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No matter how <strong>the</strong> announcement entry is made, it will appear in <strong>the</strong> router's<br />

configuration as classful <strong>network</strong> address summaries:<br />

router eigrp 99<br />

<strong>network</strong> 192.168.191.0<br />

<strong>network</strong> 192.168.12.0<br />

<strong>network</strong> 192.168.192.0<br />

<strong>network</strong> 192.168.10.0<br />

<strong>network</strong> 192.168.181.0<br />

<strong>network</strong> 192.168.0.0<br />

no au<strong>to</strong>-summary<br />

How <strong>the</strong> announcements are displayed in <strong>the</strong> configuration has no bearing on how<br />

<strong>the</strong>y will be announced; that is determined by which pro<strong>to</strong>col is used. If IGRP is in<br />

use, <strong>the</strong> <strong>network</strong>s will be announced classfully, depending on <strong>the</strong> subnet mask of<br />

<strong>the</strong> router for mask interpretation. Remember that when you subnet class A and B<br />

address spaces, you must use a consistent subnet maskfor <strong>the</strong> entire address space.<br />

If EIGRP is in use, <strong>the</strong> <strong>network</strong> will be announced along with <strong>the</strong>ir subnet masks.<br />

EIGRP by default uses classful au<strong>to</strong>-summarization; this should be disabled if VLSM<br />

and a single classful spaceis being used for addressing. Disabling classful<br />

au<strong>to</strong>-summarization can be accomplished with <strong>the</strong> router<br />

configuration subcommand. The configuration outlined thus far represents <strong>the</strong><br />

minimum required configuration <strong>to</strong> enable IP EIGRP/IGRP routing.<br />

NOTE<br />

IGRP/EIGRP will only send routing information updates on interfaces with addresses<br />

that correspond <strong>to</strong> those announced in its configuration. In situations where a large<br />

classful address space is used, this may not be desirable, especially if multiple<br />

IGRP/EIGRP processes are announcing different segments of <strong>the</strong> <strong>network</strong> that have<br />

<strong>the</strong> same classful root address (for example, 172.16.0.0 is a class B root address).<br />

In <strong>the</strong>se situations, <strong>the</strong> command can be used <strong>to</strong> suppress<br />

routing message updates from being sent out of an interface as part of <strong>the</strong> particular<br />

process.<br />

Before we look at <strong>the</strong> EIGRP specific configuration options, a quick word about<br />

default routing: IGRP and EIGRP both use <strong>the</strong> global configuration EXEC variable <strong>to</strong> set and distribute default <strong>network</strong><br />

information. This setting can be set on one gateway router and redistributed <strong>to</strong> <strong>the</strong><br />

rest of <strong>the</strong> members of <strong>the</strong> IGRP/EIGRP routing process, or it can be set on each

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