On a square-shaped network, OSPF neighbor relationships cannot be set up on P2P interfaces of S series switches. Why

Question: Four devices in the same VLAN form a square-shaped network and the type of their interfaces is P2P. Why cannot the devices establish OSPF neighbor relationships?

Answer: A Hello packet, as a multicast packet, sent by a device is transmitted within the entire VLAN since interfaces of the devices are in the same VLAN. P2P interfaces do not check masks, so when another device receives the Hello packet, it immediately establishes a neighbor relationship. However, the device neglects Hello packets sent by the other two devices since a P2P interface can only establish one neighbor relationship at a time. This causes the neighbor status of each device remaining Init.
A sends a Hello packet, and B receives the packet �?A considers B as its neighbor.
B sends a Hello packet, and C receives the packet �?B considers C as its neighbor.
C sends a Hello packet, and D receives the packet �?C considers D as its neighbor.
D sends a Hello packet, and A receives the packet �?D considers A as its neighbor.
Therefore, the devices cannot establish Full OSPF neighbor relationships.

Scroll to top