OSPF Passive Interface state

[vpalania]

Hi,

What is the part of passive interface in OSPF?

When we change the interface to passive, does the interface state changes to down ?

Does the neighbourship state changes to down ?

Kindly clear me .

Thanks,

Vijay

[pkhatri]

Hi Vijay,

In the context of the Cisco IOS, the process of making an interface passive does the following:

- prohibits the sending of OSPF Hellos on the interface

- OSPF Hellos that are received are discarded

- the interface state remains up

- the IP network of the interface is advertised as a connected network in Type-1 LSAs

So the reason you would make an interface passive is to advertise its IP network in OSPF as internal routes. The alternative is to redistributed connected networks but then they appear as external routes.

Hope that helps - pls rate the post if it does.

Paresh

[vpalania]

Hi,

If it doesn't send hellos then the neighbour router changes the neighbourship state to down , am i correct ?

If it is not even receiving hellos then what is use of this interface in OSPF ? Is the restriction only for hellos or for other OSPF packets ?

Thanks,

Vijay

[pkhatri]

Yes, that is correct. There is no neighbor relationship formed over the interface although the interface remains physically up.

This restriction applies to all OSPF packets; not just Hellos. However, the nature of OSPF is such that it will not pass any other types of packets over a link that does not have a recognised neighbor.

As I mentioned in my earlier post, the point of having passive interfaces is to advertise the IP network configured on such an interface as an internal OSPF network.

Hope that helps - pls rate the post if it does.

Paresh

[vpalania]

Hi,

In that case , what happens when a interface connected to a tranit network is made passive ?

Thanks,

Vijay

[pkhatri]

That interface will not be able to provide transit through that network, then. If there are other routers with non-passive links to that interface then they can be used to transit through that network.

Paresh