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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

    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 enabled only for iBGP sessions.

    Why it's wrong here

    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.

Visual reference

192.168.1.0 /24 256 addresses (254 usable) 192.168.1.0 /25 Subnet A 128 addr (126 usable) 192.168.1.128 /25 Subnet B 128 addr (126 usable) Borrowing 1 bit from host portion creates 2 subnets (/25)

Quick reference

IPv4 Address Class Summary

ClassFirst Octet RangeDefault MaskNetworksHosts per Network
A1–126/8 (255.0.0.0)12616,777,214
B128–191/16 (255.255.0.0)16,38465,534
C192–223/24 (255.255.255.0)2,097,152254
D224–239N/AMulticast groups
E240–255N/AReserved / experimental

127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.

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Written by Johnson Ajibi, MSc IT Security

Senior Network & Security Engineer · founder of Courseiva

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