238 lines
4.6 KiB
HTML
238 lines
4.6 KiB
HTML
<HTML
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>What Is A Bridge?</TITLE
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux BRIDGE-STP-HOWTO: About The Linux Modular Bridge And STP</TH
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CLASS="SECT1"
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><A
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NAME="WHAT-IS-A-BRIDGE"
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>3. What Is A Bridge?</A
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></H1
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><P
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>A bridge is a device that separates two or more network segments
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within one logical network (e.g. a single IP-subnet).
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</P
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><P
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>A bridge is usually placed between two separate groups of computers
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that talk with each other, but not that much with the computers in
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the other group.
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A good example of this is to consider a cluster of Macintoshes and a
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cluster of Unix machines.
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Both of these groups of machines tend to be quite chatty amongst
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themselves, and the traffic they produce on the network causes
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collisions for the other machines who are trying to speak to one
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another.
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</P
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><P
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>The job of the bridge is to examine the destination of the
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data packets one at a time and decide whether or not to pass the
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packets to the other side of the Ethernet segment.
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The result is a faster, quieter network with less collisions.
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</P
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><P
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>The bridging code decides whether to bridge data or to drop it not
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by looking at the protocol type (IP, IPX, NetBEUI), but by looking at
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the <SPAN
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CLASS="ACRONYM"
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>MAC</SPAN
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>-address unique to each NIC.
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</P
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><DIV
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CLASS="IMPORTANT"
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><BLOCKQUOTE
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CLASS="IMPORTANT"
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><P
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><B
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>Important: </B
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>It's vital to understand that a bridge is neither a router nor
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a fire-wall.
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Spoken in simple term a bridge behaves like a network switch
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(i.e. Layer 2 Switch), making it a transparent network component
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(which is not absolutely true, but nearly).
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Read more about this at <A
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HREF="rules-on-bridging.html"
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>Section 4</A
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>.
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</P
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></BLOCKQUOTE
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></DIV
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><P
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>In addition, you can overcome hardware incompatibilities with a
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bridge, without leaving the address-range of your IP-net or subnet.
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E.g. it's possible to bridge between different physical media like
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10 Base T and 100 Base TX.
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</P
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><P
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>My personal reason for starting to set up a bridge was that in my
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work I had to connect Fast Ethernet components to a existing HP
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Voice Grade network, which is a proprietary networking standard.
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</P
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><P
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></P
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><DIV
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CLASS="VARIABLELIST"
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><P
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><B
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>Features Above Pure Bridging</B
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></P
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><DL
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><DT
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>STP</DT
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><DD
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><P
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>The Spanning Tree Protocol is a nifty method of keeping
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Ethernet devices connected in multiple paths working.
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The participating switches negotiate the shortest available path
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by STP.
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This feature will be discussed in
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<A
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HREF="advanced-bridge.html#STP"
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>Section 7.1</A
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>.
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</P
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></DD
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><DT
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>Multiple Bridge Instances</DT
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><DD
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><P
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>Multiple bridge instances allow you to have more than one
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bridge on your box up and running, and to control each instance
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separately.
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</P
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></DD
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><DT
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>Fire-walling</DT
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><DD
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><P
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>There is a patch to the bridging code which allows you
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to use IP chains on the interface inside a bridge.
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More info about this you'll find at
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<A
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HREF="advanced-bridge.html#IPCHAINS"
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>Section 7.2</A
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>.
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</P
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></DD
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></DL
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