- A
BGP TTL security uses the Generalized TTL Security Mechanism (GTSM) to validate the TTL of incoming BGP packets.
Correct because GTSM checks that the TTL is within a valid range based on the configured hop count.
- B
The command 'neighbor <ip> ttl-security hops <hop-count>' is used to enable BGP TTL security on a per-neighbor basis.
Correct because this command configures the expected hop count for the BGP session.
- C
The default TTL value for eBGP packets is 64.
Why wrong: Incorrect because the default TTL for eBGP packets is 1, not 64.
- D
BGP TTL security encrypts the BGP update messages to prevent eavesdropping.
Why wrong: Incorrect because GTSM does not provide encryption; it only validates TTL values.
- E
BGP TTL security is only applicable to eBGP sessions, not iBGP.
Why wrong: Incorrect because GTSM can be used for both eBGP and iBGP sessions.
Quick Answer
The correct answer is that the command 'neighbor <ip> ttl-security hops <hop-count>' enables BGP TTL security on a per-neighbor basis, and this feature, known as GTSM (Generalized TTL Security Mechanism), protects against CPU-based attacks by validating that incoming BGP packets arrive with a TTL of 255 minus the expected hop count. This works because an attacker off-path cannot forge a packet with the precise, high TTL value required, making it a lightweight defense against spoofed BGP updates. On the ENCOR 350-401 exam, this topic tests your understanding of control-plane hardening; a common trap is confusing the default eBGP TTL of 1 with the GTSM requirement, or assuming GTSM works unilaterally—it must be configured on both peers. Remember the memory tip: "255 minus hops" ensures only legitimate neighbors with the correct hop distance are trusted.
CCNP Infrastructure Security Practice Question
This 350-401 practice question tests your understanding of infrastructure security. 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.
Which two statements about BGP TTL security are true? (Choose two.)
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
BGP TTL security uses the Generalized TTL Security Mechanism (GTSM) to validate the TTL of incoming BGP packets.
BGP TTL security (GTSM) protects against CPU-based attacks by ensuring incoming BGP packets have a TTL of 255 minus the expected hop count. The neighbor ttl-security hops command is used on Cisco IOS-XE to enable this feature, and it must be configured on both peers to be effective. Option C is incorrect because the default TTL for eBGP is 1, not 64. Option D is incorrect because GTSM does not encrypt BGP updates. Option E is incorrect because GTSM is supported for both eBGP and iBGP.
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.
- ✓
BGP TTL security uses the Generalized TTL Security Mechanism (GTSM) to validate the TTL of incoming BGP packets.
Why this is correct
Correct because GTSM checks that the TTL is within a valid range based on the configured hop count.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
The command 'neighbor <ip> ttl-security hops <hop-count>' is used to enable BGP TTL security on a per-neighbor basis.
- ✗
The default TTL value for eBGP packets is 64.
Why it's wrong here
Incorrect because the default TTL for eBGP packets is 1, not 64.
- ✗
BGP TTL security encrypts the BGP update messages to prevent eavesdropping.
Why it's wrong here
Incorrect because GTSM does not provide encryption; it only validates TTL values.
- ✗
BGP TTL security is only applicable to eBGP sessions, not iBGP.
Why it's wrong here
Incorrect because GTSM can be used for both eBGP and iBGP sessions.
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 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
Infrastructure Security — This question tests Infrastructure Security — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: BGP TTL security uses the Generalized TTL Security Mechanism (GTSM) to validate the TTL of incoming BGP packets. — BGP TTL security (GTSM) protects against CPU-based attacks by ensuring incoming BGP packets have a TTL of 255 minus the expected hop count. The neighbor ttl-security hops command is used on Cisco IOS-XE to enable this feature, and it must be configured on both peers to be effective. Option C is incorrect because the default TTL for eBGP is 1, not 64. Option D is incorrect because GTSM does not encrypt BGP updates. Option E is incorrect because GTSM is supported for both eBGP and iBGP.
What should I do if I get this 350-401 question wrong?
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 350-401 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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