200-101 · Question #211
What OSPF command, when configured, will include all interfaces into area 0?
The correct answer is A. network 0.0.0.0 255.255.255.255 area 0. In OSPF, the 'network' command uses a wildcard mask. Using 0.0.0.0 as the address with 255.255.255.255 as the wildcard mask matches every possible IP address, enabling OSPF on all interfaces.
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Options
- Anetwork 0.0.0.0 255.255.255.255 area 0
- Bnetwork 0.0.0.0 0.0.0.0 area 0
- Cnetwork 255.255.255.255 0.0.0.0 area 0
- Dnetwork all-interfaces area 0
How the community answered
(37 responses)- A95% (35)
- B3% (1)
- D3% (1)
Why each option
In OSPF, the 'network' command uses a wildcard mask. Using 0.0.0.0 as the address with 255.255.255.255 as the wildcard mask matches every possible IP address, enabling OSPF on all interfaces.
The OSPF 'network' command syntax is 'network <address> <wildcard-mask> area <id>'. A wildcard mask of 255.255.255.255 means all bits are wildcarded (ignored), so 'network 0.0.0.0 255.255.255.255 area 0' matches any interface IP address and places every interface into area 0.
The address 0.0.0.0 with wildcard 0.0.0.0 would only match interfaces with the exact IP address 0.0.0.0, which no interface has - this command would match nothing.
The address 255.255.255.255 with wildcard 0.0.0.0 requires an interface with exactly the IP 255.255.255.255, which is a broadcast address and not assignable, so no interface would match.
'network all-interfaces area 0' is not valid OSPF syntax on Cisco IOS; no such keyword exists in the OSPF network command.
Concept tested: OSPF network command with wildcard mask to match all interfaces
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
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