The scenario describes two customer sites (R1, R4) using the same AS number (65000) connected via a transit network (R2, R3). By default, BGP's loop prevention mechanism will cause R4 to reject routes advertised by R1 because R4 will see its own AS in the ASPATH, and vice-versa.

There are two primary methods to solve this, both configured on the transit routers (R2 and R3):

1. as-override (Option B): This feature is configured on the transit routers' BGP sessions facing the customer. It modifies the ASPATH by replacing all occurrences of the customer's AS number with the transit router's own AS number. This effectively hides the customer's AS from the remote site, bypassing the loop prevention check.

2. advertise-peer-as (Option C): This is an alternative feature that allows a transit router to advertise routes learned from one external peer to another external peer within the same customer AS. This is the provider-side step of a two-part solution that also requires the customer routers (R1, R4) to be configured with the loops statement to accept routes containing their own AS.

A. Configure the BGP group with the advertise-peer-as parameter on R1 and R4.

This feature is configured on the transit provider routers (R2, R3), not the customer edge routers.

D. Configure the BGP group with the as-override parameter on R1 and R4.

This feature is configured on the transit provider routers (R2, R3), not the customer edge routers.

1. Juniper Networks TechLibrary

Junos OS Routing Protocols Library

as-override: "For use in BGP sessions with external BGP (EBGP) neighbors. When you include this statement

the local routing device replaces the autonomous system (AS) number of the neighbor with its own AS number in the AS path." This directly supports Option B as a solution.

2. Juniper Networks TechLibrary

Junos OS Routing Protocols Library

advertise-peer-as: "Allows a device to advertise routes to an external BGP (EBGP) peer that were learned from another EBGP peer in the same autonomous system (AS). This feature is used when a customer is multihomed to the same service provider at different locations and the customer is using the same AS number at these locations." This identifies the feature's purpose for the exact scenario in the question

supporting Option C.

3. Juniper Networks TechLibrary

BGP Feature Guide for Routing Devices

"Allowing a PE Router to Advertise Routes to a CE Router in the Same AS": This guide explicitly presents as-override and advertise-peer-as as the two alternative solutions for this problem. It states

"To allow a PE router to advertise routes to a CE router in the same AS

you can configure either the as-override statement or the advertise-peer-as statement in the BGP group that is configured on the PE router." This confirms both B and C are valid approaches configured on the transit routers.