Question 2,092 of 2,152

SNMP Troubleshooting →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is an ACL applied to the SNMP community string that does not permit the NMS IP address. This is correct because the debug output on R2 explicitly shows packets being dropped with an "access-list violation," yet no ACL is applied to any interface. When an SNMP community string is configured with an optional ACL, that ACL filters incoming SNMP requests at the application layer before the router processes them; if the NMS IP is not permitted, the router silently drops the packets, mimicking an interface ACL violation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between interface ACLs, CoPP policies, and SNMP community ACLs during root cause analysis of intermittent SNMP polling failures. A common trap is to assume a missing interface ACL means no filtering exists, but SNMP community ACLs operate independently. Memory tip: "SNMP community ACLs are invisible gatekeepers—if the NMS can't ping but debug shows ACL drops, check the community string's access-list first."

300-410 SNMP Troubleshooting Practice Question

This 300-410 practice question tests your understanding of snmp troubleshooting. 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.

A large enterprise network is experiencing intermittent SNMP polling failures from the NMS to router R2. R1 and R2 are connected via a serial link running OSPF. R1 has the following relevant configuration: snmp-server community public RO, snmp-server community private RW, snmp-server trap-source Loopback0, snmp-server enable traps ospf. R2 shows: debug ip packet shows packets from NMS (10.1.1.100) to R2's Loopback0 (10.2.2.2) being dropped with 'access-list violation'. No ACL is applied to any interface on R2. What is the root cause?

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
An ACL is applied to the SNMP community string that does not permit the NMS IP address.
The snmp-server community command can have an optional ACL. If configured, it filters SNMP access. The debug shows ACL violation, so this is the direct cause.
B
OSPF network type mismatch between R1 and R2 causes routing blackhole.
Why wrong: OSPF network type mismatch can cause adjacency issues, but the symptom is SNMP polling failure, not routing loss. Debug shows ACL violation, not routing.
C
The NMS is using SNMPv3 with incorrect credentials, causing authentication failure.
Why wrong: The debug shows 'access-list violation', not authentication failure. SNMPv3 authentication would show different debug messages.
D
R2's loopback interface is not advertised into OSPF, making it unreachable.
Why wrong: If the loopback were not advertised, the NMS would not have a route, but the debug shows packets reaching R2 and being dropped by ACL, so routing is working.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

An ACL is applied to the SNMP community string that does not permit the NMS IP address.

The NMS is sending SNMP requests to R2's Loopback0 IP, but R2's OSPF configuration includes an implicit deny in the route map or distribution list that blocks the return path. However, the debug output indicates an ACL violation. The root cause is that R2 has a CoPP policy that rate-limits or drops SNMP traffic, but the debug shows ACL violation, so it is likely an implicit deny in the access-class applied to the VTY lines or an SNMP ACL. Given no interface ACL, the most common cause is an 'access-class' under line vty that denies the NMS IP, or an 'snmp-server community' with an ACL that implicitly denies the NMS. The correct answer is: 'An ACL is applied to the SNMP community string that does not permit the NMS IP address.'

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
An ACL is applied to the SNMP community string that does not permit the NMS IP address.
Why this is correct
The snmp-server community command can have an optional ACL. If configured, it filters SNMP access. The debug shows ACL violation, so this is the direct cause.
Related concept
OSPF neighbours must agree on key parameters.
✗
OSPF network type mismatch between R1 and R2 causes routing blackhole.
Why it's wrong here
OSPF network type mismatch can cause adjacency issues, but the symptom is SNMP polling failure, not routing loss. Debug shows ACL violation, not routing.
✗
The NMS is using SNMPv3 with incorrect credentials, causing authentication failure.
Why it's wrong here
The debug shows 'access-list violation', not authentication failure. SNMPv3 authentication would show different debug messages.
✗
R2's loopback interface is not advertised into OSPF, making it unreachable.
Why it's wrong here
If the loopback were not advertised, the NMS would not have a route, but the debug shows packets reaching R2 and being dropped by ACL, so routing is working.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

Trap categories for this question

Command / output trap
OSPF network type mismatch can cause adjacency issues, but the symptom is SNMP polling failure, not routing loss. Debug shows ACL violation, not routing.

Detailed technical explanation

How to think about this question

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

SNMP Troubleshooting — This question tests SNMP Troubleshooting — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: An ACL is applied to the SNMP community string that does not permit the NMS IP address. — The NMS is sending SNMP requests to R2's Loopback0 IP, but R2's OSPF configuration includes an implicit deny in the route map or distribution list that blocks the return path. However, the debug output indicates an ACL violation. The root cause is that R2 has a CoPP policy that rate-limits or drops SNMP traffic, but the debug shows ACL violation, so it is likely an implicit deny in the access-class applied to the VTY lines or an SNMP ACL. Given no interface ACL, the most common cause is an 'access-class' under line vty that denies the NMS IP, or an 'snmp-server community' with an ACL that implicitly denies the NMS. The correct answer is: 'An ACL is applied to the SNMP community string that does not permit the NMS IP address.'

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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