[c-nsp] Router/Switch Sizing terms

[Pete Templin]

Sami Joseph wrote:

> 1) Backplane:

The <thingy> across which packets are flung from ingress to egress.

> 2) Switch Fabric

In switches or switch-like routers, this is another term for the backplane.

> 3) Switching Capacity

L2 devices switch packets from ingress to egress.  L3 devices route, and 
then switch packets from ingress to egress.  Either method requires an 
appropriate (L2, L3, etc.) lookup, one time per packet regardless of 
packet payload size, and therefore most switches and routers are 
performance-limited by the number of frames/packets they can perform 
switching/routing lookups on (and then switch the packet to the egress 
port).

> 4) Forwarding performance

Often a measure of switching capacity multiplied by minimum, average, 
weighted, and/or maximum packet size.  Used to differentiate between 
devices that can only fill their ports if maximum-sized packets are 
used, and devices that can fill their ports if minimum-sized packets are 
used.

pt