What information in the Ethernet header is used to populate the bridging table?

Layer 2 switches, or bridges, build their forwarding tables (known as bridging tables or MAC address tables) by examining the source MAC address of incoming Ethernet frames. When a frame arrives on a port, the switch inspects the frame's header, extracts the source MAC address, and creates an entry in its table that associates this MAC address with the ingress port. This process, called MAC learning, allows the switch to intelligently forward future frames destined for that MAC address only to the correct port, rather than flooding them to all ports.

A. The destination address is used to look up an existing entry in the table for forwarding decisions, not to populate the table with new entries.

C. The type field (EtherType) identifies the Layer 3 protocol encapsulated within the frame (e.g., IPv4, ARP) and is not used for MAC learning.

D. The protocol field is not a standard field in the basic Ethernet II header; the EtherType field serves the purpose of identifying the protocol.

1. Juniper Networks TechLibrary. (2023). Understanding Bridging and VLANs for Switches. Junos OS Section: Ethernet Switching. "A switch learns MAC addresses by examining the source address field of the frames it receives and adds the address and the corresponding interface to its MAC address table."

2. Juniper Networks TechLibrary. (2023). Junos OS for QFX Series and EX Series Switches. Section: Layer 2 Learning and Forwarding. "When the switch receives a packet

it learns the source MAC address and adds it to the Ethernet switching table..."