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New Member

OSPF loopback question

                   Hi all,

I have this question on OSPF loopback.

if i config router with loopback interfaces say 10.1.1.1

Under router ospf 1

network 20.2.2.2 0.0.0.0

Will this router advertise the loopback IP address now?

As far as i understand it should not ?

Please correct me if i am wrong!

I was reading that loopback should be set to passive interface so that hellos overhead can be reduced does this mean that when we put interface

as passive then no ospf nei establish on that interface?

Thanks

MAhesh

Everyone's tags (2)
2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

OSPF loopback question

Hello,

As you did not specify the loopback ip with network command it wont participate in OSPF. And the address wont be advertised as well untill you redistribute it (as connected) or configure loopback to participate in OSPF (with network or "ip ospf area" command).

Other thing is when you have interface participating in OSPF and you want it to stop sending HELLOs out - then you configure this int as passive.

Nik

Hall of Fame Super Silver

OSPF loopback question

Note that in the situation which Mahesh asks about the loopback interface will still be chosen as the OSPF Router ID. Since it does not match the network statement OSPF will not be active on the loopback interface and will not advertise the loopback address. But it will still use the loopback as the RID. And note that there is not any requirement that the RID be advertised or have IP connectivity in the network.

The suggestion that Mahesh mentions deals with the situation where there is a network statement that matches the loopback interface. If the loopback does match the network statement then OSPF will actively run on the loopback and will go through the process of building hello messages and update messages for the interface, even though physically they can not be transmitted. So if you include passive interface for the loopback then you avoid that overhead activity.

HTH

Rick

4 REPLIES
Cisco Employee

OSPF loopback question

Hello,

As you did not specify the loopback ip with network command it wont participate in OSPF. And the address wont be advertised as well untill you redistribute it (as connected) or configure loopback to participate in OSPF (with network or "ip ospf area" command).

Other thing is when you have interface participating in OSPF and you want it to stop sending HELLOs out - then you configure this int as passive.

Nik

New Member

OSPF loopback question

Hi Nik,

Many thanks for prompt reply

Mahesh

Hall of Fame Super Silver

OSPF loopback question

Note that in the situation which Mahesh asks about the loopback interface will still be chosen as the OSPF Router ID. Since it does not match the network statement OSPF will not be active on the loopback interface and will not advertise the loopback address. But it will still use the loopback as the RID. And note that there is not any requirement that the RID be advertised or have IP connectivity in the network.

The suggestion that Mahesh mentions deals with the situation where there is a network statement that matches the loopback interface. If the loopback does match the network statement then OSPF will actively run on the loopback and will go through the process of building hello messages and update messages for the interface, even though physically they can not be transmitted. So if you include passive interface for the loopback then you avoid that overhead activity.

HTH

Rick

New Member

OSPF loopback question

Hi Rick,

Thanks again for reply back.

There is always something new to learn form you!.

Regards

MAhesh

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