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

The BGP ASPATH is a well-known mandatory attribute used for loop prevention and path selection. When a BGP speaker advertises a route to an external BGP (eBGP) peer in a different Autonomous System (AS), it prepends its own AS number to the beginning of the ASPATH list. This creates an ordered list of ASes that the route advertisement has traversed. The receiving BGP router uses this path information to ensure it is not accepting a route that has already passed through its own AS, which would indicate a routing loop.

A. The order of the AS path is critical for BGP's loop prevention mechanism and for path selection policies.

B. The local AS number is prepended (added to the beginning) of the AS path, not appended to the end.

C. The AS path is not modified between internal BGP (iBGP) peers; its primary significance is in eBGP exchanges between different autonomous systems.

1. Juniper Networks Documentation

"Junos OS Routing Protocols Library for Routing Devices"

section "BGP Path Attributes". It states

"When a routing device sends a route to an eBGP peer

it prepends its own autonomous system number to the AS path."

2. IETF RFC 4271

"A Border Gateway Protocol 4 (BGP-4)"

Section 5.1.2

"ASPATH". It specifies: "When a BGP speaker advertises a route to an external peer

the speaker shall prepend its own AS number to the ASPATH attribute."