Solved: Preventing OSPF adjacency from forming on broadbast medium

mikeleecha · ‎12-02-2005

Hello.

Does anyone know if it's possible to prevent OSPF routers from forming any sort of neighborship together? I have 4 routers on a common VLAN (Routers A,B,C,D). I don't want Router's B and C to become neighbors...even if they're on the same VLAN. Is it possible to do this? Thanks!

sundar.palaniappan · ‎12-02-2005

Yes, possible.

If I understand your question correctly, you want Router B and C to form neighbor relationship with Router A and D but not with each other.

You can configure the interface as a non-broadcast and specify the neighbors under OSPF process only for the routers that need to form neighbor relationship.

However, I don't know why you want to do this. This could create some routing problems. Let's say RouterA is the DR on this segment and it learns NET X from RouterB. RouterA will inform RouterC that send all traffic destined to NET X to Router B (who orginated this route). RouterC mightn't install this route since RouterB wouldn't show up in it's OSPF database.

Remember, OSPF is based on SPF calculations. Every OSPF router in the same AREA has to have an identical database to build the SPT and for correct routing functionality.

I didn't lab it up. But, I think this might be the behavior.

Instead, if you simply want Routers B and C (vlan int in question) not particpate in OSPF at all, make sure, the network statements under OSPF process on B and C does not include the IP of this interface. This way, these interfaces will not be particpating in OSPF process at all.

HTH,

Sundar

View solution in original post

sundar.palaniappan · ‎12-02-2005

Yes, possible.

If I understand your question correctly, you want Router B and C to form neighbor relationship with Router A and D but not with each other.

You can configure the interface as a non-broadcast and specify the neighbors under OSPF process only for the routers that need to form neighbor relationship.

However, I don't know why you want to do this. This could create some routing problems. Let's say RouterA is the DR on this segment and it learns NET X from RouterB. RouterA will inform RouterC that send all traffic destined to NET X to Router B (who orginated this route). RouterC mightn't install this route since RouterB wouldn't show up in it's OSPF database.

Remember, OSPF is based on SPF calculations. Every OSPF router in the same AREA has to have an identical database to build the SPT and for correct routing functionality.

I didn't lab it up. But, I think this might be the behavior.

Instead, if you simply want Routers B and C (vlan int in question) not particpate in OSPF at all, make sure, the network statements under OSPF process on B and C does not include the IP of this interface. This way, these interfaces will not be particpating in OSPF process at all.

HTH,

Sundar

mikeleecha · ‎12-03-2005

Thanks for the reply Sundar. In this particular case Routers B and C are routers from two different telco's that provide Metro Ethernet services to me. I want to prevent the two telco's from sending routing updates to one another. I only want them to send updates to my routers (A and D). So in this situation I don't want all routers in the same area to have an identical OSPF database.

dbokotey · ‎12-03-2005

I would enable MD5 with different password and this will prevent them to established session between

my 2 cents

Best regards

Dmitry Bokotey

amit-singh · ‎12-05-2005

Dmitry,

If you assign a MD5 authentication on the interfaces in this situation, will it now prevent all the routers forming adjacency with B and C. This is only possible in case there is a direct connection between B and C.

regards,

-amit singh