The MPLS WG Archive

[Curtis Villamizar <curtis@workhorse.fictitious.org>]

In message <000501c07722$ef469e70$a0b7010a@winstar.com>, "Yi Chu" writes:
> Please forgive me if my question is so obvious and maybe stupid, since I am
> new to this discussion.
> 
> Why 1000 nodes can have 10-15 adjacencies while IP over ATM needs 999
> adjacencies per node?  I think IP over ATM uses ATM VC just as a physical
> link, so there is no difference in adjacencies per node whether the routers
> are connected through Sonet links or ATM VCs.
> 
> So what is the reason that IP over ATM requires full VC mesh of all nodes?
> 
> Yi Chu


If you use ATM as a means of traffic engineering, you must apply a
full mesh of VCs and allow ATM to route the VCs.

If you use ATM solely as a link layer protocol, you can set up a much
more limited set of VCs that correspond more closely to the physical
topology, but ATM is no longer able to route the traffic.  The traffic
follows the set of hops dictated at layer 3.

If you use MPLS, you get a control plane that maps directly onto the
physical topology and therefore has less adjacecencies but you retrain
the ability to traffic engineer from edge to edge.

Curtis