RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001 Many vendors have already started to offer SONET and SDH products that are managed by IP instead of CLNS and a general migration from CLNS towards IP is anticipated within the industry. Part of any migration strategy from CLNS to IP should provide for the co-existence of both CLNS managed and IP managed network elements in the same network. Such a migration strategy should foresee the need to manage existing CLNS managed network elements that become isolated by a new IP network. Such a scenario may be tackled by tunnelling CLNP PDUs over IP using the existing GRE standard RFC 2784 [1] and informational RFC 1702 [2]. Networks have already been deployed that use this method. Such a migration strategy should also foresee the need to manage new IP managed network elements that are installed on the far side of existing CLNS managed network. Such a scenario requires a method for tunnelling IP over CLNS. 2. GRE over CLNS advantages Using GRE to tunnel IP over CLNS offers some advantages. In the absence of a standard for tunnelling IP over CLNS, GRE as specified in RFC 2784 [1] is the most applicable standard that exists. The move from CLNS to IP comes at a time when IP is itself migrating from IPv4 to IPv6. GRE defines a method to tunnel any protocol that has an Ethernet Protocol Type. Therefore by defining a method for CLNS to transport GRE, a method will then exist for CLNS to transport any other protocol that has an Ethernet Protocol Type defined in RFC 1700 [5]. Thus GRE over CLNS can be used to tunnel both IPv4 and IPv6. GRE is already commonly used to tunnel CLNP PDUs over IP and so using GRE to tunnel IP over CLNS gives a common approach to tunnelling and may simplify software within network elements that initiate and terminate tunnels. The only disadvantage of using GRE is the extra minimum of four bytes that will be used between CLNP header and IP payload packet. Given the large size of CLNP headers this will not make a significant difference to the performance of any network that has IP over CLNP PDUs present on it. Christian Informational