Question 1,871 of 2,152

IP SLA →hardMultiple ChoiceObjective-mapped

300-410 IP SLA Practice Question

This 300-410 practice question tests your understanding of ip sla. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.

An engineer configures IP SLA to track a route to a loopback address for BGP next-hop tracking. Router R1 has:

ip sla 4

icmp-echo 192.168.100.1 source-ip 10.0.0.1 frequency 5

ip sla schedule 4 life forever start-time now

track 4 ip sla 4 reachability

router bgp 65001
 neighbor 10.0.0.2 remote-as 65002
 neighbor 10.0.0.2 update-source Loopback0
 neighbor 10.0.0.2 ebgp-multihop 2
 neighbor 10.0.0.2 fall-over bfd

But the BGP session to 10.0.0.2 is flapping every 5 seconds. What is the root cause?

A
The IP SLA probe is sending ICMP packets that are being rate-limited by the CoPP policy, causing the BFD session to drop.
Why wrong: CoPP rate-limiting ICMP packets does not affect BFD sessions because BFD uses separate UDP packets. The flapping is not caused by IP SLA interference with BFD.
B
The ebgp-multihop 2 is not sufficient for the BFD session to establish.
Correct. ebgp-multihop 2 sets the TTL for BGP packets but does not configure BFD for multihop. BFD multihop requires explicit configuration; without it, BFD cannot establish, leading to BGP session flapping.
C
The update-source Loopback0 is not reachable from 10.0.0.2.
Why wrong: If the update-source loopback were unreachable, the BGP session would fail to establish at all, not flap periodically.
D
The IP SLA schedule is set to start now, but the frequency is too high, causing the router to crash.
Why wrong: IP SLA frequency of 5 seconds is standard and does not cause router crashes. The flapping is due to BFD, not CPU overload.

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

✓

The ebgp-multihop 2 is not sufficient for the BFD session to establish.

The BGP session flaps every 5 seconds because BFD is enabled with 'fall-over bfd', but ebgp-multihop 2 alone is not sufficient for BFD to establish over a multihop eBGP session. BFD requires explicit multihop configuration under the BGP process or interface, such as the 'bfd interval' command with the 'multihop' keyword. Without this, BFD fails to establish, causing BGP to detect a neighbor failure and flap the session. The 5-second interval matches the default BFD failure detection timing, not the IP SLA frequency.

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.

✗
The IP SLA probe is sending ICMP packets that are being rate-limited by the CoPP policy, causing the BFD session to drop.
Why it's wrong here
CoPP rate-limiting ICMP packets does not affect BFD sessions because BFD uses separate UDP packets. The flapping is not caused by IP SLA interference with BFD.
✓
The ebgp-multihop 2 is not sufficient for the BFD session to establish.
Why this is correct
Correct. ebgp-multihop 2 sets the TTL for BGP packets but does not configure BFD for multihop. BFD multihop requires explicit configuration; without it, BFD cannot establish, leading to BGP session flapping.
Related concept
OSPF neighbours must agree on key parameters.
✗
The update-source Loopback0 is not reachable from 10.0.0.2.
Why it's wrong here
If the update-source loopback were unreachable, the BGP session would fail to establish at all, not flap periodically.
✗
The IP SLA schedule is set to start now, but the frequency is too high, causing the router to crash.
Why it's wrong here
IP SLA frequency of 5 seconds is standard and does not cause router crashes. The flapping is due to BFD, not CPU overload.

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.

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?

IP SLA — This question tests IP SLA — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The ebgp-multihop 2 is not sufficient for the BFD session to establish. — The BGP session flaps every 5 seconds because BFD is enabled with 'fall-over bfd', but ebgp-multihop 2 alone is not sufficient for BFD to establish over a multihop eBGP session. BFD requires explicit multihop configuration under the BGP process or interface, such as the 'bfd interval' command with the 'multihop' keyword. Without this, BFD fails to establish, causing BGP to detect a neighbor failure and flap the session. The 5-second interval matches the default BFD failure detection timing, not the IP SLA frequency.

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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