OSPF Qs

[claudemichel]

 Hi, I have a small query. 5.6.7.8(loopback)->Router1-> Router2 ->2.2.2.2(loopback). On router 1 when I try to advertise 1.1.1.1 using the below command I don't see any  packet going out but as soon as I advertise the physical interface(ip) it sends a  packet, may I know what is the reason, can someone please explain?  router ospf 1 network 5.6.7.8 0.0.0.255 area 0       

[MHM Cisco World]

Issue with 1.1.1.1 or 5.6.7.8?

If it 5.6.7.8 check below

router ospf 1 network 5.6.7.8 0.0.0.255 area 0     

Are you config LO with 24 or 32 mask?

If you config as 24 then under LO add this

Ip ospf network point-to-point 

MHM

[Joseph W. Doherty]

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@MHM Cisco World wrote:

router ospf 1 network 5.6.7.8 0.0.0.255 area 0     

Are you config LO with 24 or 32 mask?

If you config as 24 then under LO add this

Ip ospf network point-to-point 

MHM

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Hmm, @MHM Cisco World could you explain the above?  Unclear why you suggested it.

I recall (and just confirmed with PT [for whatever that's worth]) that OSPF advertises loops backs as /32s regardless of the interface address mask.

[Richard Burts]

The OP describes 2 steps:

1) configure OSPF to run on the loopback interface. When OSPF starts to run on the loopback it can listen for advertisements (but will never receive an advertisement) but it can not send an advertisement because the interface is virtual.

2) configure OSPF on the physical interface. OSPF running on the physical interface will send and receive packets on the physical interface.

Step 1 by itself does not accomplish much. For things to work you need both steps.

[Joseph W. Doherty]

Probably the issue is due to what @Richard Burts describes, i.e. you need to establish OSPF neighbor ship between the two routers, normally done on the physical interfaces connecting the two routers sharing a subnet, but it could even be done with a tunnel between the two routers, also sharing a subnet.  I.e. you don't actually need to include the physical interfaces in OSPF, but again, that's the usual setup for OSPF neighbors.