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Re: Granularity (was Re: [RRG] ALT + NERD is inelegant & inefficient, compared to APT or Ivip)



Brian E Carpenter wrote:
The idea is to make *less* work by splitting the problem space in two.

How does that apply in the case of a site that changes from single
homed to dual homed to triple homed and back again? I expect this
will be a very common occurrence.
If the instance of triple homed is really a question of switching which second provider is being used,
then I think a suitable question to dig further along this is:

How important is it that while migrating, a site be able to use *all three* links, if only the redundancy is critical?

And, how likely is it that the uninvolved link will fail during the transition window for the other two links, as well as one of the other two links failing simultaneously? Would that be any worse than if both links failed, before or after the transition event? Is having the third link available to do a fast transition (or fast reversion) adequate to meet the needs of a site which isn't so concerned with redundancy as to operate their own EID infrastructure?

If we can generalize this to being, "It is common operational practice to have an overlap period between the provisioning of one service, and the cancellation of another service, and a 'hot' cut-over between those during the overlap duration", then I think it's okay to not support temporary triple-homing.

Making the distinction between full-on triple-homing, and having three links, two of which must always be
configured for redundancy, is IMHO a reasonable situation.

When there is one link that doesn't change, and two that are in the hot-cut mode (one add, one drop), the stability and redundancy is quite reasonable with a solution limited to N=2. IMHO.

BTW - reminder about the N=2 distinction. I should point out, that the "heavyweight" solution without host granularity, is also available even in N=2. The compromise is, no host granularity, and more administrative,
technical, financial, and operational overhead is required.

Brian Dickson

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