Question 412 of 2,015

WAN Technologies →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is to use AS path prepending on routes advertised to ISP B and set a higher local preference for routes learned from ISP A. This works because BGP traffic engineering for dual-homed MPLS L3VPN relies on manipulating path attributes to control both inbound and outbound traffic flow. For inbound traffic, AS path prepending artificially lengthens the AS_PATH attribute for routes sent to ISP B, making that path less preferred by remote routers, which steers incoming traffic toward ISP A. For outbound traffic, local preference is evaluated first within the CE router; setting a higher local preference for routes learned from ISP A ensures all outbound traffic from the company prefers that link. On the ENCOR 350-401 exam, this scenario tests your understanding of BGP path selection order and the distinction between influencing inbound versus outbound traffic—a common trap is confusing which attribute affects which direction. Remember the memory tip: “Prepend to prevent inbound, prefer local for outbound.”

350-401 WAN Technologies Practice Question

This 350-401 practice question tests your understanding of wan technologies. 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 company is using a dual-homed MPLS L3VPN connection with two different ISPs. The CE router is running eBGP with both PE routers. The engineer wants to ensure that inbound traffic from the Internet to the company's web servers uses both links, but outbound traffic from the company should prefer ISP A. The company advertises the same /24 prefix to both ISPs. What BGP configuration should the engineer apply on the CE router?

Question 1mediummultiple choice

Open the full BGP breakdown →

A
Set a lower MED for routes advertised to ISP A and a higher MED for routes advertised to ISP B.
Why wrong: Incorrect. MED is used to influence inbound traffic, but it is a multi-exit discriminator that is only considered when the same AS receives routes from multiple points. In a dual-homed scenario with different ISPs, MED is not typically used because the ISPs are different ASes.
B
Use AS path prepending on routes advertised to ISP B and set a higher local preference for routes learned from ISP A.
Correct. AS path prepending makes the path to ISP B longer, discouraging inbound traffic from using it. Setting a higher local preference for routes from ISP A makes outbound traffic prefer ISP A.
C
Advertise a more specific prefix (e.g., /25) to ISP A and a less specific prefix (/24) to ISP B.
Why wrong: Incorrect. Advertising different prefix lengths can cause suboptimal routing and may not be accepted by ISPs. It is not a standard practice for load balancing.
D
Configure the CE router to use BGP multipath with both ISPs.
Why wrong: Incorrect. BGP multipath allows load balancing of outbound traffic, but it does not influence inbound traffic. The scenario requires different behaviors for inbound and outbound.

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

✓

Use AS path prepending on routes advertised to ISP B and set a higher local preference for routes learned from ISP A.

To influence inbound traffic, the engineer can use AS path prepending to make one path less preferred. For outbound traffic, local preference can be used to prefer one ISP. Since the company wants outbound traffic to prefer ISP A, they should set a higher local preference for routes learned from ISP A. For inbound traffic, they can prepend AS path to ISP B to make that path less attractive.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

Answer analysis

Option-by-option breakdown

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

✗
Set a lower MED for routes advertised to ISP A and a higher MED for routes advertised to ISP B.
Why it's wrong here
Incorrect. MED is used to influence inbound traffic, but it is a multi-exit discriminator that is only considered when the same AS receives routes from multiple points. In a dual-homed scenario with different ISPs, MED is not typically used because the ISPs are different ASes.
✓
Use AS path prepending on routes advertised to ISP B and set a higher local preference for routes learned from ISP A.
Why this is correct
Correct. AS path prepending makes the path to ISP B longer, discouraging inbound traffic from using it. Setting a higher local preference for routes from ISP A makes outbound traffic prefer ISP A.
Related concept
Static NAT maps one inside address to one outside address.
✗
Advertise a more specific prefix (e.g., /25) to ISP A and a less specific prefix (/24) to ISP B.
Why it's wrong here
Incorrect. Advertising different prefix lengths can cause suboptimal routing and may not be accepted by ISPs. It is not a standard practice for load balancing.
✗
Configure the CE router to use BGP multipath with both ISPs.
Why it's wrong here
Incorrect. BGP multipath allows load balancing of outbound traffic, but it does not influence inbound traffic. The scenario requires different behaviors for inbound and outbound.

Common exam traps

Common exam trap: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Trap categories for this question

Scenario analysis trap
Incorrect. MED is used to influence inbound traffic, but it is a multi-exit discriminator that is only considered when the same AS receives routes from multiple points. In a dual-homed scenario with different ISPs, MED is not typically used because the ISPs are different ASes.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

Real-world example

How this comes up in practice

A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

What to study next

Got this wrong? Here's your next step.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 350-401 question test?

WAN Technologies — This question tests WAN Technologies — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: Use AS path prepending on routes advertised to ISP B and set a higher local preference for routes learned from ISP A. — To influence inbound traffic, the engineer can use AS path prepending to make one path less preferred. For outbound traffic, local preference can be used to prefer one ISP. Since the company wants outbound traffic to prefer ISP A, they should set a higher local preference for routes learned from ISP A. For inbound traffic, they can prepend AS path to ISP B to make that path less attractive.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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