The answer is that BGP auto-summary is disabled by default on Cisco IOS-XE. This default behavior means that BGP will not automatically summarize subnets to their classful boundaries, such as converting a /24 into a Class A, B, or C network. The reason for this is that modern BGP deployments, especially in MPLS L3VPN environments, require precise, classless routing to support VRF-specific prefixes and avoid route aggregation that could cause black-holing or incorrect path selection. On the Cisco CCNP ENARSI 300-410 exam, this concept often appears as a trick question where candidates assume auto-summary is enabled, as it was in older IOS versions. The common trap is confusing BGP’s default with EIGRP or RIP, which historically summarized by default. Remember the memory tip: “BGP is classless by default—no auto-summary means no forced classful boundaries.”
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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.
In MPLS L3VPN, what is the default behavior of BGP auto-summary on Cisco IOS-XE?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Auto-summary is disabled by default.
BGP auto-summary is disabled by default on Cisco IOS-XE. When enabled, it would summarize subnets to their classful boundary, but this is not default.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Auto-summary is enabled by default for all BGP sessions.
Why it's wrong here
Auto-summary is disabled by default in modern IOS-XE.
✓
Auto-summary is disabled by default.
Why this is correct
Correct. Auto-summary is off by default, preventing classful summarization.
Auto-summary is not enabled by default for any BGP session type.
✗
Auto-summary is disabled for eBGP but enabled for iBGP.
Why it's wrong here
Auto-summary is uniformly disabled by default.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
MPLS L3VPN — This question tests MPLS L3VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Auto-summary is disabled by default. — BGP auto-summary is disabled by default on Cisco IOS-XE. When enabled, it would summarize subnets to their classful boundary, but this is not default.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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