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yuchenglai
Beginner

OSPF & Point-to-Point links

Hi All,

I found documentation that OSPF will forego the same "subnet" check for adjacencies on point-to-point links.  Is this true?

David

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Peter Paluch
Hall of Fame Cisco Employee

Hello David,

I found documentation that OSPF will forego the same "subnet" check for adjacencies on point-to-point links.  Is this true?

Not entirely. On point-to-point links, OSPF will ignore the subnet mask mismatch. However, the neighbors still have to be on a common subnet, i.e. from the viewpoint of one router, the second router's IP address must fall within the interface network of the first router.

However, what you read is true for point-to-point unnumbered links. On unnumbered links, it is unavoidable that the two routers are on different subnets. Here, OSPF ignores both the common subnet and the netmask check, and proceeds to create an adjacency if the remaining criteria are met (identical timers, area numbers and types, authentication).

Best regards,

Peter

View solution in original post

2 REPLIES 2
Peter Paluch
Hall of Fame Cisco Employee

Hello David,

I found documentation that OSPF will forego the same "subnet" check for adjacencies on point-to-point links.  Is this true?

Not entirely. On point-to-point links, OSPF will ignore the subnet mask mismatch. However, the neighbors still have to be on a common subnet, i.e. from the viewpoint of one router, the second router's IP address must fall within the interface network of the first router.

However, what you read is true for point-to-point unnumbered links. On unnumbered links, it is unavoidable that the two routers are on different subnets. Here, OSPF ignores both the common subnet and the netmask check, and proceeds to create an adjacency if the remaining criteria are met (identical timers, area numbers and types, authentication).

Best regards,

Peter

View solution in original post

Peter,

Thanks for the insight.  That cleared things up quite a bit!

David