Re: router-router NHRP

[Yakov Rekhter <[e-mail]>]

Dimitry,

> I guess I misunderstood implications of the "first NHRP target
> constraint" of your proposal:
> 
>  "This document constrains an NHRP target by requiring that all the
>   destinations covered by the target must form a subset of the NLRI of at
>   least one route in the Forwarding Information Base (FIB) of the router
>   that either originates, or propagates an NHRP Request. For the rest of
>   the document we'll refer to this as the "first NHRP target
>   constraint".  A router can originate and/or propagate an NHRP Request
>   only if the NHRP target of the Request does not violate the first NHRP
>   target constraint.
> 
>   A route (from a local FIB) whose NLRI forms a minimal superset of all
>   the destinations covered by the NHRP target is called an "NHRP
>   forwarding route". Observe, that by definition the set of destinations
>   covered by an NHRP target always exhibits a subset relation to the set
>   of destinations covered by the NHRP forwarding route associated with
>   the target."
> 
> Doesn't it require a forwarder to have a route for each potential target?
> If so, in many cases where a large scale aggregation is not possible, it coul
d
> be a lot of routes.

The intention of the "first NHRP target constraint" is to preserve
the "longest match" semantics. To illustrate what was the intention
consider an NHRP Request for the 192.9/16 target. If such a Request
arrives at a router whose FIB contains a route to 192.9/16 and a route
to 192.9.200/24, and both of these routes have different next hops, then
the router should not propagate the Request, but should just send back
a Reply (with itself as the Next Hop). 

On the other hand, if the Request arrives at a router that has
a route to 192/8 in its FIB, and no routes more specific than this
route, then the router should forward the Request. The router need
not instantiate a route to 192.9/16.

Yakov.