Why does an OSPF neighbor relationship between two S series switches remain in the 2-Way state

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Question: Why are Full/DR, FULL/BDR, and 2-WAY/DR OTHER displayed in the display ospf peer command output?

A: To reduce broadcast network (such as the Ethernet, FDDI, and token ring network) media to be flooded, only the DR and BDR can establish Full neighbor relationships with DR Others. The neighbor relationship between two DR Others remains in the 2-Way state.

Other related questions:
Common reasons why an OSPF neighbor relationship between S series switches cannot change to the Full state
Question: What are common reasons why an OSPF neighbor relationship cannot change to the Full state? Answer: Common causes of this problem include the following: 1. The OSPF network is a non-broadcast multiple access (NBMA) network, and no neighbor is configured. 2. The interval at which the OSPF neighbor sends Hello packets is longer than the aging time of the neighbor relationship. 3. In a stub area or not-so-stubby area (NSSA) area, some switches are not configured as stub or NSSA. 4. The configuration of OSPF verification is incorrect. 5. The OSPF router IDs on two ends of the OSPF link are the same. 6. The network types on two ends of the OSPF link are different. 7. The MTU values on two ends of the OSPF link are different. 8. OSPF is not enabled on the local or remote device. 9. The area numbers on two ends of the OSPF link are different.

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.

An OSPF neighbor relationship between S series switches stays in the Exstart/Exchange state for a long time
Question: How can I identify the cause when an OSPF neighbor relationship between S series switches stays in the Exstart/Exchange state for a long time? Answer: The Exstart/Exchange state indicates that the local switch is exchanging Database Description (DD) packets with a neighbor. If the Exstart/Exchange state lasts for a long time, check the following items: 1. Whether the MTU values on the local interface and neighboring interface are the same 2. Whether the router IDs of the switches are the same (Two switches must have different router IDs.) 3. Whether the size of received packets exceeds the MTU of the interface (If yes, packets sent from the neighbor are dropped.)

Can an OSPF neighbor relationship be established if the network types of the two S series switches are different
Question: Can a Full neighbor relationship be established if the network types of the two ends of an OSPF link are different? Answer: Yes. For example, two devices are interconnected through an Ethernet link. On one end of the link, the default broadcast network type is adopted. The other end is configured as OSPF peer-to-peer (P2P). Neighbor relationship can be established between these two devices, which reach the Full state by exchanging their Link State Databases (LSDBs). However, no route can be learned because OSPF devices need LSDBs to construct a Shortest Path Tree (SPT). The LSDBs, however, are problematic. That is, the link-state advertisements (LSAs) generated by one end consider the peer a broadcast neighbor, while the other end considers its peer a P2P neighbor. Therefore, no SPT can be constructed correctly and the Shortest Path First (SPF) algorithm cannot calculate the right routes either.

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.

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