Splitting out IX prefixes from my default route advertisement

I operate a hobby network with edge routers in two US locations for redundancy. One router runs at Neptune Networks in NYC, while another operates at iFog in Ashburn. These edges primarily announce a default route back to the lab environment.

The NYC edge also peers with BGP.Exchange, a virtual internet exchange. To ensure certain prefixes route through Neptune regardless of primary path selection, I implemented community-based filtering.

The Solution

The solution uses BIRD's import and export filters:

Import Filter: Rejects any default routes from BGP.Exchange and tags incoming prefixes with a large community identifier (215855,0,24318).

Export Filter: Selectively announces tagged prefixes separately from the default route, ensuring more-specific routes take precedence over the catch-all default path.

Results

The resulting routing table shows a default route alongside several more-specific IPv6 prefixes from peered networks. This approach guarantees that destinations matching these prefixes always prefer the NYC path even when Ashburn handles the majority of traffic.

This technique demonstrates how BGP communities enable granular route policy control within a dual-homed network topology.