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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
Controlling Advertising and Processing in Routing Updates
You can use the distribute-list router configuration command with access control lists to suppress routes
from being advertised in routing updates and to prevent other routers from learning one or more routes.
When used in OSPF, this feature applies to only external routes, and you cannot specify an interface
name.
You can also use a distribute-list router configuration command to avoid processing certain routes listed
in incoming updates. (This feature does not apply to OSPF.)
Beginning in privileged EXEC mode, follow these steps to control t he adv ertis ing o r pro cessing of
routing updates:
Use the no distribute-list in router configuration command to change or cancel a filter. To cancel
suppression of network advertisements in updates, use the no distribute-list out router confi gurati on
command.
Filtering Sources of Routing Information
Because some routing information might be more accurate than others, you can use filtering to prioritize
information coming from different sources. An administrative distance is a rating of the trustworthine ss
of a routing information source, such as a router or group o f rout ers. In a l arge networ k, so me r outi ng
protocols can be more reliable than others. By specifying administrati ve distance values, you enable the
router to intelligently discriminate between sources of routing information. The router always picks the
route whose routing protocol has the lowest administrative distance. Table 22-11 on page 22-56 shows
the default administrative distances for various routing information sources.
Because each network has its own requirements, there are n o g en eral guide li nes for as sig ning
administrative distances.
Beginning in privileged EXEC mode, follow these steps to filter sources of routing information:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 router {rip | ospf | igrp | eigrp} Enter router configuration mode.
Step3 distribute-list {access-list-number |
access-list-name} out [interface-name | routing
process | autonomous-system-number]
Permit or deny routes from being advertised in routing
updates, depending upon the action listed in the access list.
Step4 distribute-list {access-list-number |
access-list-name} in [type-number]Suppress processing in routes listed in updates.
Step5 end Return to privileged EXEC mode.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 router {rip | ospf | igrp | eigrp} Enter router configuration mode.