Peter, > you only have two paths to reach any node.
Who says that you must be limited to two paths only ? Why not create a flooding graph such that flooding will happen over 4 paths as opposed to flooding over 16 or 32 today without optimization. And if you are worried that you loose *wisely selected* all 4 paths before you manage to distribute new flooding topology you can always flood over 6 :) Best, R. On Tue, Mar 5, 2019 at 9:17 PM Peter Psenak <[email protected]> wrote: > Robert, > > On 05/03/2019 20:12 , Robert Raszuk wrote: > > > >> 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 ... > > we are talking about the control plane convergence, not data plane one. > If the flooding topology is subset of the real topology, then at the > flooding level you don't have all the ECMPs available - you only have > two paths to reach any node. In such case it is possible that the > flooding topology gets partitioned and you want to get out of that state > quickly, as you may get out of sync with the the reality and eventually > loose all the data plane ECMPs as a consequence. > > thanks, > Peter > > > > > > > On Tue, Mar 5, 2019 at 5:42 PM Tony Przygienda <[email protected] > > <mailto:[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] > > <mailto:[email protected]>> wrote: > > > > Hi Tony, > > > > On 05/03/2019 17:16 , [email protected] <mailto:[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] <mailto:[email protected]> > > https://www.ietf.org/mailman/listinfo/lsr > > > > _______________________________________________ > > Lsr mailing list > > [email protected] <mailto:[email protected]> > > https://www.ietf.org/mailman/listinfo/lsr > > > >
_______________________________________________ Lsr mailing list [email protected] https://www.ietf.org/mailman/listinfo/lsr
