OSPF passive interface

[munawar.zeeshan]

I have a command under my ospf process as passive-interface Loopback0.

I do understand the impact of passive - interfaces , means thy r not going to take part in routing advertisments/mechanisam etc etc.I want to know why the LO interface has been declared as passive here ? What impacts it will make to the overall OSPF process, More-over what pros and cons will be there in declaring a LO interface as passive.

Much Thanks.

[husycisco]

Hi Munawar,

L0 will take part in routing advertisements if it is stated in ospf process with network command. Passive-interface command will not prevent this to happen. Passive-interface command will stop an interface from learning routes from stated interface. In distance vector routing protocols like RIP, this command will prevent router from advertising routes, but it still can learn routes from that interface. But in OSPF, passive interface command will prevent interface from receiving HELLO packets thus, interface can not learn or advertise over the stated interface, since it can not form adjecencies without HELLO packets.

Now about your question, this command might have been issued to reduce CPU utilization (Wont make much noticable difference if you dont have lots of Loopbacks). If you run an ospf debug, you will see a statement like that "Ignoring advertisiement from self L0" . Passive interface command will stop L0 to try advertising, and it is reasonable since Loopbacks cant do it infact. As passive interface is issued, router will no more have to handle ignoring advertisements to itself.

Regards

[Richard Burts]

Munawar

The impact on OSPF of configuring the loopback interface as passive is only positive. Remembering that passive interface means that OSPF will not build any hello packets for that interface and will not listen for any hello packets on that interface means that OSPF saves CPU cycles in processing interfaces that are passive. And since the loopback interface is a virtual interface there would never be a hello message that was actually sent or received on the interface. So passive interface is a good thing on the loopback interface.

So the impact is positive and there is no negative aspect of configuring the loopback interface as passive in OSPF (or in EIGRP either).

HTH

Rick