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Part 11. Transparent Bridging


Date: 2015-10-07; view: 450.


Most Ethernet LAN switches use a very cool system called transparent bridging to create their address lookup tables. Transparent bridging is a technology that allows a switch to learn everything it needs to know about the location of nodes on the network without the network administrator having to do anything. Transparent bridging has five parts:

· Learning

· Flooding

· Filtering

· Forwarding

· Aging

Here's a step-by-step description of transparent bridging:

§ The switch is added to the network, and the various segments are plugged into the switch's ports.

§ A computer (Node A) on the first segment (Segment A) sends data to a computer (Node B) on another segment (Segment C).

§ The switch gets the first packet of data from Node A. It reads the MAC address and saves it to the look-up table for Segment A. The switch now knows where to find Node A anytime a packet is addressed to it. This process is called learning.

§ Since the switch does not know where Node B is, it sends the packet to all of the segments except the one that it arrived on (Segment A). When a switch sends a packet out to all segments to find a specific node, it is called flooding.

§ Node B gets the packet and sends a packet back to Node A in acknowledgement.

§ The packet from Node B arrives at the switch. Now the switch can add the MAC address of Node B to the look-up table for Segment C. Since the switch already knows the address of Node A, it sends the packet directly to it. Because Node A is on a different segment than Node B, the switch must connect the two segments to send the packet. This is known as forwarding.

§ The next packet from Node A to Node B arrives at the switch. The switch now has the address of Node B, too, so it forwards the packet directly to Node B.

§ Node C sends information to the switch for Node A. The switch looks at the MAC address for Node C and adds it to the look-up table for Segment A. The switch already has the address for Node A and determines that both nodes are on the same segment, so it does not need to connect Segment A to another segment for the data to travel from Node C to Node A. Therefore, the switch will ignore packets traveling between nodes on the same segment. This is filtering.

§ Learning and flooding continue as the switch adds nodes to the look-up tables. Most switches have plenty of memory in a switch for maintaining the lookup tables; but to optimize the use of this memory, they still remove older information so that the switch doesn't waste time searching through stale addresses. To do this, switches use a technique called aging. Basically, when an entry is added to the look-up table for a node, it is given a timestamp. Each time a packet is received from a node, the timestamp is updated. The switch has a user-configurable timer that erases the entry after a certain amount of time with no activity from that node. This frees up valuable memory resources for other entries. As you can see, transparent bridging is a great and essentially maintenance-free way to add and manage all the information a switch needs to do its job!

In our example, two nodes share segment A, while the switch creates independent segments for Node B and Node D. In an ideal LAN-switched network, every node would have its own segment. This would eliminate the possibility of collisions and also the need for filtering.


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Part 10. Switch Configurations | Part 12. Redundancy
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