7. Interactions with Routing

This specification of source address selection assumes that routing (more precisely, selecting an outgoing interface on a node with multiple interfaces) is done before source address selection. However, implementations MAY use source address considerations as a tiebreaker when choosing among otherwise equivalent routes.

For example, suppose a node has interfaces on two different links, with both links having a working default router. Both of the interfaces have preferred (in the RFC 4862 sense) global addresses. When sending to a global destination address, if there's no routing reason to prefer one interface over the other, then an implementation MAY preferentially choose the outgoing interface that will allow it to use the source address that shares a longer common prefix with the destination.

Implementations that support Rule 5.5 of source address selection (Section 5) also use the choice of router to influence the choice of source address. For example, suppose a host is on a link with two routers. One router is advertising a global prefix A and the other router is advertising global prefix B. Then, when sending via the first router, the host might prefer source addresses with prefix A and when sending via the second router, prefer source addresses with prefix B.