When any OSPF router receives a hello packet, it first checks the source IP of the hello packet and look if the source is in same subnet as the interface on which it receives the hello packet. If yes, the following fields needs to match the local parameter:
- Area ID
- Hello Interval
- Dead Interval
- Subnet Mask
There are specific cases, where a part of the above rule can be ignored. In this article, we will see which parameters can be ignored.
Case 1: OSPF adjacency will be formed even if the Subnet mask doesn’t match.
Case 2: OSPF adjacency can be formed even if the Source IP of the hello packet is in different subnet.
As per RFC2328, on a point-to-point link and virtual link, Subnet Mask will be ignored and can form adjacency even if the subnet masks are different.
An example below will prove the statement.
When any OSPF router receives a hello packet on point-to-point link which is configured using “ip unnumbered
But when the interface is configured with a /32 address, it throws an error message saying “src not on the same network” and OSPF adjacency will not be formed.
On a PPP link, it is common to see /32 subnet mask when IP address is negotiated as part of IPCP as below,