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OSPF Passive Interface

When you use the network command in OSPF, two things will happen:

All interfaces that have a network that falls within the range of the network command will be advertised in OSPF.
OSPF hello packets are sent on these interfaces.

Sometimes it’s undesirable to send OSPF hello packets on certain interfaces. Take a look at the image below:

R1 and R2 are configured for OSPF. R1 is connected to network 192.168.10 /24 which has some computers connected to a switch. R1 wants to advertise this network to R2.

Once we use the network command to advertise 192.168.10.0 /24 in OSPF, R1 will also send OSPF hello packets towards the switch. This is a bad idea, first of all because there are no routers on this network but it’s also a security risk. If someone on the computer starts an application that replies with OSPF hello packets then R1 will try to become neighbors. An attacker could advertise fake routes using this technique.

To prevent this from happening, we can use the passive-interface command. This command tells OSPF not to send hello packets on certain interfaces. Let’s see how it works…

Configuration

Here’s the OSPF configuration of R1 and R2:

R1(config)#router ospf 1
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R1(config-router)#network 192.168.10.0 0.0.0.255 area 0

R2(config)#router ospf 1
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0

With the above configuration, R2 will learn network 192.168.10.0 /24:

R2#show ip route ospf 
O    192.168.10.0/24 [110/20] via 192.168.12.1, 00:03:21, FastEthernet0/0

This is great but a side-effect of this configuration is that R1 will send hello packets on its FastEthernet 0/1 interface. We can see this with a debug:

R1#debug ip ospf hello 
OSPF hello events debugging is on

OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/1 from 192.168.10.254

OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 192.168.12.1

Above you can see that hello packets are sent in both directions.

Let’s fix this. We will configure OSPF to stop the hello packets towards the switch:

R1(config)#router ospf 1
R1(config-router)#passive-interface FastEthernet 0/1

You only have to use the passive-interface command under the OSPF process. You can verify our work with the following command:

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Forum Replies

ivoaleksiev says:

Hi Rene!
You mention that passive-interface command is preventing an interface from sending hello packets.
I’d like to know technically does it prevent interface from RECEIVING hello packets as well?
Also, I read in an official Cisco literature that it is recommended ISP facing interfaces to be passive for security.
However I don’t understand, if this is the case how is my router going to receive routing to external destinations using OSPF if it doesn’t become neighbor
with the next hop.
Thank you in advance!
ReneMolenaar says:
Hi Ivaylo,

It won’t prevent us from receiving the hello packets but I believe it does stop processing them. Here’s a little experiment I did with two routers, directly connected to each other:
```
R1#debug ip ospf hello 
OSPF hello debugging is on
```
Now we can see we are sending and receiving hello packets:
```
R1#
OSPF-1 HELLO Gi0/1: Send hello to 224.0.0.5 area 0 from 192.168.12.1
OSPF-1 HELLO Gi0/1: Rcv hello from 2.2.2.2 area 0 192.168.12.2
```
Let’s make the interface passive:
```
R1(config)#router ospf 1
R1(config-router)#passive-interface GigabitEthernet 0/1
```
At this mo
... Continue reading in our forum
ReneMolenaar says:

Hi Ahmad,

OSPF and EIGRP have one thing in common, they both establish a neighbor adjacency before they advertise any routing information. RIP doesn’t establish a neighbor adjacency, it just advertises routing updates.

When you use the passive interface command for RIP then it stops advertising RIP routing updates on that interface. When you use it for OSPF or EIGRP, they won’t send any hello packets anymore so that it becomes impossible to establish a neighbor adjacency on the passive interface.

Rene
lagapides says:

Hello Monir

When you indicate that an interface is passive, it means that any and all OSPF related messages are never sent out that interface. The connected network does participate in the OSPF process, that is, the connected subnet is advertised to other OSPF routers, but no hellos, LSAs or any other OSPF related packets are sent out that interface.

The passive interface is configured on interfaces where you KNOW there is no OSPF router connected to it to receive any kind of OSPF information.

I hope this has been helpful!

Laz
bil-khan says:

Hello Rene,

I have a question for passive interface. If one interface having many sub interfaces for example fa0/0.1, fa0/0.2, fa0/0.3 and so on and we use the command passive interface fa0/0 under OSPF, this makes all sub interfaces passive or no ?

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