 Aging Time
In order to fully understand aging time, it's important to understand several key terms that will be used in this paper to describe how the concept of aging works:
Ethernet Switch - A switch, also known as a bridge, is a complex device with built in intelligence and memory to connect Ethernet nodes. A switch operates at the data link layer (layer 2 of the OSI model). The switch learns the MAC address of each connected device and routes the unicast packets (frames) accordingly. Broadcast packets are flooded and Multicast packet handling depends on the type of switch used. Switches also eliminate collisions that typically make Ethernet non-deterministic.
Local Area Network (LAN) - A local area network is a group of computers and associated devices (PLC's, HMI's, PC's, etc), commonly refered to as nodes, that share a common communications line or wireless link. Individual users can share data or files on a LAN as if the data or files resided on their respective computers.
Media Access Control (MAC) Address - A hardware address that uniquely identifies each device, or node, on a local area network.
Address Resolution Logic (ARL) - A table, internal to a switch, of forwarding rules based upon which MAC addresses exist on which ports of the switch. Switches use these tables to pass frames, which are destined for an address that's connected to another port on the switch.
Aging Time - The length of time that a MAC address entry can remain in the ARL forwarding table. When an entry reaches its aging time, it "ages out" and is purged from the table, effectively cancelling frame forwarding to that specific port. In other words, if the switch doesn't hear from a device after a specified period of time, the MAC entry in the ARL table is deleted.
Unicast Frame - Sent to a specific host. One to one communication.
Multicast Frame - Sent to one or more interested hosts. A host registers its interest in a particular multicast group via the Internet Group Management Protocol (IGMP). One to many Communication.
Broadcast Frame - Sent to all hosts in the local network. One to all communication.
Switch Functionality Using Address Resolution Logic (ARL)
Learning from the frames they receive, switches dynamically create tables that associate the ports with the MAC addresses of the hosts that can be reached. The port that a frame is forwarded to is based on an internal Address Resolution Logic (ARL) table that maps MAC addresses to the individual ports
connected. The switch then forwards unicast frames received on any given port to the appropriate port(s), helping reduce the overall traffic in a local area network (LAN) and providing more effective use of bandwidth with its filtering abilities. Unlike unicast frames, switches will forward all broadcasts, multicasts, and/or packets destined to unknown MAC addresses to all ports in the local area network, other than the port in which it was received.
ARL Table Entries
The following figures will show the process a switch goes through in order to learn and build ARL tables, as well as delete "aged out" entries. Keep in mind that if a switch doesn't hear from a device after the specified period of time, the MAC entry in the ARL table is deleted. In Figure 1A, the switch is adding an entry to its ARL table. The switch records the MAC address from the source address field of a frame and records which port it came in on.
In figures 1A and 1B, a switch is shown adding a second entry to the ARL table:
Conversely, figures 1D and 1E show a switch deleting an entry after the aging time has expired:
The information in this white paper was first published by N-Tron. In an effort to bring out customers the best possible information, B&B Electronics has received permission to republish this document. B&B Electronics thanks N-Tron for allowing us to distribute this information to our customers. You can visit their site at http://www.n-tron.com.
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