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Understanding Bridging and VLANs

Bridging is a frame-forwarding technique used in Layer 2. With bridging, there are no assumptions about where a particular address is located in a network, so a mixed collection of interface types and speeds can be logically grouped together.

Layer 2 on a switch uses the transparent bridging protocol. This protocol enables a node to learn information about all the nodes on the LAN, including nodes on all the different VLANs. This information is used to create address-lookup tables, called Ethernet switching tables. The switch uses these tables when forwarding traffic to or toward a destination on the LAN.

Transparent bridging uses five mechanisms to create and maintain Ethernet switching tables on the node:

• Learning is the process used to obtain the MAC addresses of all the nodes on a network. When a node is first connected to an Ethernet LAN or VLAN, it has no information about other nodes on the network. To learn MAC addresses, a node reads all detected packets on the LAN or on the local VLAN, looking for the MAC addresses of sending nodes. These addresses are added to the Ethernet switching table, along with the interface (or port) on which the traffic was received and the time when the address was learned.
• Forwarding describes how traffic passes from an incoming to an outgoing interface that leads to or toward the destination. To forward frames, the Ethernet switching table is read to determine if the table contains the MAC address corresponding to the frame destination. If the Ethernet switching table contains an entry for the desired destination address, the traffic is sent out to the interface associated with the MAC address. The Ethernet switching table is read in the same way when frames are transmitted that originate on nodes connected directly to the initiating node.
• Flooding is the mechanism by which a node learns about destinations not in its Ethernet switching table. If the table has no entry for a particular destination MAC address, traffic floods out to all interfaces except the interface on which it was received. (If traffic originates on the node, the device floods it out all interfaces.) When the destination node receives the flooded traffic, it returns an acknowledgment packet, allowing the node to learn the MAC address and add it to its Ethernet switching table.
• Filtering refers to the limiting of traffic to the local VLAN whenever possible. As the number of entries in the Ethernet switching table grows, an increasingly complete picture of the VLAN and the larger LAN is built, showing which nodes are in the local VLAN and which are on other network segments. This information is used to filter traffic. Specifically, for traffic whose source and destination MAC addresses are in the local VLAN, filtering prevents traffic forwarding to other network segments.
• Aging keeps the entries in the Ethernet switching table current. For each MAC address in the table, a timestamp shows when the information about the network node was learned. Each time traffic from a MAC address is detected, the timestamp is updated. A timer on the node periodically checks the timestamp, and if it is older than a user-configured value, that MAC address is removed from the Ethernet switching table. This aging process ensures that the device tracks only active nodes on the network and that it is able to flush out network nodes that are no longer available.