Lab 30: Passive Interfaces for RIPv2 Updates

Lab Objective:

The objective of this lab exercise is for you to learn and understand how to prevent RIPv2 from sending unnecessary updates by using passive interfaces.

Lab Purpose:

Preventing unnecessary RIPv2 updates using passive interfaces is a fundamental skill. By default, RIPv2 sends updates via multicast on all interfaces for which RIPv2 has been enabled. For example, it is not possible to ever have another device connected to a Loopback interface, so it is a waste of router processing power to have RIPv2 continuously sending updates to a Loopback interface. As a Cisco engineer, as well as in the Cisco CCNA exam, you will be expected to know how to prevent RIPv2 from sending unnecessary updates.

Certification Level:

This lab is suitable for CCNA certification exam preparation.

Lab Difficulty:

This lab has a difficulty rating of 6/10.

Readiness Assessment:

When you are ready for your certification exam, you should complete this lab in no more than 10 minutes.

Lab Topology:

Please use the following topology to complete this lab exercise:

Task 1:

This lab will use only two routers. Configure the hostnames on routers R1 and R2 as illustrated in the topology.

Task 2:

Configure a back-to-back Serial connection between R1 and R2. Configure the DCE interface Serial0/0 in R2 to provide clocking to R1 at a clock speed of 2 Mbps.

Task 3:

Configure IP addresses 192.168.0.1/30 and 192.168.0.2/30 on R1 and R2 Serial0/0 interfaces, respectively. Configure the Loopback interfaces on R1 with the IP addresses illustrated in the topology. Enable RIPv2 for the Serial0/0 interface on R2 and R1 and attached Loopback subnets.

Task 4:

First, use the show ip protocols command to see the interfaces on which RIPv2 is sending updates. Next, enable debugging on R1 and verify that RIPv2 updates are being sent on all RIPv2-enabled interfaces. When you have verified this, disable debugging.

Task 5:

Prevent RIPv2 from sending updates on the Loopback interfaces. Verify your configuration by enabling debugging. Disable debugging when done.

Configuration and Verification

Task 1:

For reference information on configuring hostnames, please refer to earlier labs.

Task 2:

For reference information on configuring DCE clocking, please refer to earlier labs.

Task 3:

For reference information on configuring Loopback interfaces and RIPv2, please refer to earlier labs.

Task 4:

R1#show ip protocols 
Routing Protocol is “rip” 
 Sending updates every 30 seconds, next due in 19 seconds 
 Invalid after 180 seconds, hold down 180, flushed after 240 
 Outgoing update filter list for all interfaces is not set 
 Incoming update filter list for all interfaces is not set 
 Redistributing: rip 
 Default version control: send version 2, receive version 2 
  Interface             Send  Recv  Triggered RIP  Key-chain 
  Serial0/0             2     2 
  Loopback100           2     2 
  Loopback101           2     2 

[Output Truncated] 

R1#debug ip rip 
RIP protocol debugging is on 
*Mar  1 03:09:46.237: RIP: sending v2 update to 224.0.0.9 via Serial0/0 (192.168.0.1) 
*Mar  1 03:09:46.237: RIP: build update entries
*Mar  1 03:09:46.237:   10.10.100.0/27 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:09:46.237:   10.10.101.0/26 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:09:53.248: RIP: sending v2 update to 224.0.0.9 via Loopback101 (10.10.101.1) 
*Mar  1 03:09:53.248: RIP: build update entries 
*Mar  1 03:09:53.248:   10.10.100.0/27 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:09:53.248:   192.168.0.0/30 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:09:53.252: RIP: ignored v2 packet from 10.10.101.1 (sourced from one of our addresses) 
*Mar  1 03:10:09.070: RIP: sending v2 update to 224.0.0.9 via Loopback100 (10.10.100.1) 
*Mar  1 03:10:09.070: RIP: build update entries 
*Mar  1 03:10:09.070:   10.10.101.0/26 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:10:09.070:   192.168.0.0/30 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:10:09.074: RIP: ignored v2 packet from 10.10.100.1 (sourced from one of our addresses) 
R1# 
R1#undebug ip rip 
RIP protocol debugging is off

NOTE: Pay particular attention to the fact that RIPv2 is sending updates via the Loopback interfaces as illustrated below:

*Mar  1 03:09:53.248: RIP: sending v2 update to 224.0.0.9 via Loopback101 (10.10.101.1) 
*Mar  1 03:10:09.070: RIP: sending v2 update to 224.0.0.9 via Loopback100 (10.10.100.1)

Loopback interfaces are logical interfaces that have the majority of the characteristics of physical interfaces. However, one important thing to remember is that no host can ever reside on a subnet configured for a Loopback interface. If you assign a Loopback interface as a /24 subnet mask, for example, you are simply wasting valuable IP address space. Given that no host can every reside on the same subnet as a Loopback interface, it is a waste of router resources to have a routing protocol send updates to a Loopback interface, as there will never be another router (or other device) that will ever respond back to these updates. Hence, when you configure Loopback interfaces, it is always considered best practice to disable routing protocols that can send updates to them using the passive-interface command as illustrated in Task 5 below:

Task 5:

R1#conf t 
Enter configuration commands, one per line.  End with CTRL/Z. 
R1(config)#router rip 
R1(config-router)#passive-interface loopback100 
R1(config-router)#passive-interface loopback101 
R1(config-router)#end 
R1# 

R1#show ip protocols 
Routing Protocol is “rip” 
 Sending updates every 30 seconds, next due in 5 seconds 
 Invalid after 180 seconds, hold down 180, flushed after 240 
 Outgoing update filter list for all interfaces is not set 
 Incoming update filter list for all interfaces is not set
 Redistributing: rip 
 Default version control: send version 2, receive version 2 
  Interface             Send  Recv  Triggered RIP  Key-chain 
  Serial0/0             2     2 

R1#debug ip rip 
RIP protocol debugging is on 
R1# 
*Mar  1 03:20:02.355: RIP: sending v2 update to 224.0.0.9 via Serial0/0 (192.168.0.1) 
*Mar  1 03:20:02.355: RIP: build update entries 
*Mar  1 03:20:02.355:   10.10.100.0/27 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:20:02.355:   10.10.101.0/26 via 0.0.0.0, metric 1, tag 0 
R1# 
*Mar  1 03:20:28.974: RIP: sending v2 update to 224.0.0.9 via Serial0/0 (192.168.0.1) 
*Mar  1 03:20:28.974: RIP: build update entries 
*Mar  1 03:20:28.974:   10.10.100.0/27 via 0.0.0.0, metric 1, tag 0 
*Mar  1 03:20:28.974:   10.10.101.0/26 via 0.0.0.0, metric 1, tag 0 
R1#undebug all 
All possible debugging has been turned off

NOTE: Suppose you have a router with one Serial interface and 600 Loopback interfaces. Given such a scenario, issuing the passive-interface command for every one of those Loopback interfaces would take a great deal of time. Fortunately, Cisco recognized this and created the passive-interface default command in Cisco IOS. When this command is issued, all interfaces are configured as passive. In order to send updates on a particular interface, you would negate that interface as not being passive by issuing the no passive-interface command followed by the interface(s) you want to send routing protocol updates to. This is illustrated below:

R1#conf t 
Enter configuration commands, one per line.  End with CTRL/Z. 
R1(config)#router rip 
R1(config-router)#passive-interface default 
R1(config-router)#no passive-interface serial0/0 
R1(config-router)#end 
R1#

The configuration above makes all interfaces configured for RIP passive with the exception of interface Serial0/0. Make sure you remember this command, not only for the purposes of the CCNA exam but also for use in the real world.


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