Switch Mechanisms for Building and Maintaining Bridge Tables

ACD

Switches build and maintain bridge tables (also known as MAC tables) to facilitate the forwarding of frames to their intended destination. The bridge table maps MAC addresses to the switch port on which they were last seen. When a frame enters a switch, the switch learns the source MAC address of the frame and the port on which it was received. Based on the destination MAC address of the frame, the switch uses its bridge table to determine the port(s) to which the frame should be forwarded.

The three mechanisms used by switches to build and maintain bridge tables are:

A. Learning: When a switch receives a frame, it learns the source MAC address of the frame and the port on which it was received. The switch then adds this information to its bridge table. If the switch already has an entry for the source MAC address, it updates the port information for that entry.

B. Aging: Switches use aging to remove stale entries from the bridge table. Each entry in the table has a timer associated with it, which is reset whenever a frame is received from the corresponding MAC address. If the timer expires without any activity from the MAC address, the entry is removed from the table. The aging timer is typically set to a default value of 300 seconds.

C. Flooding: If a switch receives a frame with a destination MAC address that is not in its bridge table, it floods the frame out to all ports (except the port on which the frame was received). This ensures that the frame reaches its intended destination, even if the switch does not know the location of the destination MAC address. Flooding can be an inefficient use of network bandwidth, so switches typically use other mechanisms (such as learning) to reduce the need for flooding.

Rewriting and blocking are not mechanisms used by switches to build and maintain bridge tables.

D. Rewriting: Rewriting refers to the modification of frame headers by a switch. Switches may modify the source and/or destination MAC address of a frame before forwarding it. This can be used, for example, to implement VLANs or to hide the identity of devices on a network.

E. Blocking: Blocking refers to the state of a port on a switch that is not forwarding frames. Blocking can be used, for example, to prevent loops in a network. When a switch detects a loop (i.e., multiple paths between two switches), it may block one or more ports to prevent frames from looping endlessly between switches.