JNCIS-ENT BGP Practice Question
You are running BGP between two routers. One router has a 4-byte ASN (e.g., ASN 65536). The remote router only supports 2-byte ASNs. How does BGP handle this compatibility issue?
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
✓
The router uses the reserved AS 23456 (AS_TRANS) in place of the 4-byte ASN for legacy peers.
BGP uses the TRANSIT_AS or AS4_PATH / AS4_AGGREGATOR capabilities negotiated during the OPEN message phase to support 4-byte ASNs across 2-byte peers using a reserved ASN (AS 23456).
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 session fails to open because 4-byte ASNs are strictly incompatible with 2-byte peers.
Why it's wrong here
Capabilities negotiation allows fallback via AS 23456.
- ✓
The router uses the reserved AS 23456 (AS_TRANS) in place of the 4-byte ASN for legacy peers.
Why this is correct
AS 23456 is used as a transition AS for legacy 2-byte AS routers.
- ✗
The session establishes as iBGP instead of eBGP.
Why it's wrong here
ASN differences still classify it as eBGP.
- ✗
The 4-byte ASN is truncated to 2 bytes by dropping the most significant bits.
Why it's wrong here
Truncation would corrupt AS paths; AS_TRANS is used instead.
About these practice questions
This JNCIS-ENT question is part of Courseiva's 506-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed August 2026 · checked against the official Juniper Networks exam blueprint
This JNCIS-ENT practice question is part of Courseiva's free Juniper Networks 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 JNCIS-ENT exam.