Cisco LAN Switching Fundamentals

Congestion and Head-of-Line Blocking

Head-of-line blocking occurs whenever traffic waiting to be transmitted prevents or blocks traffic destined elsewhere from being transmitted. Head-of-line blocking occurs most often when multiple high-speed data sources are sending to the same destination. In the earlier shared bus example, the central arbiter used the round-robin service approach to moving traffic from one line card to another. Ports on each line card request access to transmit via a local arbiter. In turn, each line card's local arbiter waits its turn for the central arbiter to grant access to the switching bus. Once access is granted to the transmitting line card, the central arbiter has to wait for the receiving line card to fully receive ...