I assume at some level it makes sense to do switching for topology >
reasons. But for performance, it is not a benefit?
I believe there are two benefits to L2 switching across the core
that are rarely mentioned, but of significant help to large
networks. Detached L1 reroute, and endpoint pair flow control.
L2 switching across the core can benefit your L3 infrastructure by
removing some of the route recalculation required by changes in L1
topology. If the L2 network can reroute without the L3 knowing
about the topology change, less overhead for the L3 network to worry
The general goal here is to 'segment the responsibility'. The same
result could be found by instantiating an additional L3 routing
domain across the backbone (a technique that many have done, and
from which many seem to be moving away).
Of course, you add an additional variable, as you must now
consider the variables inherent to L2 routing, and your complexity
goes up, though the pieces complexity go down.
That is to say, you divide the complexity up into two pieces, with
the whole of the sum being greater, but the individual pieces far
less. When one is given equipment with set capacity, this is a
As well, the PVC endpoint flows can be engineered at a level
unknown to pure L3 networks.
If one has a large amount of traffic going from Boise to Dayton,
and the traffic from Topeka to Evanston wants to take the same
path, one can build pvcs between the two endpoint pairs, and
manually route the PVCs over the preferred trunks.
These two benefits
- L3 detached reroutes
- endpoint pair flow control
I agree w/ Bradner's note that the difference in latency is
trivial in a WAN environment. However the two benefits listed
above are not.
This is not to say that L2 switching is better for everyone.
Obviously all folks must do L3 switching, at least at the edges.
But for many folk [large providers] with many problems [large
networks, weenie routers] L2 switching is helping to allow
networks to grow as stronger routers are built.