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CCNP Routing and Switching ROUTE 300-101: Official Cert Guide by Kevin Wallace

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Inbound and Outbound BGP Filtering on Prefix/Length

Enterprises that choose to use BGP benefit from both learning routes from the connected ISPs and advertising the enterprise’s public prefix to the same ISPs. However, when the eBGP connections to the various ISPs come up, the enterprise BGP routers advertise all the best routes in each router’s BGP table over the eBGP connection. As a result, the ISPs could learn a best route that causes one ISP to send packets to the enterprise, with the enterprise then forwarding the packet out to another ISP. In such a case, the enterprise AS would be acting as a transit AS.

Enterprise engineers can, and probably should, make an effort to filter inappropriate routes sent to the ISP over the eBGP peer connections ...

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