In BGP-Inter-Scenario 2, we implement a routing setup where routers from different autonomous systems (AS) exchange routes using BGP. The primary focus is on configuring AS-Path Prepending and Prefix Filtering as export policies, ensuring that:

• AS-Path Prepending influences path selection by making certain routes less preferred.
• Prefix Filtering ensures routers only advertise their own prefixes (such as /23 and /24) and do not act as transit routers for external traffic.

This scenario uses the following routers:

• R1 (AS100) : Connected to R2 and R3, with specific policies applied for BGP route exports.
• R2 (AS200) : Connected to R1 and R3, with appropriate OSPF and BGP configuration.
• R3 (AS300) : Connected to R1 and R2, sharing routes between its own network and the other ASes.

1. Export Policy - AS-Path Prepending:

• R1 applies AS-Path prepending when advertising routes to R2. This policy makes routes learned via R1 less preferred by adding extra AS numbers to the AS path, making it appear longer.

2. Export Policy - Prefix Filtering:

• R1 only exports its own prefixes (/23 and /24) and avoids advertising routes it learns from other routers. This policy prevents R1 from becoming a transit router for traffic between R2 and R3.

The topology includes the following interconnections between the routers:

• R1 (AS100) ↔ R2 (AS200) via 10.10.1.0/24
• R1 (AS100) ↔ R3 (AS300) via 10.10.2.0/24
• R2 (AS200) ↔ R3 (AS300) via 10.10.3.0/24

Each router also has its own internal networks:

• R1: 10.11.1.0/24
• R2: 10.12.1.0/24
• R3: 10.13.1.0/24

1. AS-Path Prepending: You can verify the AS-path prepending by checking the BGP route advertisements from R1 to R2. R2 will receive the routes with additional AS numbers added by R1.
2. Prefix Filtering

R2# show ip bgp neighbors 10.10.1.1 advertised-routes

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