Which two variables must match between two OSPF routers to form a neighbor adjacency? (Choose two.)
- area IDs
- K-values
- process IDs
- router priorities
- hello and dead intervals
To form a neighbor adjacency in OSPF (Open Shortest Path First), two routers must agree on several key parameters. Out of the options provided, the two variables that must match between two OSPF routers to form a neighbor adjacency are area IDs and hello and dead intervals. Below is an explanation of each option provided, detailing why it is either essential or not essential for forming an OSPF neighbor adjacency.
1. Area IDs
Correct: The area ID is one of the critical parameters that must match for two OSPF routers to form a neighbor adjacency. OSPF is a link-state routing protocol that divides the network into areas to optimize routing efficiency and limit the size of the routing table. Each OSPF router belongs to an area, and the area ID is used to identify the area in which the router is operating.
When two routers attempt to establish a neighbor relationship, they exchange OSPF Hello packets. One of the fields in the Hello packet is the area ID. If the area IDs on both routers match, they can proceed to establish a neighbor adjacency. However, if the area IDs do not match, the routers will not form a neighbor adjacency, and the OSPF neighbor relationship will not be established.
The reason for this strict requirement is that the area ID is fundamental to OSPF’s hierarchical design. It ensures that OSPF routers share the same view of the network topology within a particular area, which is crucial for consistent routing decisions. Without matching area IDs, routers would be operating in different OSPF areas, which could lead to incorrect or incomplete routing information being exchanged.
2. K-values
Incorrect: K-values are not relevant in the context of OSPF; they are specific to the EIGRP (Enhanced Interior Gateway Routing Protocol). K-values are used in EIGRP to influence the metric calculation for routing decisions, determining the best path for data packets. They adjust the weight of various factors such as bandwidth, delay, load, and reliability in the composite metric calculation.
Since K-values are not part of OSPF, they do not influence the formation of OSPF neighbor adjacencies. Therefore, mismatched or even nonexistent K-values do not affect the ability of two OSPF routers to become neighbors. This option is included as a distractor, highlighting the importance of understanding the differences between routing protocols like OSPF and EIGRP.
3. Process IDs
Incorrect: The process ID is a locally significant identifier used to distinguish between different OSPF processes running on the same router. It is configured on a per-router basis and has no significance beyond the individual router on which it is configured.
When two routers exchange OSPF Hello packets to form a neighbor adjacency, the process ID is not included in the packet. As a result, the process IDs on neighboring routers do not need to match. This allows a network administrator to run multiple OSPF processes on the same router, each identified by a unique process ID, without affecting neighbor relationships with other routers. The OSPF process ID is purely a local matter and does not impact the ability of routers to form OSPF neighbor adjacencies.
4. Router Priorities
Incorrect: Router priorities are used in OSPF to influence the election of the Designated Router (DR) and Backup Designated Router (BDR) on a multi-access network, such as an Ethernet LAN. The DR and BDR election process helps reduce the number of adjacencies in a broadcast network, minimizing the amount of OSPF control traffic.
While router priorities play an important role in determining which router becomes the DR or BDR, they are not a factor in whether or not two OSPF routers can form a neighbor adjacency. Even if the router priorities differ between two routers, they can still establish an OSPF neighbor relationship as long as other essential parameters match, such as the area ID and hello and dead intervals. The priority value is only relevant during the DR/BDR election process and does not affect the fundamental neighbor relationship between OSPF routers.
5. Hello and Dead Intervals
Correct: The hello and dead intervals are two critical OSPF parameters that must match between routers to form a neighbor adjacency. The hello interval defines how frequently an OSPF router sends Hello packets to its neighbors. The dead interval defines the time period during which a router must receive a Hello packet from its neighbor; if no Hello packet is received within this period, the neighbor is considered unreachable, and the adjacency is torn down.
When two OSPF routers attempt to form a neighbor relationship, they exchange Hello packets that include the hello and dead interval values. If the hello and dead intervals match on both routers, they can proceed to establish a neighbor adjacency. However, if these intervals do not match, the routers will not form a neighbor adjacency, as they would be operating on different timelines for sending and expecting Hello packets.
Matching hello and dead intervals is crucial because they ensure that both routers are in sync regarding the frequency of communication and the tolerance for missed Hello packets. If these intervals were mismatched, one router might consider the other unreachable too quickly, leading to unstable or incomplete neighbor relationships and potential routing inconsistencies.
Summary
In summary, to form a neighbor adjacency in OSPF, the area IDs and hello and dead intervals must match between the two routers. These parameters ensure that the routers belong to the same OSPF area and operate on the same communication schedule, enabling them to exchange routing information effectively. Other parameters like K-values (irrelevant to OSPF), process IDs (locally significant), and router priorities (relevant to DR/BDR election) do not need to match and do not impact the formation of the OSPF neighbor adjacency. Understanding these requirements is crucial for network engineers who configure and troubleshoot OSPF in complex network environments.