Benefits of switching at L2

[Alan_Hannan4]

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
  about.

  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
  reasonable choice.

  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

  are significant.

  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.

  -alan

[Dorian_Kim]

  These two benefits
  
    - L3 detached reroutes
  
  - endpoint pair flow control

  are significant.

And as significant as these benefits are, one must not also forget about the
significant disadvantages of L2 switching that comes with current
implementations. TANSTAAFL.

By having two seperate intelligent networks that do not communicate with each
other, one is incurring the cost of greatly increased systemic complexity as
well as chances of failure mode amplification and obfuscation between L2 and
L3 infrastructure.

By adding complex devices to L2, one addes more candidate for failures, and
shorten mean time between failures of one's network elements.

Furthermore, L3 detached reroutes has it own share of problems.

Whether this trade off makes sense is up to the individual networks, but like
all things each choice has it's own pains and pleasures.

I personally prefer to keep L2 as simple as possible, and leave L3 to do its
thing. I guess it's a different interpretation of "segment the
responsibility."

  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.

This is the key point. If there were real routers to buy, much of the fun
games that are played by various folks would be unnecessary. Furthermore,
there is no reason why L3 network can't implement things like end pair traffic
management much the same way L2 networks can given correct implementations.

-dorian