📄 eigrptheory.htm
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<p><b><font size="4" color="#008000" face="Arial">EIGRP Overview</font></b></p>
<p><font face="Arial">Enhanced Interior Gateway Routing Protocol (EIGRP) is an Advanced distance-vector,
interior gateway protocol (IGP) used by routers to exchange routing
information. EIGRP uses a composite metric of bandwidth and delay to determine the best path between two
locations. The metric can also be administratively configured to factor in
the Maximum Transmission Unit (MTU), Reliability, and load for the link. In a
EIGRP network, each router
sends <b>incremental</b> updates of its routing table to its neighboring routers
whenever there is a change. Because of this EIGRP has very fast
convergence. When a router receives a neighbor's routing table update it uses the
information provided to run an algorithm (DUAL algorithm) and update its own routing table and then sends the updated
information to its neighbors. This procedure is repeated by each router and results in
a state referred to as network convergence, in which all routers have an
identical view of the internetwork topology.</font></p>
<font face="Arial">
<p>EIGRP can route IP, IPX and Appletalk. Along with IS-IS, it is one of the
few multi-protocol routing protocols. The Diffusing Update Algorithm
(DUAL) is the heart of EIGRP. In essence, DUAL always keeps a backup route in
mind, in case the primary route goes down. DUAL also limits how many routers
are affected when a change occurs to the network.</p>
<p>There is no maximum allowable number of hops. In a EIGRP network, each
router multi-casts "hello" packs to discover its adjacent neighbor. This
adjacency database is shared with other routers to build a topology database.
From the topology database the best route (Successor) and the second best route
(Feasible Successor) is found.</p>
<p>EIGRP is classless, meaning it does include the subnet mask in routing
updates. However, by default 'auto-summary' is enabled. You must disable
it if
you want subnet information from other major networks. The EIGRP metric is
a complex calculation, but by default it only uses bandwidth and delay
to determine the best path.</p>
</font>
<p> </p>
<p><font face="Arial">IP Addresses: Please set these IP addresses on the interfaces of your routers.<a name="ip"></a></font></p>
<table border="0" width="100%">
<tr>
<td width="25%"></td>
<td width="25%"><font face="Times New Roman">eRouter1</font></td>
<td width="25%"><font face="Times New Roman">eRouter2</font></td>
<td width="25%"><font face="Times New Roman">eRouter4</font></td>
</tr>
</table>
<table border="0" width="100%">
<tr>
<td width="25%"><font face="Times New Roman">Interface Ethernet 0</font></td>
<td width="25%"><font face="Times New Roman">10.1.1.1 255.255.255.0</font></td>
<td width="25%"><font face="Times New Roman">10.1.1.2 255.255.255.0</font></td>
<td width="25%"><font face="Times New Roman">Not Available</font></td>
</tr>
<tr>
<td width="25%"><font face="Times New Roman">Interface Serial 0</font></td>
<td width="25%"><font face="Times New Roman">172.16.10.1 255.255.255.0</font></td>
<td width="25%"><font face="Times New Roman">Not Available</font></td>
<td width="25%"><font face="Times New Roman">172.16.10.2 255.255.255.0</font></td>
</tr>
</table>
<p> </p>
<p><font face="Arial">Lets connect to eRouter1 and get it configured. We will be using the
table above for our IP addresses.</font><font color="#FF0000" face="Arial"><br>
</font><font face="Arial">
<br>
</font><font face="Terminal" size="2">eRouter>en<br>
eRouter#conf t<br>
Enter configuration commands, one per line. End with CNTL/Z.<br>
eRouter(config)#int e0<br>
eRouter(config-if)#ip address 10.1.1.1 255.255.255.0<br>
eRouter(config-if)#no shut<br>
eRouter(config-if)#exit<br>
00:35:15: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up<br>
eRouter(config)#hostname eRouter1<br>
eRouter1(config)#int s0<br>
eRouter1(config-if)#ip address 172.16.10.1 255.255.255.0<br>
eRouter1(config-if)#no shut<br>
00:35:16: %LINK-3-UPDOWN: Interface Serial0, changed state to up<br>
eRouter1(config-if)#exit<br>
00:35:16: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up</font><font color="#FF0000" face="Times New Roman"><br>
</font>
</p>
<p><b><font face="Arial" size="4" color="#008000">Configuring Routers to use
EIGRP</font></b></p>
<p><font face="Arial">We first want to configure eRouter1 for EIGRP. To enable EIGRP as the routing protocol we only need to type: <font color="#FF0000">router
EIGRP AS. </font> The AS stands for an Autonomous System number. An
Autonomous System is defined as a network under a common administration with a
common routing policy. You will need to use the SAME autonomous system
number on every router that you would like to share its routing table
with. We can see this below in the router output. Notice the new mode
we have entered eRouter1(config-router)# that tells us we are configuring the
router. </font></p>
<p><font face="Terminal" size="2">
eRouter1(config)#router EIGRP 100<br>
eRouter1(config-router)#</font></p>
<p><font face="Arial">Now that we have EIGRP running on our Router we need to tell the router which
networks it is connected to. We do this by using the network statement.
What this means is every interface of our router that is directly connected to an
active network needs a network number. We will have some networks using the same
IP
addressing schemes with different subnets, and some are using entirely different
addressing schemes. Look at the diagram below. In this diagram we have
three different kinds of addressing schemes. Lets look at these in more
detail. On eRouter 1 we have an IP address of 10.1.1.1 with a /24 subnet
mask. When entering the network statement you are only required to enter the class part
of the address. For example on eRouter1 we have
already issued the command router EIGRP, we then need to specify the directly
connected networks to eRouter1 so the router can advertise these routes in its routing table. To do this we would only need to type: <font color="#FF0000">network
10.0.0.0 </font>now we have not told the router about the network
on its Serial interface, to do this we would type: <font color="#FF0000">network
172.16.0.0 </font>Lets look at eRouter 2 what network statement would we need to use on this router ______________________________________(see
the answer below the diagram.) </font> </p>
<p> </p>
<p align="center">
<font face="Arial">
<img border="0" src="default.jpg" width="524" height="431"></font></p>
<p><font face="Arial">The answer is <font color="#FF0000">network 10.0.0.0. </font>The network statement for the
Ethernet link is the same for eRouter1
and eRouter2. On eRouter1 what network statement would you need for the
serial link? For this network
statement you used the classful portion of the address 172.16.10.1 which would be
just <font color="#FF0000">network 172.16.0.0</font>.</font></p>
<p><font face="Arial">Now that we understand the network command lets enter it on our eRouter1.</font></p>
<p><font face="Arial" color="#FF0000"><br>
</font><font face="Terminal" size="2">eRouter1(config-router)#network 172.16.0.0<br>
eRouter1(config-router)#network 10.0.0.0<br>
eRouter1(config-router)#</font></p>
<p><font face="Arial">If you notice we only needed to enter 10.0.0.0 for our network statement, this is
because 10.0.0.0 is a Class A address and EIGRP only uses the classful portion
of the address. Now we have configured eRouter1 for EIGRP lets connect to
eRouter2 and get it setup.</font></p>
<p><font face="Arial"> We need to
connect to eRouter2 and follow the same instructions. Lets select eRouter2
from the Window pull down menu. When we connect we are going to set a hostname to eRouter2, then set the
IP addresses to the <a href="#ip">table above</a> and configure
EIGRP. </font></p>
<p><font color="#FF0000" face="Arial"><br>
</font><font face="Terminal" size="2">eRouter>en<br>
eRouter#conf t<br>
Enter configuration commands, one per line. End with CNTL/Z.<br>
eRouter(config)#hostname eRouter2<br>
eRouter2(config)#int e0<br>
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