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Atonomous system configuration in OSPF


I know that OSPF is used to distribute the routes inbetween the same autonomous system and not between the different autonomous systems. But In the OSPF configuration we dont include the autonomous system number then how the ospf configured router will find it's boundary (different autonomous system router).

But In IGRP configuration, we include the AS number, so when IGRP finds different AS number it will not forward the update.IGRP will keep updating the routers as long as it fins the same AS number.

Please tell me how ospf differentiate the Autonomous system.



Re: Atonomous system configuration in OSPF


OSPF does not have a real concept af an AS. So it is up to the administrator of the network to configure OSPF only on routers where it should be exchanging routes. On interfaces where another AS is found, like the customer ISP interface, just do not enable OSPF.

You could still do it and OSPF - not knowing of any AS - would be routing across AS boundaries if the required OSPF parameters match.

Regards, Martin

Re: Atonomous system configuration in OSPF

Hello Jaffer,

not sure if that is what you are asking, but you can only add networks to OSPF areas when you are in routing protocol configuration mode, and you can only enter that mode when you specify an AS number. So, as an example, if you specify:

router ospf 1

You specify that everything you configure after entering that command, will belong to AS 1.

So, an OSPF number is required, and hence will form the boundary.

Does that make sense ?



Hall of Fame Super Silver

Re: Atonomous system configuration in OSPF


Your explanation is incorrect. The required number in the router ospf 1 command is not an AS number but is a process ID. On the surface it may be confusing since in router eigrp 1 it is an AS number. They look the same but the function is different.

If I were to configure this:

router ospf 1

network area 1

router ospf 2

network area 2

I have created 2 separate OSPF processes but I have not indicated anything about AS numbers.

As Martin pointed out OSPF is intended to run withing an autonomous system but does not have any ability to configure the AS. It is a matter of administrative control to only configure it on router interfaces that are within a common AS.



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