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Managing Routing Protocols


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Managing Routing Protocols



Configuring Passive Interfaces



Filtering Routing Updates



Managing Redistribution



Resolving Issues with VLSM and Classful Routing Protocols



Leveraging Default Routing



Configuring Route Summarization



Deploying Policy Routing with Route Maps




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Enhanced IP Services for Cisco Networks

From: Enhanced IP Services for Cisco Networks
Author: Donn Lee
Publisher: Cisco Press (53)
More Information

3. Managing Routing Protocols

Routing, introduced in Chapter 2, “Deploying Interior Routing Protocols,” is crucial to the proper operation of your network. The routing function of a network is like the circulatory system of the human body: It is responsible for moving vital elements (data or blood cells) through a network efficiently. When those vital elements fail to reach their intended destinations, you can expect the systems that feed off the network to suffer (this applies to both internetworks and humans).

Chapter 2 covered basic routing configuration, but routing involves much more than simply enabling RIP, IGRP, OSPF, and EIGRP and letting them run. In a real-world network, routing protocols must be managed, extended, and optimized to promote overall network stability, flexibility, and efficiency. This chapter covers the key techniques that help you achieve these objectives.

The main topics of this chapter are

  • Configuring Passive Interfaces

  • Filtering Routing Updates

  • Managing Redistribution

  • Resolving Issues with VLSM and Classful Routing Protocols

  • Leveraging Default Routing

  • Configuring Route Summarization

  • Deploying Policy Routing with Route Maps

Configuring Passive Interfaces

A common way of controlling routing information is to make an interface passive. A passive interface is a silenced interface: an interface on which you deliberately suppress the advertising of routing updates. You might want to do this in certain situations. For example, for security reasons you might have to block routing updates sent to a particular department or company because the updates reveal the topology of your network. In another case, when redistributing from one routing protocol to another, passive interfaces localize updates for efficiency and stability (see “Managing Redistribution,” later in this chapter). Also, in dial-on-demand routing setups, passive interfaces prevent routing updates from triggering dial-up lines that are billed per minute—this controls operational costs.

A simple use of a passive interface is for silencing chatty protocols such as RIP on networks that do not require routing updates. Figure 3-1 illustrates such a scenario.

Figure 3-1. Using a Passive Interface to Block Routing Information

As depicted in Figure 3-1, Router A is using RIP to learn and advertise the subnets in major net However, Router A is the only RIP device on the Ethernet LAN (assume the clients are not RIP-enabled). Therefore, it is unnecessary and wasteful for Router A to send RIP updates out its Ethernet0 interface—no other routers are on the LAN and none of the clients care to receive any RIP information. By configuring Ethernet0 as passive, you can prevent all RIP advertisements from being sent out the interface and to the clients. The IOS command that does this is passive-interface, a router configuration mode command. The following is Router A's RIP configuration with the enhancement:

router rip
 passive-interface Ethernet0

Router A is running RIP—it just isn't sending any RIP packets out Ethernet0. In fact, Router A still listens to RIP on the passive interface and would not block any RIP updates arriving on it. To prevent the router from listening to RIP, you must use route filters (see the following section, “Filtering Routing Updates”).

To verify that the passive-interface command is working, you can look at the debugging messages for the routing protocol. Here's an output of debug ip rip for the scenario in Figure 3-1:

RTA#debug ip rip
RIP protocol debugging is on

RIP: sending v1 update to via Serial0 (      subnet, metric 1      subnet, metric 2      subnet, metric 1 RIP: sending v1 update to via Serial1 (      subnet, metric 1      subnet, metric 3      subnet, metric 3      subnet, metric 4      subnet, metric 4      subnet, metric 4      subnet, metric 1

The preceding output is one of Router A's periodic waves of RIP updates. The output confirms that Router A is sending updates from Serial0 and Serial1, but not from Ethernet0. Ethernet0 is noticeably absent from the debug output.


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