Peter, > From: Peter Psenak <[email protected]> > Sent: Wednesday, November 9, 2022 2:13 PM > > 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.
I'm a bit surprised since even draft-ppsenak-lsr-igp-ureach-prefix-announce seems to say the contrary: Old nodes: " Existing nodes in a network which receive UPA advertisements will ignore them." New nodes: "As per the definitions referenced in the preceding section, any prefix advertisement with a metric value greater than 0xFE000000 can be used for purposes other than normal routing calculations. Such an advertisement can be interpreted by the receiver as a UPA." https://datatracker.ietf.org/doc/html/draft-ppsenak-lsr-igp-ureach-prefix-announce-01#section-2.1 So draft seems to clearly state that the UPA is a new interpretation leading to a new behavior. To me, the difference of opinions expressed on the list is the following: a) draft-ppsenak-lsr-igp-ureach-prefix-announce is fine with the specific metric value being locally interpreted as UPA even though that's not a standard/global behavior b) multiple other persons on the list have preference for an explicit signaling with a standardized meaning being "UPA" I agree that "a" can be made to work, with a local interpretation through local config or new code. But: - that is nonetheless a change which is non backward compatible with people currently using such high metric without the intention to mean UPA - to differentiate different usages (e.g. your UPA, my other usage such as "graceful shutdown" (still reachable but will disappear soon), endpoint CPU load is 80%...) one would need to use different metric values that would need to be at least locally registered. So why not have the IANA register a flag and avoid each network operator to do that job? In all cases, I don't see a reason for UPA to change the meaning of all the metric values >0xFE000000. You can pick a single value (e.g. 0xFE000001) and that would equally work for your use case. Regards, --Bruno > > > > > 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 > > > Orange Restricted _________________________________________________________________________________________________________________________ Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc pas etre diffuses, exploites ou copies sans autorisation. 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