Chris, > unreachable routes in the IP routing table
I don't see anywhere in the UPA spec even a hint that those unreachable pulses would be installed in the IP routing table. It seems to be a local implementation choice how ISIS or OSPF would inform other protocols about them. In fact the quote you provided specifically "other than building the normal IP routing table" IMO endorses quite verbatim what Peter claims. They can be used for other purposes then building a reachability table. Thx, R. On Sat, Nov 12, 2022 at 6:47 AM Christian Hopps <[email protected]> wrote: > > > > On Nov 9, 2022, at 2:13 PM, Peter Psenak <ppsenak= > [email protected]> wrote: > > > > On 09/11/2022 14:56, David Lamparter wrote: > >> On Wed, Nov 09, 2022 at 01:27:41PM +0000, Acee Lindem (acee) wrote: > >>> I guess I'd like to understand what one would accomplish with further > >>> specification of prefix reachable? What requirement would this > >>> satisfy? For the use case UPA is designed to handle (triggering BGP > >>> PIC or other local action) , I can't see that there would be any case > >>> where you wouldn’t want to take this action for an unreachable prefix. > >> The problem is that a prefix with metric > 0xfe000000 isn't actually an > >> unreachable prefix, it's a prefix that doesn't have specific routing > >> information associated with it, which in turn allows sticking other data > >> into it that might be routing-related but not quite a reachability. > > > > well, that is your interpretation. For me a prefix with metric > > 0xfe000000 is unreachable. Implementations use the max-metric today to > signal the prefix unreachability - to avoid reaching > local/leaked/redistributed prefixes in cases where OL-bit is set on the > originator. So we are not doing anything new here really. > > [as wg-member] > > But his interpretation seems correct. RFC5305 says specifically that the > prefix is not to be used for building the normal IP routing table, that > would include not creating/installing blackhole/reject routes in the normal > IP routing table. > > If a prefix is advertised with a metric larger then MAX_PATH_METRIC > (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered > during the normal SPF computation. This allows advertisement of a > prefix for purposes other than building the normal IP routing table. > > Do the implementations you’re referring to install unreachable routes in > the IP routing table, seemingly in violation of this? > > Thanks, > Chris. > > > >> I vaguely remember several years back we did indeed implement something > >> (seriously no memory on details) that resulted in the creation of a new > >> prefix reachability TLV with some experimental/local sub-TLVs. These > >> prefixes did not exist in the IS-IS domain beforehand. I have no idea > >> what the operational reality is on the existence of such things, but I > >> know that /some/ code exists that does this. > >> To boil this down into the core of the essence and be explicit, > >> - you can create an IS-IS prefix reachability for some arbitrary prefix, > >> and stick > 0xfe000000 into the metric, and that won't have any effect > >> on the existing IS-IS domain > >> - this has in fact been done to carry custom bits of information that > >> for one reason or another were decided to be routing-related and thus > >> make sense to put there > >> - the assumption for the use case is that there are indeed less specific > >> covering prefixes around, providing actual reachability > >> - any setup doing that would now suddenly have fresh "unreachable" > >> semantics attached to something that didn't have them before, which > >> breaks things (or rather: prevents enabling/deployment of the UPA > >> feature) > > > > and why that would be a problem? Such prefix would never be used to for > resolution of the BGP prefix. So the presence of such unreachable prefix > would never trigger any action even of the UPA processing was enabled on > the receiver. I don't see a problem. > > > >> - (if those extra prefixes are created with 0xffffffff metric, a > >> configurable >= limit for UPA does not help either.) > > > > again, what is the problem? > > > >> Making IS-IS UPA explicit with a bit, sub-TLV, or whatever else is > >> (IMHO) not a significant cost, and completely eliminates this issue. > >> The only reason against it (that I can think of) is that the > >> advertisement might be a little bit larger; a new sub-TLV or flag bit > >> should be completely invisible to existing implementations (= I don't > >> see how this would create compatibility or rollout problems.) > > > > I'm afraid, you forgot to consider an operational aspect of the solution. > > > > thanks, > > Peter > > > >> Cheers, > >> -David > > > > _______________________________________________ > > 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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