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

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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.

Other related questions:
Must two S series switches be on the same network segment to set up an OSPF neighbor relationship
Question: Must the interfaces on two ends of an OSPF link be on the same network segment and the mask digits on the neighboring interfaces be consistent? Answer: When two S series switches are establishing an OSPF neighbor relationship, the interfaces on the broadcast network, NBMA network, or P2MP network must be on the same network segment and the mask digits on the neighboring interfaces must be consistent. The limitations do not apply to interfaces on a P2P network. On an OSPF P2P network, if the link-layer protocol is PPP, OSPF neighbors with IP addresses on different network segments can be in the Full state and correctly calculate routes. However, if the link-layer protocol is not PPP (for example, HDLC), devices with IP addresses on different network segments cannot establish an OSPF neighbor relationship. Through PPP negotiation, the local device can obtain the IP address of the remote device and have a route to the remote device without extra configuration.

Prevent an S series switch from setting up an OSPF neighbor relationship with a device connecting to an interface on the switch
Run the silent-interface command on an interface of an S series switch supporting OSPF. Then the switch cannot set up an OSPF relationship with the device connecting to the interface. To enable OSPF on many interfaces while disabling OSPF neighbor relationship establishment on most interfaces, run the silent-interface all command and then run the undo silent-interface command to enable OSPF neighbor relationship establishment on specified interfaces.

Why does an IS-IS level-1 neighbor relationship fail to be set up
Q: Why does an IS-IS level-1 neighbor relationship fail to be set up? A: An IS-IS Network Entity Title (NET) of a switch is in the X…X.XXXX. XXXX.XXXX.00 format. X…X indicates the area address, the 12 Xs in the middle indicate the system ID of the switch, and 00 indicates the selector (SEL). The area address length ranges from 1 bytes to 13 bytes. Switches in the same area must be configured with the same area address. The 6-byte system ID of a switch is unique in the entire area and backbone area. The SEL is the last byte and its value must be 00. IS-IS devices can set up a level-1 neighbor relationship only when the following conditions are met: 1. The devices have the same area address. 2. The devices have different system IDs. 3. The SEL of the devices is 00. If any of the preceding conditions is not met, the level-1 neighbor relationship cannot be set up.

Why is the setup of an OSPF neighbor relationship between S series switches on a broadcast network slow
On a broadcast network, when two devices establish a 2-Way OSPF neighbor relationship, they elect the DR and BDR. Generally, the DR and BDR are elected after the Waiting timer expires, and the default value of the Waiting timer is 40s. Therefore, the setup of a Full OSPF neighbor relationship requires about 1 minute. Advertising a route to a loopback interface is useless. This is because a loopback interface cannot connect to a physical network, and traffic to a network segment cannot be routed to a loopback interface.

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