[Lsr] Prefix Unreachable Announcement Use Cases

"Acee Lindem (acee)" <acee@cisco.com> Sat, 14 November 2020 21:33 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: "lsr@ietf.org" <lsr@ietf.org>
Thread-Topic: Prefix Unreachable Announcement Use Cases
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Date: Sat, 14 Nov 2020 21:33:03 +0000
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Subject: [Lsr] Prefix Unreachable Announcement Use Cases
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Speaking as WG member…

With respect to the use cases in section 3:

  3.1 Inter-Area Node Failure Scenario – With respect to this use case, the node in question is actually unreachable. In this case, the ABRs will normally install a reject route for the advertised summary and will send an ICMP unreachable when the packets are received for the unreachable prefix. This is the expected behavior and there really isn’t that much of advantage to move the point of unreachable detection a couple hops closer. If faster detection is required, BFD or other mechanisms are available.

  3.3 Intra-Area Node Failure Scenario – In the first place, multiple areas with overlapping summaries is just a bad network design. If the prefix is unreachable, the case digresses to getting the ICMP unreachable from the ABR with the invalid overlapping summary.

3.2 Inter-Area Links Failure Scenario – This is the case where the prefix is reachable but only through a subset of the area ABRs. This is really the only valid use case. IMO, it is better to solve this case with intra-area tunnels through the backbone as described in section 6.1. I think this is preferable to the complexity proposed in this draft and especially section 6. It is “interesting” when non-implementors specify implementation details.

Thanks,
Acee