Re: r2r NHRP - Target Size

["M.J. Robinson" <[e-mail]>]

Dimitry and other ROLCers,

... (text cut)
> 
> One example:
> 
> Let's assume a forwarder A wants to build a single shortcut to a default
> router which mainains a complete routing table and can act as a traffic
> de-multiplexer for A.  The domain topology may look like that:
> 
>                      E
>                      |
>       A -- B -- C -- D -- F
>                      |
>                      G
> 
> where D is a proxy router for E, G, and F exit forwarders.  D injects a default
> into IGP.  To resolve the shortcat destination, A sends request for the default 
> route (i.e. 0.0.0.0/0) along ABCD path.  When request reaches D, D would be obliged
> to reply to A since it has more specific routes than the requested default route. D
> specifies itself as the shortcut endpoint.
> 
> Dimitry

	I'm assuming that all the routers in the above diagram are on the 
same NBMA. Is this correct ?
	If so, then the behaviour suggested isn't quite what I expected
but can be made to happen. I'm interested in a fairly simple multi-area
OSPF arrangement of routers as the "underlay" to NHRP. A diagram can be
made available if you like, but for now I'll try and describe my scenario:
	Network is broken up into OSPF areas of approx. 50 routers each. 
Two ABRs are allocated for each _pair_ of areas. Each ABR links to area 0
and to _both_ these areas, thus providing redundant connection between
each area and the backbone.
	Each area is in effect a smaller "large cloud"  and so in the
above diagram routers B and C don't exist.
	The ABRs perform address aggregation and consequently other
routers do not know what size LANs are attached to routers in other
networks. Just sending NHRP requests for a single host address is liable
to be inefficient. On the other hand, establishing a "short-cut"  to the
ABR is not normally useful since NHRP has then done nothing. 

	From the current spec. (Yakov's email of 20th Oct.) I'm under the 
impression that if A sends a Request for a Target covering (say) E,F and 
G then D should in effect return an error:
	"If the Second NHRP target constraint is violated then the router
[D in this case] sends back an NHRP Reply and terminateas the query. The
Reply should indicate that the second NHRP target constraint was violated.
The Reply contains IP and NBMA addresses of the router." 
	The upshot of this is that the "error" reply lets you use D as a
forwarder for E,F and G if you really want.
	However, please could you give some examples where this is
desirable (I'm under the impression that it's advisable to avoid sending
data via D since sending traffic between D and the NBMA network will often
cost money ). 

	Matthew.