Hi, Robert:
Answers to your comments are inline below. Best Regards Aijun Wang China Telecom From: [email protected] <[email protected]> On Behalf Of Robert Raszuk Sent: Wednesday, October 13, 2021 3:52 AM To: Acee Lindem (acee) <[email protected]> Cc: [email protected] Subject: Re: [Lsr] "Prefix Unreachable Announcement" and "IS-IS and OSPF Extension for Event Notification" Hi Acee, I would like to make a comment on your point #1. You said: "Is this a problem that needs to be solved in the IGPs? The use case offered in both drafts is signaling unreachability of a BGP peer. Could this better solved with a different mechanism (e.g., BFD)..." That is not a very precise statement - simply due to the fact that to signal anything in any solution requires a peer to detect giben network event. In both cases such an event will likely be detected using BFD (if we are talking about unreachability of a BGP or IGP peer). [WAJ] No. Link or Node Failures are not detected via BFD within one IGP domain. It just rely on the normal IGP flooding procedures. Once the ABRs receive the updated LSA, they will make the calculation and find the missing prefixes. Neither of the drafts mentioned here replaces local BFD detection. [WAJ] Deploy such mechanisms can replace the BFD for BGP configuration between PEs that located in different IGP domains. I do infer from your message that what was meant to be said was an alternative to support BFD sessions across areas/levels for example between PEs. Clearly that would get very ugly very quickly. However IMO the natural signalling in this case would be to use BGP itself. Here are the few reasons: [WAJ] How the BGP peer be detected that the peer has been unreachable? Via BFD for BGP? * detection time will be the same for IGP or BGP [WAJ] It is the ABR’s summarization action hides the unreachable prefixes, then BGP peers located in different IGP domains can’t know the failure as quickly as IGP within the same domain. * only those network elements which keep "interesting" state will be notified [WAJ] To be more accurate, your above statement should be 【* only those network elements which keep "interesting" state will act upon the PUA messages.】 * speed of withdraw can be argued to be as fast as IGP flooding especially considering hierarchical IGP design [WAJ] When we let ABR send the PUA messages, not hide them, the speed of withdraw can certainly be accelerated. * withdraws can be easily aggregated (when we loose PE single prefix can be used to remove all paths advertised by given PE) [WAJ] This is the effects PUA mechanism. when the BGP peer receives the PUA message that includes the PE itself, all the path advertised by the given PE can certainly be removed. * withdraws can be injected as the next hop /32 or /128 prefixes and remote next hop validation can be set not to consider less specific routes to resolve next hops (in any case due to MPLS data plane host routes are used in many networks today to resolve BGP service routes to LSPs in spite of efforts to make this more scalable). [WAJ]This the effect of PUA mechanism. Only when we prove that BGP based solutions are not sufficient we could/should explore moving such signalling to IGPs. [WAJ] Do the above responses prove the BGP based solution are not sufficient? Kind regards, R. On Tue, Oct 12, 2021 at 9:06 PM Acee Lindem (acee) <[email protected] <mailto:[email protected]> > wrote: Speaking as WG Chairs: The authors of “Prefix Unreachable Announcement” have requested an adoption. The crux of the draft is to signal unreachability of a prefix across OSPF or IS-IS areas when area summarization is employed and prefix is summarised. We also have “IS-IS and OSPF Extension for Event Notification” which can be used to address the same use case. The drafts take radically different approaches to the problem and the authors of both drafts do not wish to converge on the other draft’s method so it is understandable that merging the drafts really isn’t an option. Before an adoption call for either draft, I’d like to ask the WG: 1. Is this a problem that needs to be solved in the IGPs? The use case offered in both drafts is signaling unreachability of a BGP peer. Could this better solved with a different mechanism (e.g., BFD) rather than flooding this negative reachability information across the entire IGP domain? 2. Assuming we do want to take on negative advertisement in the IGP, what are the technical merits and/or detriments of the two approaches? We’ll reserve any further discussion to “WG member” comments on the two approaches. Thanks, Acee and Chris _______________________________________________ Lsr mailing list [email protected] <mailto:[email protected]> https://www.ietf.org/mailman/listinfo/lsr
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