The MPLS WG Archive

["Cheng-Yin Lee" <leecy@nortelnetworks.com>]

Curtis,

Curtis Villamizar wrote:

> There were a number of motivations for getting rid of ATM in IP
> networks.  One was the cell tax which at a typical 20% would not alone
> kill ATM.  Another was SAR.  You can't get fast ATM router interfaces
> because SAR speed and SAR buffering becomes a problem.  This alone was
> enough to kill ATM.
>
> Perhaps even larger was the problems of independent control planes and
> the effect on IGP scaling.  Flooding is needed for reliability in the
> IGP and flooding in a full mesh of N routers has some N^3 properties
> with regard to messages sent when a router in the mesh fails and N^2
> when a VC fails.  Attempts to constrain the flooding tended to slow
> convergence and sometimes cause long lived IGP inconsistency when
> change occurred.  This is not a problem is the flooding follows the
> physical interconnections as it does with MPLS since the number of
> adjacencies per node drops dramatically.

Over time , would there be any inclinations to add direct connectivity between B
and H, and then over F and D, and so on, leading to perhaps densely meshed
networks in the long run ? What is your opinion on this?

                          B-------C-------D
                        /                                          \
                      /                                               \
                 A                                                    E
                    \                                                /
                      \                                            /
                       F--------G--------H

One could use a common control plane and may still run into IGP scaling issues
because the network is densely meshed, right?

> The IGP scaling problem was address by ISPs by partitioning their
> network into core and regions but this reduced the effectivenes of TE.

Could you kindly elaborate your thoughts on this?

thanks,
cyl