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

Peter Psenak <ppsenak@cisco.com> Tue, 06 August 2013 18:45 UTC

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Date: Tue, 06 Aug 2013 20:45:09 +0200
From: Peter Psenak <ppsenak@cisco.com>
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To: David Lamparter <equinox@diac24.net>
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Subject: Re: [OSPF] OSPFv3 Extended LSAs TLV-level "disposition-if-unsupporetd indicator"?
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David,

00 - ignoring unrecognized TVLs is a standard thing to do

01 - non-MT routers would ignore all TLVs related to MT-ID other then 0 
anyway, there is nothing special you need to do. If the non-MT capable 
router tries to do anything with TLV which is bound to a non-0 MT, or 
interpret any data that relates to non-0 MT, its a bug on that router.

10 - you can achieve this by advertising the SADR capability in the RI 
LSA and do it for any prefix coming from the non-SADR router, no per TLV 
granularity is required.

11 - looks like a router STUB advertisement

BTW, we should set the U-bit on all Extended LSAs and I believe Acee has 
agreed on that already.

thanks,
Peter



On 8/6/13 15:49 , David Lamparter wrote:
> Hi ospf WG,
>
>
> 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.
>
> 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.
>
> Note that this is supposed to work in conjunction with other ways of
> communicating per-router capabilities, e.g. Router Information LSAs.
> A router may well need to take a different action when, on calculating,
> it notices that it passed by a router without support for feature XYZ.
> Since that applies to routers that *do* implement a particular
> extension, the exact behaviour for this needs to be specified in that
> extension.
>
> Comments?
>
>
> -David
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