affects the information that is exchanged between routers A and B and between them and router C. Unless you configure another routing protocol and redistribute between it and RIP, router A does not tell router C that it has a way to reach the networks advertised by router B via RIP. Likewise, router B does not tell router C that it has a way to reach the networks advertised by router A via RIP. Redundancy is built into this network. However, the three routers are not able to use the redundancy effectively. For example, if the link between router C and router A fails, router C does not know that it has an alternate route through router B. In this situation, route filtering should be configured. Web Links Filtering Routes
http://www.cisco.com/en/US/tech/tk365
/technologies_tech_note09186a0080208748.shtml EIGRP Passive Interface
http://www.cisco.com/en/US/tech/tk365
/technologies_tech_note09186a0080093f0a.shtml
Content 5.3 Controlling Routing Update Traffic 5.3.4 Configuring Route Filtering Using Distribute Lists The passive interface technique prevents all routing updates from being advertised out of an interface. However, in many cases you do not want to prevent all routing information from being advertised. You might want to block the advertisement of only certain specific routes. For example, you could use such a solution to prevent routing loops when you are implementing two-way route redistribution with dual redistribution points. Some ways to control or prevent dynamic routing updates are as follows: Another way to control routing updates is with a distribute list, which allows an access control list (ACL) to be applied to routing updates. You may be familiar with ACLs associated with an interface and used to control IP traffic. However, routers can have many interfaces, and route information can also be obtained through route redistribution, which does not involve an interface at all. Additionally, ACLs do not affect traffic that is originated by the router, so applying one to an interface would have no effect on outgoing routing advertisements. When you link an ACL to a distribute list, routing updates can be controlled no matter what their source is. Configure ACLs in global configuration mode, and then configure the associated distribute list under the routing protocol. The ACL should permit the networks that will be advertised or redistributed and deny the networks that will remain hidden. The router then applies the ACL to routing updates for that protocol. Options in the distribute-list command allow updates to be filtered based on three factors: A distribute list gives the administrator great flexibility in determining exactly which routes the router distributes.
Content 5.3 Controlling Routing Update Traffic 5.3.5 Implementing the Distribute List You can filter routing update traffic for any protocol by defining an ACL and applying it to a specific routing protocol. You use the distribute-list command and link it to an ACL to complete the filtering of routing update traffic. (The inbound distribute-list command allows the use of a route map instead of an ACL.) A distribute list enables the filtering of routing updates coming into a specific interface from neighboring routers using the same routing protocol or going out of the interface toward the routers. A distribute list also allows the filtering of routes redistributed from other routing protocols or sources. To configure a distribute list using an ACL, use the following procedure: Step 1 Identify the network addresses that you want to filter and create an ACL. Step 2 Determine whether you want to filter traffic on an incoming interface, an outgoing interface, or routes being redistributed from another routing source. Step 3 Use the distribute-list out command to assign the ACL to filter outgoing routing updates or to assign it to routes being redistributed into the protocol. Figure displays the command parameters.
Note
The distribute-list out command cannot be used with link-state routing protocols for blocking outbound link-state advertisements (LSAs) on an interface. Step 4 Use the distribute-list in command to assign the ACL to filter incoming routing updates coming in through an interface. This command prevents most routing protocols from placing the filtered routes in their database. When this command is used with OSPF, the routes are placed in the database but not the routing table. Figure displays the command parameters. Figure provides an example of an outbound distribute list. The configured distribute list will deny the advertising of network 10.1.1.0 from exiting the serial 2 interface on router RTA. Figure provides an example of an inbound distribute list. The configured distribute list will deny the advertising of network 10.1.1.0 from entering the serial 0 interface on router RTZ.
Content 5.3 Controlling Routing Update Traffic 5.3.6 Filtering Routing Updates with a Distribute List The distribute-list 7 out s0 command in Figure applies ACL 7 as a route filter for EIGRP routing updates sent out from interface serial 0 to other routers running this routing protocol. ACL 7 is a standard ACL that permits routing information regarding network 172.16.0.0 only. The implicit deny any at the end of the ACL prevents routing updates about any other networks from being advertised. As a result, network 10.0.0.0 is hidden from the rest of the network.
Content 5.3 Controlling Routing Update Traffic 5.3.7 Controlling Redistribution with Distribute Lists With mutual redistribution, using a distribute list helps prevent route feedback, which also helps prevent routing loops. Route feedback occurs when routes originally learned from one routing protocol are redistributed back into that protocol. As shown in Figure , two-way redistribution is completed between RIP and OSPF. Networks 10.1.0.0 to 10.3.0.0 redistribute from RIP into OSPF. Route feedback could occur if another redistribution point is configured (router D) and OSPF then redistributes those networks back into RIP. ACL 2 allows the original RIP routes and denies all others. The distribute list configured under OSPF refers to this ACL. The result is that networks 10.8.0.0 to 10.11.0.0, originated by OSPF, cannot be redistributed back into OSPF from RIP. Redistribution into RIP from OSPF is filtered with ACL 3. Router D will have a similar configuration to Router B. A distribute list hides network information, which could be considered a drawback in some circumstances. In a network with redundant paths, the goal of using a distribute list may be to prevent routing loops. The distribute list permits routing updates that enable only the desired paths to be advertised. Therefore, other routers in the network do not know about other ways to reach the filtered networks.
Content 5.4 Policy-based Routing 5.4.1 Defining Route Maps Route maps are similar to complex ACLs, but are much more powerful. They are also much more flexible than ACLs and can handle situations that are not possible with ACLs. Route maps might also use complex ACLs. They allow conditions to be tested against a