91 lines
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91 lines
4.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>The Routing Table</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node32.html">Metric Values</A>
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<B>Up:</B> <A HREF="node27.html">IP Routing</A>
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<B> Previous:</B> <A HREF="node30.html">Gateways</A>
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<BR> <P>
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<H2><A NAME="SECTION004440000">The Routing Table</A></H2>
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<A NAME="tcpiproutingtable"></A>
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We are now focusing our attention on how IP chooses a gateway to use
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when delivering a datagram to a remote network.
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<P>
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We have seen before that erdos, when given a datagram for
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quark, checks the destination address and finds it is not on the
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local network. It therefore sends it to the default gateway,
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sophus, which is now basically faced with the same task.
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sophus recognizes that quark is not on any of the networks
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it is connected to directly, so it has to find yet another gateway to
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forward it through. The correct choice would be niels, the
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gateway to the Physics Department. sophus therefore needs some
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information to associate a destination network with a suitable gateway.
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<P>
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The routing information IP uses for this is basically a table linking
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networks to gateways that reach them. A catch-all entry (the <em>default
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route</em>) must generally be supplied, too; this is the gateway associated
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with network 0.0.0.0. All packets to an unknown network are sent
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through the default route. On sophus, this table might look like
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this:
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<pre>
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-----------------------------------------
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+-----------+-------------+------------+
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|Network | Gateway | Interface |
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+------------+-------------+------------+
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+------------+-------------+------------+
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|149.76.1.0 | - | fddi0 |
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|149.76.2.0 | 149.76.1.2 | fddi0 |
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|149.76.3.0 | 149.76.1.3 | fddi0 |
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|149.76.4.0 | - | eth0 |
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|149.76.5.0 | 149.76.1.5 | fddi0 |
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|... | ... | ... |
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|0.0.0.0 | 149.76.1.2 | fddi0 |
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+------------+-------------+------------+
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+------------+-------------+------------+
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</pre>
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Routes to a network that sophus is directly connected to don't
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require a gateway; therefore they show a gateway entry of ``-''.
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<P>
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<A NAME="1177"></A>
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<A NAME="1178"></A>
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<A NAME="1179"></A>
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<A NAME="1180"></A>
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Routing tables may be built by various means. For small LANs, it is
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usually most efficient to construct them by hand and feed them to IP
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using the route command at boot time (see chapter-<A HREF="node58.html#iface"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif"></A>).
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For larger networks, they are built and adjusted at run-time by
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<em>routing daemons</em>; these run on central hosts of the network and
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exchange routing information to compute ``optimal'' routes between the
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member networks.
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<P>
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<A NAME="1184"></A>
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<A NAME="1185"></A>
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<A NAME="1186"></A>
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<A NAME="1302"></A>
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<A NAME="1303"></A>
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<A NAME="1304"></A>
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Depending on the size of the network, different routing protocols will
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be used. For routing inside autonomous systems (such as Groucho Marx
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campus), the <em>internal routing protocols</em> are used. The most
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prominent one is RIP, the Routing Information Protocol, which is
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implemented by the BSD routed daemon. For routing between
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autonomous systems, <em>external routing protocols</em> like EGP
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(External Gateway Protocol), or BGP (Border Gateway Protocol) have to be
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used; these (as well as RIP) have been implemented in the University of
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Cornell's gated daemon.<A HREF="footnode.html#1305"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node32.html">Metric Values</A>
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<B>Up:</B> <A HREF="node27.html">IP Routing</A>
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<B> Previous:</B> <A HREF="node30.html">Gateways</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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