typically specific to an APN or some PGW internal routing primitive/function construct. Table 4.8 lists the possible changes in the request PDN Type and what actually gets created. A dash in the DAF column means that the value of the DAF has no meaning. Note 1 The PGW respons with a cause code #18 (for GTPv2) or #129 (for GTPv1) 'New PDP/PDN Type due to network preference', which a UE will eventually see as a received (E)SM cause #50 'PDN/PDP Type IPv4 only allowed' [12, 13]. The UE must not attempt to establish a second parallel PDN Connection of a different PDN Type in order to mimic dual-stack behavior. Note 2 The PGW responds with a cause code #18 (for GTPv2) or #129 (for GTPv1) 'New PDP/PDN Type due to network preference', which a UE will eventually see as a received (E)SM cause #51 'PDN/PDP Type IPv6 only allowed' [12, 13]. The UE must not attempt to establish a second parallel PDN Connection of a different PDN Type in order to mimic dual-stack behavior. Note 3 The PGW responds with a cause code #19 (for GTPv2) or #130 (for GTPv1) 'New PDP/PDN Type due to single address bearer only', which a UE will eventually see as a received (E)SM cause #52 'single address bearers only allowed' [12, 13]. The UE is allowed for/should establish a second parallel PDN Connection of a different PDN Type in order to mimic dual-stack behavior. When a PDN Connection activation fails, for example, due to a UE requesting a PDN Type that is not supported or provisioned in the PGW for the requested APN, then the Table 4.8 The PGW configuration and Dual Address Bearer Flag's effect on the PDN Type selection Requested Configured Result Remarks Reject IPv4v6 Reject IPv4v6 IPv4v6 IPv4 and IPv6 IPv4 or IPv6 Note 3 IPv4v6 IPv4v6 unset IPv4 or IPv6 Note 3 IPv4v6 Note 1 IPv4v6 Note 2 IPv4v6 IPv4v6 IPv4v6 IPv6 in 3GPP Networks PGW can send back an error cause code #83 'Preferred PDN Type not supported'. The MME may eventually translate this to either (E)SM cause codes #50 or #51 and send those to the UE over the NAS signaling. 4.5.5 Fallback