Source-specific routing

The main application of source-specific routing is to allow a cheap form of multihoming without the need for provider-independent addresses or any cooperation from upstream ISPs.

Both edge routers announce a default route, meaning that they are willing to accept packets destined for the Internet.

The effect is that each edge router only attracts packets that have a source address in that provider's prefix.

With source-specific routing, each host interface has multiple addresses, one per provider-dependent prefix.

For outgoing traffic, host software must choose the right source address.