lect-07 - Internetworking: LAN Switches (Bridges) ECE/CS...

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CS/ECE 438 1 Internetworking: LAN Switches (Bridges) ECE/CS 438 Fall 2011 Slides from Peterson & Davies
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Outline n Switching and Bridging ¡ Datagram switching ¡ Virtual Circuits ¡ Source Routing n Basic Internetworking (IP) n Routing
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Bridges and LAN Switches n Bridges and LAN Switches ¡ Class of switches that is used to forward packets between shared- media LANs such as Ethernets n Known as LAN switches n Referred to as Bridges ¡ Suppose you have a pair of Ethernets that you want to interconnect n One approach is put a repeater in between them ¡ It might exceed the physical limitation of the Ethernet n No more than four repeaters between any pair of hosts n No more than a total of 2500 m in length is allowed n An alternative would be to put a node between the two Ethernets and have the node forward frames from one Ethernet to the other ¡ This node is called a Bridge ¡ A collection of LANs connected by one or more bridges is usually said to form an Extended LAN
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Bridges and LAN Switches n Simplest Strategy for Bridges ¡ Accept LAN frames on their inputs and forward them out to all other outputs ¡ Used by early bridges n Learning Bridges ¡ Observe that there is no need to forward all the frames that a bridge receives
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n Consider the following figure ¡ When a frame from host A that is addressed to host B arrives on port 1, there is no need for the bridge to forward the frame out over port 2. ¡ How does a bridge come to learn on which port the various hosts reside? Learning Bridges and LAN Switches
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Learning Bridges and LAN Switches n Solution ¡ Download a table into the bridge ¡ Who does the download? n Human ¡ Too much work for maintenance A Bridge B C X Y Z Port 1 Port 2 Host Port -------------------- A 1 B 1 C 1 X 2 Y 2 Z 2
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Bridges and LAN Switches n Can the bridge learn this information by itself? YES! n How: ¡ Each bridge inspects the source address in all the frames it receives ¡ Record the information at the bridge and build the table ¡ When a bridge first boots, this table is empty ¡ Entries are added over time ¡ A timeout is associated with each entry ¡ The bridge discards the entry after a specified period of time n To protect against the situation in which a host is moved from one network to another n If the bridge receives a frame that is addressed to host not currently in the table ¡ Forward the frame out on all other ports
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This note was uploaded on 02/08/2012 for the course ECE 438 taught by Professor Luo during the Fall '08 term at University of Illinois, Urbana Champaign.

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lect-07 - Internetworking: LAN Switches (Bridges) ECE/CS...

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