hardMultiple Choice
300-410 Practice Question: In MPLS L3VPN, what is the default behavior of…
In MPLS L3VPN, what is the default behavior of BGP auto-summary on Cisco IOS-XE?
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
✓
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.
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
IOS-XE ships with auto-summary disabled by default across all address families and session types, so no session has it enabled unless explicitly configured. The option tempts because auto-summary was enabled by default in very old IOS releases, and remains configurable per address family for classful network summarisation.
- ✓
Auto-summary is disabled by default.
Why this is correct
Cisco IOS-XE ships with BGP auto-summary disabled, so classful network summarisation does not occur unless explicitly enabled. This default preserves the exact prefixes advertised across the MPLS L3VPN, satisfying the stem's question about out-of-the-box behaviour rather than configured behaviour.
- ✗
Auto-summary is enabled only for iBGP sessions.
Why it's wrong here
Auto-summary is disabled by default for iBGP as well as eBGP on IOS-XE, so restricting the default to iBGP sessions misstates the platform behaviour. The option tempts because iBGP carries internal routes where summarisation concepts feel relevant, yet the default remains off for both session types.
- ✗
Auto-summary is disabled for eBGP but enabled for iBGP.
Why it's wrong here
Cisco IOS-XE disables auto-summary by default for every BGP session, eBGP and iBGP alike, so splitting the default by session type contradicts the platform's actual behaviour. The option tempts because eBGP and iBGP do differ in other defaults, such as next-hop handling and administrative distance.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
Learn chapter
MPLS L3VPN Basics
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.