke) it is likely that we would have to make all these links >> part of the flooding topology anyway, because the spoke is typically >> dual attached to two hubs only. And the incremental adjacency bringup is >> something that an implementation may already support. >> >> > >> > >> > >> >> given that the flooding on the LAN in both OSPF and ISIS is done as >> multicast, there is currently no way to enable flooding, either permanent >> or temporary, towards a subset of the neighbors on the LAN. So if the >> flooding is enabled on a LAN it is done towards all routers connected to >> the it.. >> > >> > >> > Agreed. >> > >> > >> >> Given that all links between routers are p2p these days, I would vote >> for simplicity and make the LAN always part of the FT. >> > >> > >> > I’m not on board with this yet. Our simulations suggest that this is >> not necessarily optimal. There are lots of topologies (e..g., parallel >> LANs) where this blanket approach is suboptimal. >> >> the question is how much are true LANs used as transit links in today's >> networks. >> >> thanks, >> Peter >> >> > >> > Tony >> > >> > . >> > >> >> _______________________________________________ >> Lsr mailing list >> [email protected] >> https://www.ietf.org/mailman/listinfo/lsr >> > _______________________________________________ > Lsr mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/lsr >
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