Here,s why:
BGP Community Attributes: This is a standard BGP feature specifically designed for controlling which routes get advertised to other BGP peers. By attaching specific community attributes to advertised routes from the on-premises router, the developer can precisely control which VCNs within the DRG receive those routes.
Granular Control: Unlike other options, community attributes offer fine-grained control over route advertisement. They can be used to selectively advertise routes to specific VCNs, groups of VCNs based on specific criteria, or even control how OCI prioritizes incoming routes based on their associated communities.
Efficiency: Modifying community attributes directly at the source (on-premises router) is efficient and avoids unnecessary configuration changes within the DRG or individual VCNs.
Comparison with other options:
DRG Security List Rules (A): Security lists control traffic flow within a VCN, not routes advertised to the VCN via BGP.
VCN Route Tables (C): Route tables within VCNs control how traffic within the VCN is routed, not which routes are advertised to the VCN.
IPSec Tunnels with Route Filters (D): While IPSec tunnels support route filters, this approach is less efficient and less standardized compared to BGP community attributes, requiring configuration on both sides of the tunnel.