There are several methods you can use to minimize routing updates

• prevent updates being
  generated on the interface
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• eliminate routing protocols
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• filter routing updates
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• minimize route advertising
  with redistribution
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You use the Passive-Interface command to prevent routing protocol updates from being generated on the interface.
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Because updates are not sent, the router cannot discover any neighbors on that interface.
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This means that the router does not learn about networks that are linked to a passive interface.
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The syntax is

  passive-interface £Binterface-name£b
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You specify the interface type and number with the Interface-Name parameter.
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You can eliminate the routing protocol Network command for an interface using an address from a separate network.
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Do not use this command to configure the routing protocol on the interfaces that connect to that network.
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You can set up route filters on any interface to prevent it inappropriately learning or propagating routing information.
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This is a useful security feature in Enhanced IGRP.
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Route filtering means that certain routes are not advertised to some parts of the network.
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To do this you use access lists to filter routing updates on one or more interfaces.
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You use the Distribute-List command to filter networks received in updates.
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The syntax for the command is

  distribute-list £Baccess£b-£Blist-number£b
     {£Uout£u|£Uin£u} [£Binterface - £b
     £Bname£b|£Brouting£b-£Bprocess£b]
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The Access-List-Number parameter is a standard IP number.
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The list explicitly specifies which networks are to be received and which are to be suppressed.
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The optional Interface-Name parameter is an interface on which the access list should be applied to incoming updates.
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If no interface is specified, the access list is applied to all incoming updates.
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The £Brouting-process£b parameter can contain the optional name of the particular routing process.
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Or it can contain the keywords static or connected.
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The Distribute-List command can be applied to transmitted (outbound) or received (inbound) routing updates.
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In this route filtering example you apply access list 7 to outbound packets.
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The access list does not allow routing information from network 10.0.0.0 to be distributed out of the S0 interface.
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As a result, network 10.0.0.0 is hidden on the S0 interface.
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This command applies access list 7 as a route redistribution filter on routing updates sent on Serial 0.
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The £UPermit£u keyword allows traffic matching the parameters to be forwarded.
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The network number and wildcard mask are used to verify source addresses.
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The first two address octets must match and the rest are don't care.
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You can minimize route advertising using redistribution.
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Redistribution allows routing information discovered through one routing protocol to be distributed in the update messages of another routing protocol.
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You do this using the Distribute-List command with access lists.
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Let's look at this redistribution filtering example.
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It filters the redistribution of routing updates between different routing processes such as IP RIP and Enhanced IGRP.
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This command enables routes learned from Enhanced IGRP on autonomous system 1 to be redistributed into IP RIP.
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This command specifies that all routes learned from Enhanced IGRP will be advertised by RIP as reachable in 3 hops.
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Here the command applies to routes defined by distribute list 7.
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These routes leaving the Enhanced IGRP process will be filtered prior to being given to the RIP process.
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This distribute list redistributes all routing information except updates from network 10.0.0.0.
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You can reduce routing updates over an interface by defining a static route on that interface.
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A static route is explicitly configured and entered into the routing table.

Static routes take precedence over routes chosen by dynamic routing protocols.
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You can configure static route redistribution on one router only.
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This eliminates the possibility of routing loops created by static route redistribution on routers with parallel routes between networks.
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And it allows updates to converge on the configured router.
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You define a static route using the IP Route command.
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The router configuration command

  ip route £Bnetwork£b[£Bmask£b]
    £Baddress£b[£Bdistance£b]

defines a path to an IP destination network or subnet.
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The default administrative distance is 1.
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This configuration requires redistribution.
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This router configuration

  ip route £Bnetwork£b[£Bmask£b]
    £Binterface£b[£Bdistance£b]

has a default administrative distance of 0.
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You use the interface parameter instead of the address parameter when the router is directly connected to the network.
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This configuration is automatically redistributed.
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Here's an example of a configuration for static route distribution.
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The Redistribute Static command applies to routes learned from static entries in the routing table.
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It redistributes them into Enhanced IGRP.
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This command filters routes, specified in distribute list 3, learned from static entries.
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This occurs before the routes are passed to the Enhanced IGRP process.
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Packets from source IP addresses that match the first two octets of 172.16, and the other octets that are don't care, will be forwarded.
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Route summarization procedures condense routing information.
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Using summarization, routers can reduce some sets of routes to a single advertisement.
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This reduces the load on the router and the perceived complexity of the network.
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Without summarization each router in a network must retain a route to every subnet in the network.
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Some routing protocols summarize automatically.
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Other routing protocols require manual configuration to support route summarization.
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With Enhanced IGRP, subnet routes of directly connected networks are automatically summarized at network number boundaries.
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In addition, a network administrator can configure route summarization at any interface with any bit boundary.
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This allows ranges of networks to be summarized arbitrarily.
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Network level route summarization - class A, B, or C - is automatic and is enabled by default.
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But you can turn off automatic route summarization using the No Auto-Summary command.
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You use the IP-Summary-Address Enhanced IGRP command to specify summary routes on a particular interface.
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It is useful if you want to advertise summary routes on a subnet boundary, rather than class A, B, or C addresses.
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The £BAs-Number£b parameter specifies the autonomous system number of the network being summarized.
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The £BAddress£b parameter is the IP address being advertised as the summary address.
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The address does not need to be aligned on class A, B, or C boundaries.
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The £BMask£b parameter contains the IP subnet mask being used to create the summary address.
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In this route summarization example, automatic route summarization has been disabled to allow discontiguous subnets 172.17.1.0 and 172.17.2.0 to communicate.
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The command Router Enhanced IGRP 1 starts an Enhanced IGRP process for autonomous system 1.
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The No Auto-Summary command disables automatic summarization at the point between major network numbers, such as 172.168.5.0 and 172.16.0.0.
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In this configuration £Beigrp 1£b specifies that routes learned from Enhanced IGRP autonomous system 1 will be manually summarized at the interface Serial 0.
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Your company's internetwork has been configured to run Enhanced IGRP.
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You want to monitor the Enhanced IGRP operation on the network.
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You enter the command £Rshow ip eigrp£r to display general information about the Enhanced IGRP routing process.
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You examine the contents of the routing table by entering the command £Rshow ip route£r.
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You enter £Rshow ip protocol£r to display the parameters and current state of the active routing process.
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You can turn on debugging by entering £Rdebug ip-eigrp events£r.
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As Enhanced IGRP updates are sent or received they are echoed to the screen.
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Enhanced IGRP debugging remains on until you turn it off.
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You turn it off by entering £Rundebug ip-eigrp events£r.
