The IP Over NBMA (ION) Archive

[etxbeos@kk.ericsson.se (ETX-B-DGI Olsson Bengt J)]

>                                             I think there are cases
> where NHRP can add value, even in the presence of CSR's.  For example
> if there are more ATM switches in the WAN than CSRs, then one might
> find a better route across the WAN via NHRP than the CSRs can find.

Yes, this is clearly the case. But it comes at a cost. If the WAN is
big enough to have to use NHRP instead of ATMARP, it is also big enough
to have to use dynamic routing on both the IP layer and ATM layer.

I think this is the fundamental trade-off. Put in a table form:

    CSR                  NHRP
    ---                  ----

 + Global IP          - Global IP and
   routing only         ATM routing

 - Does not find      + Finds the shortest 
   shortest way in      way in the mixed CSR -
   a mixed CSR - ATM    ATM cloud network
   cloud network

It might be difficult to decide which approach is most beneficial
from this table. However, going to the extreme networks pictures 
(only CSR's or only ATM cloud + NHS's) CSR wins because both find
the same path (equivalence between IP and ATM routing protocols) 
but CSR's do without the adress translation. 

In the mixed scenario (CSR's embedded in or connecting ATM clouds) 
there is an analogy to routers and ordinary LAN's. CSR's maps to
routers and ATM clouds to LAN's. There is a reason we connect hosts
to LAN's and not to routers. LAN is optimising connectivity cost in
the local area. But LAN's have a scalability issue just like ATMARP
and NHRP.

I think we have to reason in the same way with CSR's and NHRP. Maybe
the best trade-off is CSR's interconnecting ATM clouds which are 
small enough to do with ATMARP and "cheap" ATM routing? 

Which make me somewhat of an CSR proponent...

/Bengt

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