Method of routing implemented by a core router in a first autonomous system, this core router being connected to a nominal output router (ASBR1) of said first autonomous system, said nominal output router (ASBR1) being able to forward traffic intended for a second autonomous system, the first autonomous system furthermore comprising at least one backup output router (ASBR2), able to forward a traffic intended for the second autonomous system, said nominal output router and said backup output router being border routers of the first autonomous system which announce BGP routes for which they are compulsory transit point, said method comprising: - a step of receiving the announcements made by a router (ASBR2, ASBRP) of the first autonomous system which is either the backup output router (ASBR2), or a so-called protector router (ASBRP) of the first autonomous system able to redirect the traffic intended for the second autonomous system via said at least one backup output router (ASBR2), the announcements being protection functions as well as the identifier of a protection group to which the router belongs, so that the core router can identify a router able to forward the traffic intended for the second autonomous system in case of fault affecting the traffic bound for said nominal output router (ASBR1), - a step of exchanging a contextual label between the core router and each of said routers (ASBR2, ASBRP) having announced that they belong to a protection group of like identifier, the contextual label having to be used upon a fault affecting the forwarding of the traffic from said core router to the nominal output router (ASBR1), - a step of configuring a switching table (FIB_P) which comprises, for a destination D in the second autonomous system, a first output to the nominal output router and a second output to a predetermined route in a tunnel (1T, 2T) not passing through the nominal output router (ASBR1) and whose end consists of the identified router, - a step of activating said first output, - a step of detecting a fault affecting the forwarding of the traffic to said nominal output router (ASBR1); - a step of activating said second output in response to said detection step, and - a step of adding said contextual label to the packets of the traffic sent through said tunnel. - a step of receiving the announcements made by a router (ASBR2, ASBRP) of the first autonomous system which is either the backup output router (ASBR2), or a so-called protector router (ASBRP) of the first autonomous system able to redirect the traffic intended for the second autonomous system via said at least one backup output router (ASBR2), the announcements being protection functions as well as the identifier of a protection group to which the router belongs, so that the core router can identify a router able to forward the traffic intended for the second autonomous system in case of fault affecting the traffic bound for said nominal output router (ASBR1), - a step of exchanging a contextual label between the core router and each of said routers (ASBR2, ASBRP) having announced that they belong to a protection group of like identifier, the contextual label having to be used upon a fault affecting the forwarding of the traffic from said core router to the nominal output router (ASBR1), - a step of configuring a switching table (FIB_P) which comprises, for a destination D in the second autonomous system, a first output to the nominal output router and a second output to a predetermined route in a tunnel (1T, 2T) not passing through the nominal output router (ASBR1) and whose end consists of the identified router, - a step of activating said first output, - a step of detecting a fault affecting the forwarding of the traffic to said nominal output router (ASBR1); - a step of activating said second output in response to said detection step, and - a step of adding said contextual label to the packets of the traffic sent through said tunnel.