When configuring OSPF on a router with loopback0 10.10.10.10 and loopback1 5.5.5.5, which address will be selected as the router ID?

When configuring OSPF on a router with loopback0 10.10.10.10 and loopback1 5.5.5.5, which address will be selected as the router ID?

• A. 8.8.8.8
• B. 5.5.5.5
• C. 1.1.1.1
• D. 10.10.10.10

Answer: (D)

OSPF picks a router ID to uniquely identify the router in the routing domain. If you don’t set it manually, the router ID is chosen as the highest IPv4 address on any active interface. Here, both loopback addresses are up: 10.10.10.10 on one loopback and 5.5.5.5 on the other. The highest of these is 10.10.10.10, so that becomes the router ID. Loopback addresses are often used for this purpose because they’re stable and always up. If you did configure a specific router ID, that value would override the automatic selection. The other addresses would only be chosen automatically if they happened to be the highest address on an active interface.

OSPF picks a router ID to uniquely identify the router in the routing domain. If you don’t set it manually, the router ID is chosen as the highest IPv4 address on any active interface. Here, both loopback addresses are up: 10.10.10.10 on one loopback and 5.5.5.5 on the other. The highest of these is 10.10.10.10, so that becomes the router ID. Loopback addresses are often used for this purpose because they’re stable and always up. If you did configure a specific router ID, that value would override the automatic selection. The other addresses would only be chosen automatically if they happened to be the highest address on an active interface.