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Key Bridging Concepts 163
Key Bridging
Concepts
Before you configure bridge-wide or bridge port settings on your module,
you may find it helpful to review the following concepts.
Learning Addresses Bridges learn addresses so that they can, over time, more effectively
determine which frames to forward on which ports. A bridge learns
addresses simply by processing the network traffic that it receives. For a
bridge to learn the address of a station on the network that station must
transmit a frame. The bridge learns the
source address in the frame, not
the destination address, and records it with the port on which the frame
arrived. The next time that the bridge receives a frame that contains that
learned address as its destination address, the bridge knows exactly to
which port it should forward the frame.
Learned addresses that are called dynamic addresses , because they are
learned on the fly and no human intervention is required. This term
contrasts with
static addresses , which are addresses that are manually
configured.
A bridge maintains a database, called the address table , which lists all
static and dynamic addresses and associates them with appropriate port
numbers.
For more information about the module address tables, see “MAC
Address Table Design” later in this chapter.
Aging Addresses A dynamic address remains in the bridge’s address table as long as the
station to which it relates regularly transmits frames through the bridge. If
the station does not transmit within a specified period of time, the source
address is deleted, or
aged out , from the table. Accelerated address aging
may also occur if ports become disabled.
The process of aging addresses in combination with learning addresses
ensures that, if a station moves to a different segment on the network or
if the network topology changes, frames will continue to be forwarded
correctly. Address aging is also necessary because a bridge can store only
a finite number of addresses in its memory.
For information about how aging works in your modules, see “Address
Aging” later in this chapter.