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8.1 | ![]() |
Controlling Routing Update Traffic | |
8.1.3 | ![]() |
Filtering routing updates with distribute-list |
Configuring an interface as passive
prevents it from sending updates entirely. However, sometimes there is
a need to suppress only certain routes in the update from being sent or
received. If RTA is configured with the
network 10.0.0.0
command, all four directly connected subnets will be advertised in
the updates from RTA, along with any dynamically learned routes.
![]() This may be needed to enforce a routing policy that is based on some external factor such as link expense, administrative jurisdiction, or security concerns. In some cases, there is the desire to reduce needless overhead by preventing access routers from receiving the complete, and possibly immense, core routing table. Just assume that for one of these reasons, RTZ is not to learn the route to 10.1.1.0 from RTA. Use the distribute-list command to pick and choose which routing updates a router will send or receive. By referencing an access list, the distribute-list creates a route filter. This is a set of rules that precisely controls what routes a router will send or receive in a routing update. This command is available for all IP routing protocols and can be applied to either inbound or outbound routing updates. When applied to inbound updates, the syntax for configuring a route filter is as follows:
When applied to outbound updates, the syntax can be more complicated as shown in the following:
The routing-process and as-number options are invoked when exchanging routes between different routing protocols. This will be covered later in the module, in the section titled Route Redistribution. In Figure
Conversely, RTZ could have been told to globally filter network
10.1.1.0 from any incoming updates.
Or, 10.1.1.0 could have been precisely filtered from the specific interface on RTZ, shown as follows:
The distribute-list command can filter any routes in either an outbound or an inbound update globally, or for a specific interface. The Cisco IOS permits one incoming and one outgoing global distribute-list for each routing process. It also permits one incoming and one outgoing distribute-list for each interface involved in a routing process. Keep track of which routing filters are applied globally and which are applied on specific interfaces with the show ip protocols command.
Configuring a Passive EIGRP Interface Using
distribute-list
Command
With this configuration, RTA can send EIGRP Hello packets and establish adjacencies, but no routes will appear in any updates sent out s0.
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