Hi all,
With RC5305, in IS-IS we can advertise two states for a prefix IP1:
* Positive reachability (state "1"), by advertising IP1 in TLV 135 with a
metric lower than 0xFE000000
* No reachability (state "0") by either:
* Not advertising IP1 in TLV 135
* Advertising IP1 in TLV 135 with a metric larger than 0xFE000000
For unreachable prefix announcement, we need to advertise a new state: negative
reachability (state "-1") because advertising no reachability (state "0") is
not enough since there is a covering prefix advertised.
Hence we need a new signaling.
draft-ppsenak-lsr-igp-ureach-prefix-announce is proposing to advertise IP1
unreachability by advertising IP in TLV 135 with a metric larger than
0xFE000000.
This does not work as this signal is already defined to advertise state "0" (no
reachability) while we want to advertise negative reachability (state "-1"):
one can't signal two different states with a single signal.
This seems easy to fix in the last version of the draft: one just have to
signal state "-1" with the flags already defined and advertised.
In short, negative reachability would be signal by advertising IP1 in TLV 135
with a metric larger than 0xFE000000 and flag U (Unreachable Prefix) or UP
(Unreachable Planned Prefix) set.
On the receiving side, state "-1" is learned from the reception of either flag.
This is a very simple change, and it would address my concern.
Can this be done?
The drawback is that the existing pre-standard implementation would need to be
updated. But that's the rule for a pre-standard implementation.
The faster we do the change, the easier it is, especially for other
implementations.
--Bruno
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