Re: [OSPF] OSPFv3 Extended LSAs TLV-level "disposition-if-unsupporetd indicator"?

"Russ White" <russw@riw.us> Tue, 06 August 2013 14:24 UTC

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From: "Russ White" <russw@riw.us>
To: "'David Lamparter'" <equinox@diac24.net>, <ospf@ietf.org>
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Date: Tue, 6 Aug 2013 10:24:48 -0400
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Subject: Re: [OSPF] OSPFv3 Extended LSAs TLV-level "disposition-if-unsupporetd indicator"?
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> looking at the Extended LSA draft from the various use cases, I believe it
> would be advantageous to repurpose the topmost two bits of the TLV type
> to indicate what should happen if the TLV is not supported by a router.
I'm
> thinking of 3-4 possible handlings:
> 
> (these all only apply to parent TLVs or LSAs that specify a route in some
way,
> e.g. currently Inter/Intra/AS-Ext-Prefix LSAs.  Though the first two make
> sense in a generic way.)
> 
> 00 - ignore TLV
>   this can be used for all "hint" TLVs, stuff like maybe communicating
>   the origin ASN for external routes or whatever you can dream up.
> 
> 01 - ignore parent
>   on calculating SPF, completely ignore the resulting route.  This is
>   useful for MT-OSPF (if it ever happens), to be used on a MT-ID TLV
>   with an MT-ID != 0.  Basically, non-MT routers can ignore all nonzero
>   MT topologies this way.
> 
> 10 - strong unreachable
>   mark the route's destination prefix as unreachable and install a
>   corresponding blackhole/... route.  This is the right thing to do on
>   SADR routes when they hit a non-SADR router.  Even if we have the same
>   prefix reachable on a non-SADR route with a lower metric, we can't
>   ensure that it's loop-free for a particular source address.

These seem useful to me...

> 11 - weak unreachable (?)
>   treat the route as "unreachable", adding it to the SPF result as such,
>   if there is no shorter path to the same prefix so far.  This is
>   probably the least useful type, I can only come up with something like
>   "route that requires special encapsulation (tunnel?)" - no idea on the
>   reality here.

This is really just max metric --I'm not certain we need another mechanism
to achieve the same thing.

:-)

Russ