Bruno,
On 25/07/2023 14:39, [email protected] wrote:
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:
o Not advertising IP1 in TLV 135
o Advertising IP1 in TLV 135with a metric larger than 0xFE000000
right, one being implicit the other one explicit.
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 135with a metric larger than 0xFE000000 and flag U (Unreachable
Prefix) or UP (Unreachable Planned Prefix) set.
that is exactly what the draft describes. I'm not sure what is your point.
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?
I'm not sure what exactly are you asking for as the draft is already
describing what you are asking for.
The drawback is that the existing pre-standard implementation would need
to be updated. But that’s the rule for a pre-standard implementation.
not really, the handling of the metric higher that 0xFE000000 is already
defined in the existing spec.
regards,
Peter
The faster we do the change, the easier it is, especially for other
implementations.
--Bruno
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