8. How CEs Learn Routes from PEs

8. How CEs Learn Routes from PEs

In this section, we assume that the CE device is a router.

If the PE places a particular route in the VRF it uses to route packets received from a particular CE, then in general, the PE may distribute that route to the CE. Of course, the PE may distribute that route to the CE only if this is permitted by the rules of the PE/CE protocol. (For example, if a particular PE/CE protocol has "split horizon", certain routes in the VRF cannot be redistributed back to the CE.) We add one more restriction on the distribution of routes from PE to CE: if a route's Site of Origin attribute identifies a particular site, that route must never be redistributed to any CE at that site.

In most cases, however, it will be sufficient for the PE to simply distribute the default route to the CE. (In some cases, it may even be sufficient for the CE to be configured with a default route pointing to the PE.) This will generally work at any site that does not itself need to distribute the default route to other sites. (E.g., if one site in a corporate VPN has the corporation's access to the Internet, that site might need to have default distributed to the other site, but one could not distribute default to that site itself.)

Whatever procedure is used to distribute routes from CE to PE will also be used to distribute routes from PE to CE.