Obtaining the bridge table
A bridge relies on its bridge table to forward data. A bridge table consists of two parts: MAC address list and interface list. Once connected to a physical LAN segment, a bridge listens to all Ethernet frames on the segments. When it receives an Ethernet frame, it extracts the source MAC address of the frame and creates a mapping entry between this MAC address and the interface on which the Ethernet frame was received.
As shown in Figure 101, Hosts A, B, C, and D are attached to two LAN segments. Host A and Host B are connected to bridge interface 1. When Host A sends an Ethernet frame to Host B, both bridge interface 1 and Host B receive this frame.
Figure 101: Host A sends an Ethernet frame to Host B on LAN segment 1
As shown in Figure 102, when the bridge receives the Ethernet frame on bridge interface 1, it determines Host A is attached to bridge interface 1 and creates a mapping between the MAC address of Host A and bridge interface 1 in its bridge table.
Figure 102: The bridge determines Host A is attached to interface 1
When Host B responds to Host A, the bridge also hears the Ethernet frame from Host B. As shown in Figure 103, when the frame is received on bridge interface 1, the bridge determines Host B is also attached to bridge interface 1, and creates a mapping between the MAC address of Host B and bridge interface 1 in its bridge table.
Figure 103: The bridge determines Host B is also attached to interface 1
As shown in Figure 104, finally, the bridge obtains all the MAC-interface mappings (assume all hosts are in use).
Figure 104: The final bridge table
