Re: [Idr] Progressing draft-bhatia-bgp-multiple-next-hops-01.txt

[Curtis Villamizar <curtis at occnc.com>]

In message <92c950310609010921hbc8357fn4fff8b0e1c3179eb at mail.gmail.com>
"Glen Kent" writes:
>  
> Curtis,
>  
> > > Path 1 - NLRI 192/8 AS_PATH {10 20} LOCAL_PREF 100 NEXT_HOP 1.1.1.1
> > >
> > > Path 2 - NLRI 192/8 AS_PATH {10 20} LOCAL_PREF 189 NEXT_HOP 1.1.1.1
> >
> > If this came from the same router then complain because the sender of
> > this is broken.
>  
> This is exactly what a receiver will receive if add-paths is
> implemented, because you dont differentiate between paths. You blindly
> flood all the paths that you receive.

It would not receive this from one router.  That one router would have
to pick the route with the better LOCAL_PREF.  I'm assuming we are not
modifying the way LOCAL_PREF works.

If two routers send this then it is a normal BGP case.  Since
LOCAL_PREF is included this MUST be IBGP.  Pick the higher LOCAL_PREF.
If the one router sending the route with LOCAL_PREF 100 sees the
better advertisement it will generally withdraw its own advertisement.
Neither router would be advertising a multipath if this was two
routers.

> > On the priciple of "being liberal in what you accept" just toss the
> > second path.
>  
> If the receiver has to ignore all such paths then why is add-paths
> sending them in the first place?

[Oops should have sent "pick the second".  You missed that.]

The receiver has to ignore lower LOCAL_PREF based on section 5.1.5 of
RFC-4271 (BGP).  Are you suggesting we change BGP to ignore
LOCAL_PREF?  I didn't see that in either add-path or multiple-next-hops.

> > > How does this router determine which route is being used by the
> > > upstream? I dont think we should progress any proposal without
> > > incorporating ways to indicate which path is being actually used in
> > > the forwarding.
> >
> > If its a multipath it is considering both to be valid.  This may be
> > possible at an IX if next-hop-self is not set and there is an odd set
> > of routing policies.  btw- no where in BGP does it require that
> > routing policies be sane (and they can be seemingly quite wierd).  If
> > the AS are 65010, 65020, and 65030, this is perfectly valid in an AS
> > confederation setup.  In AS confederations you'd just forward to
> > 1.1.1.1 but retain the AS paths for readvertising.
>  
> No, you didnt get my question at all. My point is that we need to
> explicitly mark the route being used in fwding, as otherwise the peer
> receiving multiple paths can never know which path is being used by
> the router advertising these routes. You need this information for
> obvious reasons. See Jeffs mail for more details.

This is a different usage.  I'm assuming that a multipath means ECMP.
You seem to be assuming that all advertisements are being sent
including ones that are not being used and then complaining that there
is no way to determine which one is being used.

> None of the proposals currently do this which to me is rather blasphemous.
>  
> Glen

Maybe thats because neither of the proposals intend to send paths that
are not being used.  I think that the WG needs to decide whether that
usage is within scope for either of these proposals.  It should be
safe to assume that since it is not mentioned by either one and since
there is no support for it that it is not an intended usage.

Curtis

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