Question Related to OSPF P2P configuration

Answered Question
Jun 10th, 2009

Small text from a book "Usually the IP source of an OSPF packet is set to the interface on the router.IT IS POSSIBLE TO USE IP UNNUMBERED INTERFACES WITH OSPF.On unnumbered interfaces , the ip source address is set to the ip address of another interface of the router."

Please can anyone explain this text.

Thanks

Prasad

Correct Answer by Laurent Aubert about 7 years 8 months ago

If you configure this static route on the router which is on the other side of the unnumbered link, then you need a dynamic routing protocol or another static route so the router will know that this next-hop is reachable via the pt2pt interface:

ip route sharedip mask serialx/y

Both static routes can also be replaced by the following one:

ip route destinationip mask serialx/y

HTH

Laurent.

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Laurent Aubert Wed, 06/10/2009 - 08:17

Hi Prasad,

Instead of addressing each p2p link with a dedicated subnet, you can "borrow" the ip address of another interface.

We can't configure the same IP address explicitly on two interfaces so we use the ip unnumbered interface command instead on the 2nd interface.

OSPF packets generated over this interface will use the IP address of the interface as source address which is in this case the address of another interface.

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml

HTH

Laurent

Manish Prasad Wed, 06/10/2009 - 09:44

Thanks Laurent.

This link is really helpful.

Three questions more related to this.

1)It can only be configured on P2P link ,its okay.Is this fixed that interface can borrow ip from Multiaccess interfaces only?

2)Do the subnetmask of both the interfaces will be the same(Configured ip interface and borrowed ip interface)?

3)If the shared ip is the next hop ip address of any packet then how the router will decide through which interface it will be processed if destination address is reachable from both the interfaces.

Thanks

Prasad

Richard Burts Wed, 06/10/2009 - 09:53

Prasad

1) The original implementation of ip unnumbered was strictly limited to point to point interfaces (such as HDLC or PPP serial links).

It is not restricted to borrow the IP from Multiaccess interfaces. It is fairly common to borrow the IP from loopback interfaces (which are sometimes /32)

2) the subnet mask will be the same on both interfaces.

3) I am not sure that I understand your question here. Perhaps we can make a small example to figure out how it works. Lets assume that routerA has a serial interface with ip unnumbered which borrows the IP from Fa0/0. So both the serial interface and the Fa0/0 interface will have the same IP address. Now your question asks what if they are the nex hop address, so there needs to be routerB which connects to both the serial interface and to Fa0/0. Is that really what you are asking? If so then it is somewhat obvious that it does not matter because routerB can send over the serial or over the FastEthernet and the packet will get to routerA.

If that is not what you are asking then please clarify.

HTH

Rick

Manish Prasad Wed, 06/10/2009 - 10:20

Thank you Rick for answering these questions.

Third question i want to ask in a different way.

Suppose i have added a static route using the shared ip address

"ip route destinationip mask sharedip"

so how the router will get to now through which interface it will forwarded the packets?

And one more question that ip can be borrowed from serial interface(P2P) also?

Thanks

Prasad

Thanks for replying on some of my stupid questions.

Correct Answer
Laurent Aubert Wed, 06/10/2009 - 10:54

If you configure this static route on the router which is on the other side of the unnumbered link, then you need a dynamic routing protocol or another static route so the router will know that this next-hop is reachable via the pt2pt interface:

ip route sharedip mask serialx/y

Both static routes can also be replaced by the following one:

ip route destinationip mask serialx/y

HTH

Laurent.

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