2.1.9 Check Your Understanding – OSPF Router ID Answers

1. True or False? In the router ospf process-id command, the process ID value, which can any number between 1 and 65,535, is locally significant. It must be the same on all routers in the OSPF area.

  • True
  • False

Explanation: The process-id value is locally significant. It does not have to the be the same on all routers. However, using the same value avoids confusion and provides consistency across configurations.

2. Which of the following applies to the router ID? (Choose two)

  • The router ID cannot be defined by an administrator.
  • The router ID is not used to determine the BDR.
  • The router ID is used to determine the DR.
  • The router ID uniquely identifies the router.
  • The router ID is not required.

Explanation: The answer is True. The router ID is used by the OSPF-enabled router to uniquely identify the router and for participation in the DR election. In a multiaccess LAN environment, the router with the highest router ID is elected the DR. The routing device with the second highest router ID is elected the backup designated router (BDR).

3. Which of the following is the order of precedence for choosing the router ID?

  • 1. Router ID that is explicitly configured
    2. Lowest IPv4 loopback address
    3. Lowest active configured IPv4 address
  • 1. Router ID that is explicitly configured
    2. Lowest active configured IPv4 address
    3. Lowest IPv4 loopback address
  • 1. Router ID that is explicitly configured
    2. Highest IPv4 loopback address
    3. Highest active configured IPv4 address
  • 1. Highest IPv4 loopback address
    2. Router ID that is explicitly configured
    3. Highest active configured IPv4 address
  • 1. Highest IPv4 loopback address
    2. Highest active configured IPv4 address
    3. Router ID that is explicitly configured

Explanation: The correct order of precedence for choosing the router ID is as follows:

1. The router ID is explicitly configured using the OSPF router-id rid router configuration mode command.
2. If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of the configured loopback interfaces.
3. If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces.

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