Information Technology Reference
In-Depth Information
Bridges and STP
Ethernet bridging occurs in the data-link layer of the OSI model. Switches perform the same
function as bridges. For the rest of the chapter, the term switches refers to bridges. Bridges (and
switches) forward frames from one interface to another based on the destination MAC address.
For any incoming frame, bridges forward the frame out a specific port, if the destination MAC
address is known, or it is flooded out all ports if the MAC address is unknown. If the destination
MAC is unknown, the bridges forward the frame out all ports. This is known as flooding .
Bridges have three primary functions:
To learn the MAC addresses of all nodes and their associated port
To filter incoming frames whose destination MAC addresses are located on the same
incoming port
To forward incoming frames to the destination MAC through their associated port
Bridges keep a bridge table to track the MAC addresses available out each port.
An example of the table on a Catalyst switch is shown in Example 4-3. The switch creates a
table that lists the MAC address and port for the stations. For example, the station with MAC
address 00-10-7b-80-ba-d5 is located out port 2/10. This MAC table was created dynamically.
Bridge MAC Table
Example 4-3
cat5000: show cam dynamic
VLAN Dest MAC/Route Des Destination Ports or VCs / [Protocol Type]
---- ------------------ --------------------------------------------
710 00-10-7b-80-ba-d5 2/10 [ALL]
34 00-10-7b-1b-6f-9c 2/3 [ALL]
59 00-10-7b-7e-cc-c3 2/5 [ALL]
59 00-e0-b0-64-6e-47 2/9 [ALL]
1 00-e0-b0-64-6e-49 2/12 [ALL]
1 00-10-7b-7e-cc-cd 2/11 [ALL]
710 00-e0-b0-64-6e-07 2/7 [ALL]
34 00-50-3e-e4-58-59 2/4 [ALL]
Total Matching CAM Entries Displayed = 8
00-10-7b-80-ba-d5
Routers can also be configured to bridge protocols. By default, if bridging is configured on an
interface, the router bridges any protocol that is not routed on that interface.
The configuration in Example 4-4 bridges between Ethernet 1 and Ethernet 0. Bridge group 1
is configured for the IEEE STP with the bridge 1 protocol ieee command. Each interface is
configured to bridge with the bridge-group 1 command.
Configuration of Bridge Between Ethernet 0 and Ethernet 1
Example 4-4
bridge 1 protocol ieee
!
interface ethernet 0
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