Cisco LAN Switching Fundamentals

Buffering Data

Frames must wait their turn for the central arbiter before being transmitted in shared bus architectures. Frames can also potentially be delayed when congestion occurs in a crossbar switch fabric. As a result, frames must be buffered until transmitted. Without an effective buffering scheme, frames are more likely to be dropped anytime traffic oversubscription or congestion occurs.

Buffers get used when more traffic is forwarded to a port than it can transmit. Reasons for this include the following:

• Speed mismatch between ingress and egress ports

• Multiple input ports feeding a single output port

• Half-duplex collisions on an output port

• A combination of all the above

To prevent frames from being dropped, two common types of memory ...