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Question 17 - JN0-280 discussion

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Which statement is correct about the BGP AS path when advertising routes?

A.

The order of the AS path is not significant.

Answers
A.

The order of the AS path is not significant.

B.

The local AS number is added to the end of the AS path.

Answers
B.

The local AS number is added to the end of the AS path.

C.

The order of the AS path is only significant in IBGP.

Answers
C.

The order of the AS path is only significant in IBGP.

D.

The local AS number is added to the beginning of the AS path.

Answers
D.

The local AS number is added to the beginning of the AS path.

Suggested answer: D

Explanation:

The BGP AS (Autonomous System) path attribute is crucial in path selection and loop prevention. Each BGP router appends its local AS number to the beginning of the AS path when it advertises a route to an external BGP (eBGP) peer.

Step-by-Step Breakdown:

AS Path Attribute:

The AS path is a sequence of AS numbers that a route has traversed to reach a destination. Each AS adds its number to the front of the path, allowing BGP to track the route's history.

Why the Local AS is Added at the Beginning:

When advertising a route to an eBGP neighbor, a BGP router adds its own AS number to the beginning of the AS path. This ensures that the AS path reflects the route's journey accurately from the origin to the destination, and prevents loops in BGP. If the route returns to the same AS, the router will detect its AS number in the path and reject the route, preventing routing loops.

Order of the AS Path:

The order is significant because BGP uses it to select the best path. A shorter AS path is preferred, as it indicates fewer hops between the source and destination.

Juniper

Reference:

AS Path Attribute: Junos devices append the local AS at the start of the AS path before advertising the route to an external peer.

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asked 10/10/2024
Wilson Sigcha
31 questions
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