passive interface command

By default, Routing Information Protocol (RIP) sends its routing table updates on all interfaces on which Routing Information Protocol (RIP) is enabled every 30 seconds. However, RIP updates must only be sent to interfaces connecting to other RIP-enabled routers. This required us to stop sending an update on interfaces where not needed. The passive-interface can stop sending updates where not needed. Sending out un-needed updates on all interfaces affects the network in the following ways:

• Security Risk—Advertising routing table updates in the form of broadcasting is a security risk. Anyone can intercept the routing table updates using packet sniffing tools and software. So, the attacker can use routing table updates to corrupt the routing table with false metrics and route the traffic to the wrong path.
• Wasted Resources–All network devices received the routing table update, and they processed the update, which wasted the network devices’ resources.
• Wasted Bandwidth—An unnecessary update is broadcast to each device on the network, consuming the link’s bandwidth and causing bandwidth waste.

To overcome all the above-mentioned problems, we can use the passive-interface command. The passive interface prevents the transmission of routing updates through a router-specific interface. The command stops broadcasting routing table updates to the specified interface where they are not needed.

Configurations of Passive-interfaces and verify the interface

Referring to the reference topology, there is no need for Router0 to forward RIP updates to the LAN interface. However, Router0 is sending RIP updates to all computers on the LAN. We can verify this by enabling a debug on Router0 using the <debug ip rip> command in privileged exec mode. We can verify the interface passive configuration using the show ip protocol command.

So, we need to configure the fast Ethernet 0/0 interface connected to the LAN as a passive interface. The process to configure the passive-interface is as follows:

• Router0>enable
• Router0#config terminal
• Router0(config)#router rip
• Router0(config-router)#passive-interface FastEthernet 0/1

We can also configure the passive-interface for all dynamic protocols. If we want to cancel the passive-interface configuration from any interface, we can use no passive-interface <interface-Id> command. We can also use the passive-interface default command to configure all interfaces as passive.