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New Member

/30 route advertised with /24 mask by OSPF

I have three three routers connected in a straight line. For sake of simplicity I'll call them my left,


Left runs only RIPv2, Middle runs OSPF and RIPv2 and mutually redistributes bewteen the two, and Right runs only OSPF.

I don't think it matters, but Middle is in the backbone, and Right is in Area 1. Left can see Right and vice versa.

The point to point serial connection to Right is using a /30 mask. (The subnet is

My problem is, Middle advertises its route to Left with a /24 mask instead of /30, so nodes beyond Left cannot get to Right.

I have other /30 subnets off Middle (0.4, 0.8, etc.) that have the same problem.

Anyway, is there a way to configure Middle so the advertisements are not classful, and Left receives the correct /30 route?

Or, is my problem perhaps something else altogether?

Thanks in advance,

Matt Heffernan

Super Bronze

Re: /30 route advertised with /24 mask by OSPF


I think you need to put the 'subnets' keyword in the end of your 'redistribute...' command under OSPF on your ASBR.

This makes OSPF advertise subnets instead of major networks...


Please rate helpful posts...

Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
Hall of Fame Super Silver

Re: /30 route advertised with /24 mask by OSPF

Using the subnets keyword is certainly a good suggestion. But there is something about the problem that I do not understand: if middle is advertising then why is left not using that route and forwarding for,, etc to middle? Is there perhaps some subnet of 192.168.0 that is on left (or is advertised to left by someone in RIPv2? If left believes that it knows any subnet of 190.168.0 and if left is configured with ip classless then it would have the behavior described. Is that the case on left?



New Member

Re: /30 route advertised with /24 mask by OSPF

Got it figured out.

I needed to add no auto-summary to my RIP configuration. Turns out I had a NAT issue on Middle as well, but now that is working okay as well.



Re: /30 route advertised with /24 mask by OSPF


When you redistrubute OSPF/RIP , at that time you can put " Subnet " work in the command. Like Redistrubute rip subnet this will resolve the problem.

. because when you do not put subnet word in the commnad, it will generate entire classful network ( like /8, /16,/24 ), that is why you will get the /24 routes in routing table.

i hope this will solve your problem. please rate.