an interface with secondary ip configuring in OSPF

[Majid Jalinousi]

Hi,

There is an interface with 2 ip, when i configure this port in ospf, how is it react?

Is it like a connection with 2 link?

Thanks,

[Joseph W. Doherty]

To answer your questions, exactly how were you going to configure the port for OSPF?

[Majid Jalinousi]

You know, I saw this configuration on an interface of a router like below:

int vlan 100

 ip address 10.99.64.1 255.255.255.192 secondary
 ip address 10.197.0.2 255.255.255.192

router ospf 100

 network 10.0.0.0 0.255.255.255 area 0

I want to know with this configuration, Should i expect there are 2 routers connected to this link? Are there another possible scenarios for this kind of configuration?

Thanks,

[Richard Burts]

The additional information is helpful. No you should not expect two routers on this link. With this configuration OSPF will advertise both subnets. But OSPF hello messages are sent using the IP address of the primary address. So if there were a second router using the secondary address as its subnet then the router would not become OSPF neighbors. So what you should expect is a single router where the other router probably also has a secondary address configured.

There are multiple reasons why routers may be configured with secondary addresses. These include the original subnet was getting full and a second subnet was configured to provide additional address capacity, or a secondary address may be configured to aid in changing IP addressing. Perhaps a network was configured with one subnet and then a decision was made to transition that network to a new address. You could configure the new subnet as secondary address so that the transition could be gradual. In this configuration devices with original addresses co-exist with devices with new addresses and all are functional. When the transition is complete and no devices are using the original subnet then you remove the primary IP and make the secondary address into the primary address.

HTH

Rick

[Joseph W. Doherty]

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Posting

The configuration, alone, will not tell you how many router are connected to that link.

As Rick has already noted, OSPF neighbor adjacency will only form on the primary IP.  So, if there are any adjacent OSPF routers, they would likely be configured like what you posted, but that's not guaranteed.

Since the primary IP subnet is not point-to-point, there could be zero, one, two, three, ..., or up to the whole address range might comprise routers.