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New Member

Same interface in multiple OSPF process

Dear All,

Can i have same network statement under different OSPF process on the same router? Will the router accept it?

Regards,

8 REPLIES

Same interface in multiple OSPF process

Well technically when you create OSPF processes they are like there own little routing domains. So I would think that you would be able to do that without any concerns for loops, unless you started to redistribute routes. I have never tested this, but I would think the answers would be Yes.

Hall of Fame Super Silver

Same interface in multiple OSPF process

The specific question in the original post is whether you can configure the same network statement in multiple OSPF processes on the same router. And John is quite correct in saying that the answer is yes.

But be aware that it might not work the way that you expect. You can have the same network statement under multiple OSPF processes. But an interface can only be active in one OSPF process at a time. So if you configure multiple OSPF processes, and configure the same network statement(s) for each process then the result is that an interface will be active in only one of them (the first process that initializes the interface - typically the first process configured).

HTH

Rick

Same interface in multiple OSPF process

I didn't know that an interface could only be active for one OSPF process at a time. Thanks for the information Richard.

New Member

Same interface in multiple OSPF process

Thanks Richard, i have seen similar statements in other sites while doing google search. Do you have any reference link to cisco site refering you statement that an interface can only be active in one process?

In my case, my aim is to only advertise the subnet and the interface in that subnet is passive and not used for any ospf neighborship

New Member

Re: Same interface in multiple OSPF process

Hi

 

What if OSPF process is different for same interface but with different area ID? 

Cisco Employee

Re: Same interface in multiple OSPF process

Dear friends,

To my best knowledge, in OSPFv2, an interface can belong to a single process only. Even if it is configured to belong into several OSPF processes, only one of them is going to really own the interface - the first one that was started.

See the following configuration:

router ospf 1

router-id 1.1.1.1

log-adjacency-changes

network 192.0.2.0 0.0.0.255 area 0

!

router ospf 2

router-id 2.2.2.2

log-adjacency-changes

network 192.0.2.0 0.0.0.255 area 0

!

router ospf 3

router-id 3.3.3.3

log-adjacency-changes

network 192.0.2.0 0.0.0.127 area 0

!

router ospf 4

router-id 4.4.4.4

log-adjacency-changes

network 192.0.2.1 0.0.0.0 area 0

!

router ospf 5

router-id 5.5.5.5

log-adjacency-changes

network 192.0.2.1 0.0.0.0 area 0

R1#show ip ospf interface brief

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Se1/0        1     0               192.0.2.1/24       64    P2P   0/0

R1#

This should also result from the fact that if an interface could belong to several OSPFv2 processes, all processes would need to communicate through the same interface, and there would be no way to distinguish which messages were produced by which process.

This is exactly what OSPFv3 is supposed to solve with its Instance ID. Each process in OSPFv3 has assigned a particular instance ID for link-local communication purposes, and this instance ID is included in headers of all OSPFv3 packets sent on that interface, theoretically allowing the administrator to run several OSPFv3 processes over a common link.

Now, I wanted to configure an interface in Cisco IOS 12.4T with multiple OSPFv3 processes and different instance IDs to show that it is possible - and to my utmost surprise, I was actually not capable of doing that Even though I assigned a different Instance ID to each OSPFv3 process run over an interface, the IOS CLI always left only the last entered ipv6 ospf area command on an interface. Anyone to tell me what I did wrong? Perhaps it's just the limitation of the particular IOS I was using.

Anyway, at least according to all textbooks and RFCs, OSPFv3 is equipped with means to run several instances over a single interface, and hence allowing multiple OSPFv3 processes to own a particular interface.

My two cents...

Best regards,

Peter

New Member

Re: Same interface in multiple OSPF process

Thanks Peter for the details.

In my case the interface is a Vlan SVI which is configured as a /24 subnet. I dont need to establish any ospf neighborship peering through this SVI interface, my aim is to advertise the /24 subnet through OSPF (through both of my ospf process)to my uplink router (my uplink router has 2 vrf's, each one connected to each of the ospf process on the downlink router) and i dont prefer to do re-distribution between the 2 ospf process.

Re: Same interface in multiple OSPF process

What if under each interface you did a 'ip ospf process x' or if instead of network 192.168.10.0 0.0.0.255 area x you did 192.168.10.1 0.0.0.0 area x? I agree with what you say saying about the first one that is setup will be the active ospf process on that interface.

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