The Routing Over Large Clouds Mailing List Archive by date

[Scott Marcus <smarcus@bbnplanet.com>]

...

>> NHRP MAY help us to avoid either the
>> obvious router bottlenecks
>
>"obvious router bottlenecks"? What are they?
>
>Isn't it obvious that cell-switching IP routers, whose cell switching
>fabrics are setup by RSVP to have VCs for each flow, are just as fast
>as legacy ATM switches?

I don't want to get embroiled in your overall argument, but I think I might
take Masataka's claim a step further:  there is really no inherent reason
why routers have to be significantly slower than switches in general in the
WAN environment.  It's merely the way that the market evolved to date.






>> or the administrative complexity of
>> overlaying a mesh of IP connections over a simple ATM star topology.
>
>If the mesh of IP connections are necessary only within a subnet,
>a small cloud between the cell-switching IP routers, it's administration
>is only as complex as the current configuration.
>
>And, are you aware that if you have a large cloud with N nodes and
>a mesh of IP connections over a simple ATM star topology, most
>links will have O(N^2) VCs? That is, if you use CBR, each VC can't
>have high bandwidth. ...

I think you folks just talked past each other.  Sam said, "over a simple
ATM star topology".  He was, if I understand him correctly, assuming that
the VCs converge at a single point in the "logical center" of the network.

This is the simplest way to structure and configure an internetwork over an
NBMA, and avoids the need for (n * (n-1))/2 VCs to which Masataka
implicitly refers.  In doing so, it also avoids the fragmentation
associated with having large numbers of distinct PVCs over each link (as
Masataka says, "each VC can't have high bandwidth"), but at the cost of an
extra hop for most if not all of the traffic.

In the Frame Relay environment, this continues to be a very common and
practical way to construct internetworks, notwithstanding the hype you read
in the trade rags.  Whether it plays out the same in the ATM case is
primarily an economic question -- at higher bandwidths, the underlying
circuits are more expensive, and therefore the cost penalty of an extra hop
may be greater.

I'm not trying to argue that meshes are better than stars, or vice versa --
like most things, it depends. I'm just trying to help you get your arms
around one aspect of your disagreement.

Cheers,
-- Scott Marcus