200-101 · Question #213
Refer to the exhibit. If the router Cisco returns the given output and has not had its router ID set manually, what value will OSPF use as its router ID?
The correct answer is D. 2.2.2.2. OSPF Router ID selection follows the priority: manual configuration first, then highest loopback IP, then highest active physical interface IP. With no manual ID set, the highest loopback address is chosen.
Question
Options
- A192.168.1.1
- B172.16.1.1
- C1.1.1.1
- D2.2.2.2
How the community answered
(16 responses)- B6% (1)
- C6% (1)
- D88% (14)
Why each option
OSPF Router ID selection follows the priority: manual configuration first, then highest loopback IP, then highest active physical interface IP. With no manual ID set, the highest loopback address is chosen.
192.168.1.1 is likely a physical interface address; physical interface IPs are only considered for Router ID selection when no loopback interfaces exist, and even then only the highest is chosen.
172.16.1.1 is a lower IP address than 2.2.2.2; OSPF selects the highest available loopback IP, not a lower one.
1.1.1.1 is numerically lower than 2.2.2.2; since OSPF picks the highest loopback IP and 2.2.2.2 is higher, 1.1.1.1 would not be selected.
When the OSPF Router ID is not manually configured, Cisco IOS selects it automatically by first checking all loopback interfaces and choosing the highest IP address among them. If the output shows 2.2.2.2 as the highest loopback IP on the router, OSPF will use 2.2.2.2 as the Router ID, taking precedence over any physical interface addresses like 192.168.1.1 or 172.16.1.1.
Concept tested: OSPF Router ID automatic selection from loopback interfaces
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
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