On Sat, Nov 09, 2013 at 08:36:09PM +0000, Acee Lindem wrote: > On 11/7/13 2:49 PM, "David Lamparter" <[email protected]> wrote: > >The downside is that mixing LSA types opens really nice ways of messing > >it all up with mismatching E-LSAs/LSAs, either by bugs or maliciously. > >Also, race conditions may start popping up if LSAs arrive from flooding > >before E-LSAs. Either way, if a router is identified as E-LSA capable, > >LSAs originated by it should never be used by mode-1 routers. > > I'm not so worried about bugs as I am about carrying the complexity of > mixed-mode LSA complexity forever. If you need to use both LSA types in > the SPF computation, you will have more overhead. I'm inclined to always > use the OLD, aka, LSAs for the SPF computation if any backward > compatibility mode is configured.
[- Unless I'm overlooking something somewhere:] The problem with an old-only compatibility mode is moving from backward compatibility to native E-LSA only operation. Assume you have a network where all routers are in compatibility mode, using old LSAs only but advertising both. Now move one router to native mode. Unless the other routers consider the E-LSAs of that router, it will effectively vanish for them at that point. As a result of this, any operation with 3 modes (old,compat,new) requires mixing of LSA types in SPF calculation. In the presence of both, either can be preferred. Also, as a result from the mixing, any 3-mode compat can exhibit LSA<>E-LSA mismatch problems. For not mixing LSA types in calc, a total of 4 modes are required. Mismatches can still occur - when the middle 2 modes are used. But then again, that should only ever occur while moving from compat to native. Personally, 4-mode seems cleaner to me, but I don't have a strong opinion either way (and lack a foundation to build a strong opinion on.) -David _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
