The answer is that the interface is missing the `ip ospf authentication message-digest` command. This is correct because OSPF MD5 authentication requires two distinct configuration layers: the area-level `area 0 authentication message-digest` command enables authentication for the entire area, but each participating interface must also be explicitly told to use that authentication mode. Without the interface-level command, the router ignores the configured key and defaults to no authentication, causing OSPF neighbor relationships to fail. On the ENCOR 350-401 exam, this is a classic trap—candidates often assume that configuring the area authentication and the key is sufficient, but the missing command is the interface authentication activation. This tests your understanding that OSPF authentication is a two-step process: area enablement plus interface enforcement. A helpful memory tip is "Area enables, interface enforces"—you must apply both the area and interface authentication commands for MD5 to work.
CCNP OSPF Practice Question
This 350-401 practice question tests your understanding of ospf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Router R5 has the following OSPF configuration:
router ospf 1
router-id 5.5.5.5
network 10.0.0.0 0.255.255.255 area 0
area 0 authentication message-digest
!
interface GigabitEthernet0/0
ip address 10.1.1.5 255.255.255.0
ip ospf message-digest-key 1 md5 cisco123
What is missing from this OSPF authentication configuration?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface needs the 'ip ospf authentication message-digest' command.
Option B is correct because OSPF authentication configuration requires two components: an area-level authentication type (configured via `area 0 authentication message-digest`) and an interface-level authentication mode (configured via `ip ospf authentication message-digest`). The interface command tells the OSPF process to actually use the key defined with `ip ospf message-digest-key`. Without it, the interface defaults to no authentication, even though the area is configured for authentication.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The configuration is complete and correct.
Why it's wrong here
The interface is missing the 'ip ospf authentication message-digest' command to enable authentication.
✓
The interface needs the 'ip ospf authentication message-digest' command.
Why this is correct
This command activates MD5 authentication on the interface, required for the area authentication to take effect.
Related concept
Read the scenario before looking for a memorised answer.
✗
The 'area 0 authentication' command should be 'area 0 authentication md5'.
Why it's wrong here
The correct syntax is 'area 0 authentication message-digest' for MD5 authentication.
✗
The 'network' command should include the area authentication keyword.
Why it's wrong here
The network command does not include authentication parameters; it is configured under the interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume configuring the area authentication and the key is sufficient, overlooking the mandatory interface-level `ip ospf authentication message-digest` command that activates authentication on the specific interface.
Trap categories for this question
Command / output trap
The interface is missing the 'ip ospf authentication message-digest' command to enable authentication.
Detailed technical explanation
How to think about this question
OSPF authentication uses a two-tier model: the area command sets the authentication type for all interfaces in that area, but each interface must explicitly enable authentication via `ip ospf authentication` (or `ip ospf authentication null` to override). The MD5 key ID and password are configured separately with `ip ospf message-digest-key`. If the interface-level authentication command is missing, the router will not append the authentication data to OSPF hello packets, causing adjacency failures with neighbors that expect authentication.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The interface needs the 'ip ospf authentication message-digest' command. — Option B is correct because OSPF authentication configuration requires two components: an area-level authentication type (configured via `area 0 authentication message-digest`) and an interface-level authentication mode (configured via `ip ospf authentication message-digest`). The interface command tells the OSPF process to actually use the key defined with `ip ospf message-digest-key`. Without it, the interface defaults to no authentication, even though the area is configured for authentication.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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