Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

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

Everyone's tags (1)
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

OSPF & Point-to-Point links

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

2 REPLIES
Cisco Employee

OSPF & Point-to-Point links

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

New Member

OSPF & Point-to-Point links

Peter,

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

David

749
Views
0
Helpful
2
Replies
CreatePlease to create content