Common Causes of OSPF Neighbor Adjacency Failures

ACG.

OSPF (Open Shortest Path First) is a link-state routing protocol that is widely used in enterprise networks. In order for OSPF routers to form neighbor relationships and exchange routing information, there are several requirements that must be met. Here are the three issues that can cause OSPF neighbors to not be able to establish an adjacency:

1. Duplicate Router IDs (RID): Each router in an OSPF domain must have a unique Router ID (RID). If two or more routers have the same RID, OSPF neighbors will not form. When OSPF routers initially start up, they send hello packets to discover their neighbors, and if multiple routers respond with the same RID, a conflict will arise and the adjacency will not form.

2. Hello values: OSPF routers use hello packets to discover and establish neighbor relationships with each other. If the hello interval or hello timer values are not the same on both routers, the neighbors will not form. The hello interval defines how frequently the OSPF router sends hello packets, while the hello timer defines how long the router waits for a response before considering the neighbor down.

3. MTU: OSPF neighbors must have the same maximum transmission unit (MTU) size in order to form an adjacency. The MTU defines the maximum packet size that can be transmitted over a network, and if two routers have different MTU sizes, they will not be able to exchange packets, causing the adjacency to fail.

Therefore, options A (duplicate RIDs), C (hello values), and G (MTU) are the three issues that can cause OSPF neighbors to not be able to establish an adjacency. Options B (Feature Multicast not enabled), D (Interface cost is not set), E (OSPF instance tag), and F (PIM) are not relevant to this issue.