The Routing Over Large Clouds Mailing List Archive by date

[rajeev@trillium.com]

> From: "Andrew G. Malis" <malis@nexen.com>
> 
> Given the existence of CSRs, does the need for NHRP go away?  My own
> opinion is that the need for NHRP probably goes away when ALL routers
> on an NBMA network are integrated into the NBMA-layer switching fabric
> (so, for example, every ATM switch is also a router), and router hops
> are no more expensive than non-router hops (because there AREN'T any
> non-router hops).

One important aspect of CSRs, which I haven't seen addressed, is that
the switched cells flow *through the switch/router box*. This may not be
the most optimum routing of the cells between the communicating
end-points.  In other words, the cells may be making extra "hops" across
switches that can be eliminated with a more direct connection between
the endpoints. The same argument also applies when the router needs to
map the flow into a QoS sensitive VCC (currently, it seems Ipsilon only
supports best effort VCCs) - the path through the CSR may not even
support the required QoS of the new VCC.

Effectively, the CSR is saying "I know this flow can be switched and the
VCC will go through me".

NHRP eliminates this restriction by providing the source endpoint with
the destination ATM address and allowing the source to set up the QoS
based VCC. This VCC will be routed through the ATM network (say, using
P-NNI) along a (hopefully optimal) path that supports the request QoS
and traffic parameters.

Going a step further, in MPOA, it is envisioned that the MPOA host will
detect flows, then use NHRP to resolve the destination layer 3 address,
set up a VCC as desired and then send the data flow on that VCC. The
routers (data forwarders), when they detect flows that are using default
paths, will use a cache imposition protocol to inform the MPOA hosts
that they should establish a direct VCC between the endpoints. In
effect, they are saying "I know this flow can be switched, but the VCC
does not have to go through me, so why don't you (the host) establish
the best VCC you can".

+-------------------------------------------------------------------------+
|Rajeev Gupta                               Trillium Digital Systems, Inc.|
|rajeev@trillium.com                               Los Angeles, California|
|                  * Opinions expressed are mine alone *                  |
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