# Connection of IPv6 Islands Across Multiple IPv4 Domains Overview

When the IPv6 islands are separated by multiple IPv4 domains, the autonomous system boundary routers between the IPv4 domains must be DS-BGP routers (Figure 113).

Figure 113: IPv6 Tunneled Across IPv4 Domains

Each of these AS boundary routers establishes a peer relationship with the DS-BGP routers in its own domain, creating a separate mesh of tunnels among the DS-BGP routers of each domain. Routing between PE 1–ASBR 1 in AS 1 and between PE 2–ASBR 2 in AS 2 is accomplished by means of label-switched paths.

IPv6 unlabeled routes are exchanged through the external MP-BGP session between ASBR 1 and ASBR 2. Interdomain MPLS tunnels spanning multiple ASs are not supported.