[cisco-bba] ATM : VP & VCI

[Doug McIntyre]

On Fri, Jan 02, 2009 at 08:05:35PM +0530, raghuram chary wrote:
> Hi all,I'm unable to understand/imagine the VP and VC concept in ATM
> technology.Can anyone explain the concept in detail?Any website pointers or
> animated videos will also be appreciated.

Its much like a DLCI in frame relay or a VLAN tag in ethernet networks.. 

An identifier to mark this cell/packet/frame/whatever is identified
with this number and we differentiate that from other cells that could
be for other things.

As to the reason for splitting the identifier into two seperate ones,
ATM allows you to do path switching as well as VPI switching. You can
take everything within the same VPI and push it out a different port
than another.

You could have all of customers X traffic on VPI 10, and all of
customer Y's traffic on VPI 11. Now, no matter how many VPI's customer
X lights up, your switching setup continues to work without additional
setup on your switching gear. 

There's reserved VCI's so that also comes into play (ie. why the first
PVC may be on 0/32 instead of 0/0 like you'd expect).