[Lsr] draft-ppsenak-lsr-igp-ureach-prefix-announce

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Hi Peter, authors, all

Thanks for the draft. I find it a useful contribution to the problem space.

IMHO the use of MAX_PATH_METRIC is a good idea in particular since it can be made backward compatible and provides incremental benefits with incremental deployment.

I also have two disagreements on the current draft. Both are minor IMO, but authors may have a different opinion.


  1.  This draft defines a new signaling from an egress ABR to all ingress ABR/PE. As such, this require all these nodes to agree on this signaling. So I'd call for a STD track document.
  2.  IMO, the behavior defined in this draft is not backward compatible with the specification of MAX_PATH_METRIC in an IP prefix.

RFC5305 says:
   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.

As per the above, one (ABR) may (is allowed and free to do so) already advertise both an aggregate prefix IP1/N with a regular metric and a more specific prefix IP2/32 with MAX_PATH_METRIC.
With the above advertisement, all nodes compliant with RFC 5305 will install IP1/N in their FIB and not consider IP2/32 during their SPF and as a consequence not install it in their FIB.
In term of reachability, all nodes have IP reachability to all IP @ in IP1 including IP2.

If one node now implements the draft, it will remove reachability for IP2. And hence all my BGP routes using IPv2 for next-hop will be removed.
This looks clearly like a change in behavior to me, plus one which introduce loss of reachability in an existing network.

3) The solution that I would suggest is:
- advertise the "unreachable prefix" with metric MAX_PATH_METRIC
- define a new  "Extended Reachability Attribute Flags" ([RFC 7794]) explicitly signaling that the reachability to this prefix has been lost:

    Unreachable Flag (U_flag). Set if this prefix is to be considered unreachable.

This would allow for explicit signaling of the reachability (vs implicit currently) and would be backward compatible with existing specification and deployments.

Regards,
--Bruno


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