The answer is exceeding the Direct Connect BGP route limit of 100 prefixes, which causes AWS to reject additional routes from the on-premises router. By default, each Direct Connect virtual interface only accepts up to 100 BGP prefixes; when the on-premises router advertises more than this limit, the BGP session drops the extra prefixes, preventing them from reaching the VPCs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the default BGP route limit on Direct Connect virtual interfaces, a common trap where candidates confuse route advertisement issues with path manipulation attributes like AS_PATH prepending or MED. Remember that while AS_PATH and MED influence route preference, they do not block prefix advertisement—only the hard 100-prefix cap does. A helpful memory tip: think of the Direct Connect BGP route limit as a "hundred hurdle"—once you cross 100, your routes get stuck at the gate.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 has a hybrid network with multiple AWS Direct Connect connections to different VPCs. The on-premises network uses BGP to advertise prefixes to AWS. The network team notices that some on-premises prefixes are not being received by the VPCs. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The on-premises router is advertising more than 100 prefixes over the BGP session
Option A is correct because the maximum number of routes advertised over a Direct Connect virtual interface is 100 by default, and exceeding this limit causes BGP to reject additional routes. Option B is wrong because AS_PATH prepending would affect route preference, not advertisement. Option C is wrong because the MED attribute affects path selection, not advertisement. Option D is wrong because BGP communities are optional and not required for prefix advertisement.
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.
✓
The on-premises router is advertising more than 100 prefixes over the BGP session
Why this is correct
AWS limits the number of prefixes per BGP session to 100 by default. Exceeding this causes rejection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The on-premises router is not using BGP communities
The on-premises router is using AS_PATH prepending
Why it's wrong here
AS_PATH prepending influences path selection, not advertisement.
✗
The on-premises router is not setting the MED attribute
Why it's wrong here
MED is optional and not required for prefix advertisement.
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.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 ANS-C01 NAT questions on configuration and troubleshooting.
Network Management and Operations — This question tests Network Management and Operations — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The on-premises router is advertising more than 100 prefixes over the BGP session — Option A is correct because the maximum number of routes advertised over a Direct Connect virtual interface is 100 by default, and exceeding this limit causes BGP to reject additional routes. Option B is wrong because AS_PATH prepending would affect route preference, not advertisement. Option C is wrong because the MED attribute affects path selection, not advertisement. Option D is wrong because BGP communities are optional and not required for prefix advertisement.
What should I do if I get this ANS-C01 question wrong?
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 ANS-C01 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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