[c-nsp] Layer 2 question

[Brett Frankenberger]

On Tue, Jul 29, 2008 at 12:43:10PM -0400, Mike Johnson wrote:
>
> What happens if a layer 2 switch receives a frame that needs to be
> forwarded out the same port that the incoming frame was received on?

It will discard it.   

Back when Layer 2 switches were typically used to connect multiple
shared-media Ethernet segments, this happened all the time -- every
time two nodes on the same shared-media Ethernet segment communicated.

These days, most switch ports connect only to a single device, but the
operation of Layer 2 switching (which is just bridging) hasn't changed.

> Im realize that a typical layer 2 switch would never forward that
> packet upstream in the first place. However, If it did in fact happen
> what does the upstream L2 switch do?

Consider the case of a hub (not a switch) connected to a switch port,
and multiple devices on that hub communicating with each other.  The
switch sees all the packets, and doesn't forward them.

     -- Brett