Controlling Advertising And Processing In Routing Updates; Filtering Sources Of Routing Information - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring Protocol-Independent Features
Use a network monitoring privileged EXEC command such as show ip ospf interface to verify the
interfaces that you enabled as passive, or use the show ip interface privileged EXEC command to verify
the interfaces that you enabled as active.
To re-enable the sending of routing updates, use the no passive-interface interface-id router
configuration command. The default keyword sets all interfaces as passive by default. You can then
configure individual interfaces where you want adjacencies by using the no passive-interface router
configuration command. The default keyword is useful in Internet service provider and large enterprise
networks where many of the distribution routers have more than 200 interfaces.

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 the advertising or processing of
routing updates:
Command
Step 1 configure terminal
Step 2 router {bgp | rip | igrp | eigrp}
Step 3
distribute-list {access-list-number |
access-list-name} out [interface-name | routing
process | autonomous-system-number]
Step 4
distribute-list {access-list-number |
access-list-name} in [type-number]
Step 5 end
Step 6 copy running-config startup-config
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 configuration
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 trustworthiness
of a routing information source, such as a router or group of routers. In a large network, some routing
protocols can be more reliable than others. By specifying administrative 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.
the default administrative distances for various routing information sources.
Because each network has its own requirements, there are no general guidelines for assigning
administrative distances.
Catalyst 3750 Switch Software Configuration Guide
34-72
Chapter 34
Purpose
Enter global configuration mode.
Enter router configuration mode.
Permit or deny routes from being advertised in routing
updates, depending upon the action listed in the access list.
Suppress processing in routes listed in updates.
Return to privileged EXEC mode.
(Optional) Save your entries in the configuration file.
Configuring IP Unicast Routing
Table 34-12 on page 34-63
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