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How Transparent Bridging Works
A transparent bridge is a
data-link layer (layer 2) relay device that connects two or more networks
or network systems. When a transparent bridge powers up, it automatically
begins learning the network topology by examining the media access
control (MAC) source address of every incoming packet. The bridge
then creates an entry in the forwarding table consisting of the address
and associated interface where the packet was received.
More specifically, a transparent bridge performs
all of the following actions to learn the network topology:
- Learning—The bridge examines the MAC address of
every incoming packet, records the MAC address and associated interface
in the forwarding table, and manages the database of MAC addresses
and their associated interfaces.
- Flooding—When a packet’s destination address
does not match any entries in the forwarding table, the bridge transmits
(floods) the packet on all bridge interfaces to all network segments
except the interface on which the packet was received.
- Forwarding—Once the bridge has learned a packet’s
destination address (that is, has a matching entry in its forwarding
table), the bridge uses the associated port and interface information
to send the packet toward its destination.
- Filtering—If the bridge detects that a packet’s
source and destination addresses are on the same network segment,
it ignores (filters) that packet. Filtering is
the process by which the bridge can screen network traffic for certain
characteristics and determine whether to forward or discard (drop)
that traffic based on user-defined criteria. On E-series routers,
filtering criteria can include the MAC source address, MAC destination
address, and protocol type.
- Aging—When a bridge adds a dynamic (learned) MAC
address entry to the forwarding table, it assigns an age to the entry.
The bridge updates this age each time it receives a packet. To manage
MAC entries more efficiently, you can configure an entry’s aging
time, which is the maximum time that an entry can remain in the forwarding
table before it “ages out.”
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