North American Network Operators Group

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Re: MPLS VPNs or not?

  • From: David Charlap
  • Date: Tue Aug 07 17:18:37 2001

Craig Partridge wrote:
> the notion MPLS is faster to switch than IP reflects a poor
> knowledge of router innards, or a poor router design.

Or just outdated information.  It wasn't too long ago that IP best-
match lookup hardware couldn't do line-rate at the high speeds needed in
the cores of large networks.  At that time, ATM was able to do line-
rate forwarding at those speeds.  This is one of the reasons that ATM
was adopted in those networks.  One of the big reasons for ATM's speed
is its use of fixed-length VPI/VCI header lookups instead of variable-
length IP best-match lookups.  MPLS, thanks to its use of a fixed-
length label header lookup has the same advantage.

Over time, however, this advantage disappeared.  IP best-match lookup
chips were developed that could do a proper IP lookup at full line rate
for OC-48 and even OC-192.  With line-rate IP lookup, it's no longer
possible for something else to be faster.

It is possible that the pendulum may swing back the other way in time,
of coruse.  It is possible that IP lookup chips that can handle the next
generation of line rate may not become available in a timely manner,
which would once again give the advantage to fixed-length header lookup
chips.  But not having a crystal ball at hand, I have no idea if this is
actually going to happen or not.

-- David