The Cisco ePDG supports selection of P-GW based on the allocated UE-Usage-Type (UUT) of the subscriber. UE-Usage-Type is stored in the HSS (Home Subscriber Server) within the subscription information of the UE. Each UE can have no more than one UE-Usage-Type. This functionality of selecting P-GW based on UE-Usage-Type is used to latch the subscribers to different cores by assigning an appropriate UE-Usage-Type value.

There is no change in the fallback mechanism, priority, and preference order of selection based on various criteria between AAA provided IP, DNS, and Static as in the existing P-GW selection, and same is applicable to UUT associated P-GWs.

This feature supports the UE-Usage-Type AVP with type Unsigned32 in the Diameter EAP Answer (DEA) message. This value indicates the usage characteristics of an UE that enables the selection of a specific Dedicated Core Network (DCN). The allowed value of UE-Usage-Type is in the range 0–255.

The ePDG considers UE-Usage-Type AVP from the AAA/CPAR in the Diameter EAP Answer (DEA) message for the selection of P-GW to which the session has to be latched.

Note	Subscribers not associated with any UE-Usage-Type are treated as per the current implementation. They will not be assigned any default UE-Usage-Type value.

To select the P-GW to be latched on based on the UE-Usage-Type:

• Enable the UE-Usage-Type feature through the CLI without 5G Interworking and ensure that UE-Usage-Type AVP is received for a subscriber from the AAA server in the DEA (Diameter EAP Answer).

When the feature is enabled and UE-Usage-Type AVP is received from the AAA or CPAR in the DEA message, the ePDG uses the S-NAPTR procedure with the x-s2b-gtp+ue-<uut value> service parameter in the following scenarios:

• AAA provided FQDN based PGW selection

• APN-FQDN based PGW selection

• Local FQDN based PGW selection

The following process happens:

• ePDG first initiates the SNAPTR query against the AAA provided PGW-ID(FQDN) or Local FQDN or APN-FQDN to the server, to get the PGW IP address.

• The DNS server returns the NAPTR Resource Records (RRs) with “s” flag.

• RRs with service-parameter x-3gpp-pgw: x-s2b-gtp+ue-<uut value1> are considered by the ePDG to match one of the uut-value with the received UE-Usage-Type value.

From DNS responses, if ePDG selects the P-GW based on the service parameter x-s2b-gtp+ue-<uut-value1>.<uut-value2>.<uut-value3> , where one of the UUT value in the Service parameter of the received DNS records matches with the UE-Usage-Type value received from AAA for a subscriber, the following selection order applies:

1. If DNS response has records for the given UUT, ePDG selects the P-GW. If none of the selected P-GWs are not reachable, fallback to static P-GW selection works based on local configuration, with the given UUT.

2. If DNS response has no matching UUT records, but has P-GW records without UUT, then ePDG ignores the P-GW list and fallback based on local configuration.

3. If the DNS query fails, or there are no PGW records matching with the given UUT value, or DNS is not reachable then, ePDG fallback to static P-GW selection based on the local configuration. The appropriate DNS-related failures get incremented.

In case of Local Static selection:

• If P-GW with the matching UUT value is configured, that will be considered

• If weight is defined, then, the Weight algorithm similar to the existing P-GW selection is applied to UUT-based selection.

• If no weight is configured, P-GW is selected in a round robin manner.

• If no P-GW with subscriber’s UUT is configured, but configured with P-GWs without UUT, or different UUTs, then ePDG ignores the P-GW lists and P-GW selection fails, a call gets terminated with appropriate disconnect reasons.

This feature has the following limitations:

• The P-GW selection based on UE-Usage-Type (UUT) support is limited only to GTPv2 based s2b interface. All the three PDN types are supported including the IPv4, IPv6 and IPv4v6.

• ePDG does not send the value of UE-Usage-Type to P-GW in the Create Session Request.

• ePDG considers only the UUTs received in DEA message and not in any other diameter messages.

• This feature is not supported for Emergency calls. P-GW selection for emergency calls is as per the current implementation.

Use the following configuration commands to enable or disable the UE-Usage-Type.

configure 
     context context_name 
        epdg-service service_name 
            [ no ] pgw-selection ue-usage-type 
            end 

NOTES:

• pgw-selection ue-usage-type : Enables P-GW selection based on the UE-Usage-Type.

• no pgw-selection ue-usage-type : Disables P-GW selection based on the UE-Usage-Type.

  By default, the P-GW selection based on the UE-Usage-Type feature is disabled. If UE-Usage-Type is not enabled or not associated with the subscriber, then the Gateway selection is performed as per the existing implementation.

Use the following configuration command to configure the P-GW with a specific UE-Usage-Type (UUT).

Note	A single P-GW can serve multiple UE-Usage-Type and single UE-Usage-Type can be served by multiple P-GW.

configure 
     apn-profile apn_name 
        pgw-address ip_address ue-usage-type value 
        end 

NOTES:

• pgw-address ip_address ue-usage-type value : Configures the UE-Usage-Type for the gateway service.

  The UE-Usage-Type integer value must be 1–255. If the P-GW is configured in either primary or secondary mode, then you cannot configure more than two IPs for the P-GW regardless of the UE-Usage-Type.

This section provides information to monitor and troubleshoot this feature using show commands.

ePDG supports handling of 4G and 5G Capable User Equipment (UE) for VoWIFI functionality along with Interworking-5G subscription data provided by HSS/AAA.

With Interworking-5G enabled on ePDG, for a seamless handover with NR, customer requests ePDG to select PGW/ SMF+PGW-C based on UE-Usage-Type for both initial attach and handover scenarios. ePDG determines the type of gateway PGW/SMF+PGW-C based on the existing Interworking-5G decision matrix. For more information on decision matrix, see the Selecting P-GW or SMF+PGW-C Based on UE-Usage-Type.

When both UE-Usage-Type and Interworking-5G features are enabled and if the subscriber is allocated with UE-Usage-Type in the subscriber profile stored in HSS, ePDG determines the gateway type as PGW/SMF+PGW-C using the Interworking-5G decision matrix.

Based on the selected gateway type and UUT values (if received from AAA), ePDG either applies the appropriate DNS service parameter to select the gateway from the DNS responses received or selects the gateway serving the UUT through local configuration.

Upon enabling UE-Usage-Type feature with the Interworking-5G feature disabled, the selection of P-GW would be as per current implementation. For more information, refer P-GW Selection based on UE-Usage-Type.

Existing Interworking-5G license shall be used to enable Interworking-5G feature, no separate license would be required for UE-Usage-Type Interworking with 5G feature.

For more details, see the License Requirements section in the ePDG Interworking with SMF+P-GW-IWK Support chapter .

The gateway selection between PGW/SMF+PGW-C with the help of the AVP UE-Usage-Type happens in the following way:

• The CLI commands implemented under epdg-service for both UE-Usage-Type and Interworking-5G (licensed feature) features should be enabled for UUT Interworking-5G functionality.

• The existing CLI to configure SMF+PGW-C with the associated,UE-Usage-Type under APN profile shall be enhanced to be supported in ePDG as well. Same SMF+PGW-C IP can be configured to service more than one UE-Usage-Type. More than one SMF+PGW-C can be associated with a particular UE-Usage-Type. The valid range for UE-Usage-Type is 1-255.

  Note	Range 1-255 is as per current implementation, by MME. As per the standard, the range is 0–255, where the range 0–127 is reserved.

• If the UE-Usage-Type feature is not enabled, ePDG will not consider UE-Usage-Type, received in DEA, for the selection of PGW/SMF+PGW-C.

• Subscribers not associated with any UE-Usage-Type, would be treated as per the existing Interworking-5G implementation. Such subscribers with no UUT value from AAA, would not be assigned any default UE-Usage-Type value.

• Upon enabling both the features (Interworking-5G and UE-Usage-Type), ePDG uses the existing decision matrix to decide whether to latch on SMF+PGW-C/PGW. If the UE-Usage-Type is received from the AAA, then ePDG shall use this UE-Usage-Type and compare it against the UE-Usage-Type provided by DNS or local configuration based on the selection mechanism and select PGW or SMF+PGW-C serving the specific UE-Usage-Type.

• If the selected gateway is PGW, ePDG does not share either PDU session ID to SMF+PGW-C or S-NSSAI, PLMNID to UE, which remains the same as per the existing implementation.

• If the UE-Usage-Type is not received or if both UE-Usage-Type and Interworking-5G features are not enabled, the subscriber will be treated as per the current implementation.

• If the selected PGW/SMF+PGW-C IP is provided by the AAA server, then ePDG assumes that the provided PGW or SMF+PGW-C serves the UE-Usage-Type of the connecting UE and ePDG does not validate the same.

• Based on the decision matrix:

  • If the ePDG decides the selected gateway type as SMF+PGW-C, then DNS records with the service parameter x-3gpp-pgw:x-s2b-gtp+nc-smf+ue-<value> shall be filtered.

  • If the ePDG decides the selected gateway type as PGW then DNS records with the service parameter x-3gpp-pgw:x-s2b-gtp+ue-<value> shall be filtered.

• If PGW/SMF+PGW-C is selected, e-PDG shall use the S-NAPTR procedure in the following scenarios:

  • AAA provided FQDN-based PGW/ SMF+PGW-C selection.

  • APN-FQDN based PGW/SMF+PGW-C selection.

  • Local FQDN-based PGW/SMF+PGW-C selection.

  The service parameters used are x-3gpp-pgw:x-s2b-gtp+ue-<value> for PGW and x-3gpp-pgw:x-s2b-gtp+nc-smf+ue-<value> for SMF+PGW-C.

• ePDG will first initiate the S-NAPTR query against AAA provided FQDN or Local FQDN or APN-FQDN to DNS server to get the PGW/SMF+PGW-C IP address. DNS server shall return NAPTR Resource Records (RRs), which should be fed to a filter where only RRs having one of the UUT value matching the UUT associated with the subscriber, shall be considered by ePDG. When there are more than one matching PGWs/ SMF+PGW-Cs, priority and weight will be applied to choose the PGW/ SMF+PGW-C as per the existing implementation. The service parameters used are x-3gpp-pgw: x-s2b-gtp+ue-<value1>.<value2>… for PGW and x-3gpp-pgw: x-s2b-gtp+nc-smf+ue-<value1>. <value2>… for SMF+PGW-Cs.

• If the DNS query fails or there is no matching DNS record for the given UUT or DNS provided PGW/SMF+PGW-C is not reachable or DNS provided the SMF+PGW-C list is exhausted, ePDG shall fallback to locally configured SMF+PGW-C with matching UUT from AAA, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons. ePDG will not select DNS records without UE-Usage-Type in the service parameter, when the UE-Usage-Type feature is enabled.

• For static PGW/SMF+PGW-C selection with UE-Usage-Type, if PGW/SMF+PGW-C is not configured with the matching UUT or configured PGWs or SMF+PGW-Cs without UUT or different UUTs, ePDG ignores the other locally configured PGWs/SMF+PGW-Cs. The ePDG shall fallback to other PGW selection mechanism like DNS or AAA provided IP, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons.

• In local configuration, under APN-profile, if the PGW/SMF+PGW-C is configured in primary/secondary mode, you cannot define more than two IPs (Prim/Sec) for the PGW/SMF+PGW-C. The UE-Usage-Type configured along with primary and secondary mode shall be the same or unique and you cannot configure multiple sets of UE-Usage-Type in primary/secondary mode under the same APN profile.

If both UE-Usage-Type and Interworking-5G features are enabled and UE-Usage-Type is associated for the subscriber, ePDG would either compare it with the locally configured UE-Usage-Type or with the UE-Usage-Type value received from DNS service parameter based on the local configuration.

• If ePDG decides to choose SMF+PGW-C based on the decision matrix, then ePDG shall use the below service parameters in case if the selection mechanism is DNS.

  Service Parameters: x-3gpp-pgw:x-s2b-gtp+nc-smf+ue-<ue usage type>

  Or

  Service Parameters: x-3gpp-pgw:x-s2b-gtp+ue-<ue usage type>+nc-smf

  Or

  Service Parameters: x-3gpp-pgw:x-s2b-gtp+ue-<ue usage type1>.<ue usage type2>..+nc-smf

• If ePDG decides to choose PGW based on the decision matrix, then ePDG shall use the below service parameters in case if the selection mechanism is DNS.

  Service Parameters: x-3gpp-pgw:x-s2b-gtp+ue-<ue usage type>

  Or

  Service Parameters: x-3gpp-pgw:x-s2b-gtp+ue-<ue usage type1>.<ue usage type2>

• NOTE 1:

  Default Behavior: SMF+PGW-C supports Rel-16 functionality to support 4G-only UEs, that is, the SMF+PGW-C shall generate PDU Session ID for 4G-only UEs.

  Custom Behavior: To handle the case where SMF+PGW-C is Rel-15 and cannot support 4G only UEs.

• NOTE 2:

  Default Behavior: When Interworking-5GS APN-Configuration is set to not allowed, the APN configuration is in UDR, but handover to 5G SA is not allowed.

  Custom Behavior: When Interworking-5GS APN-Configuration is set to not allowed, the APN configuration is in SPR and not in UDR, hence P-GW needs to be selected.

• NOTE 3: Whenever SMF+PGW-C IP or FQDN is not preferably upgraded in the DNS server or in the local ePDG config, in such a case if the CLI (smf-not-configured ) is configured, then ePDG ignores the SMF+PGW-C selection and always choose P-GW in all the scenario.

Note

The service parameters from DNS are updated considering that UE-Usage-Type is enabled and received from AAA. If UE-Usage-Type is either not enabled or not received from AAA, then the service parameters would remain the same as the existing implementation, that is, x-s2b-gtp in case of PGW and x-s2b-gtp+nc-smf in case of SMF+PGW-C.

In case of dynamic selection mechanism, ePDG would first initiate the S-NAPTR query against AAA provided FQDN or Local FQDN or APN-FQDN to DNS server, to get the PGW/SMF+PGW-C IP address based on UE-Usage-Type. DNS server shall return NAPTR RRs (Resource Records) and these RRs shall be fed to a filter where only RRs with the service-parameter x-3gpp-pgw: x-s2b-gtp+ue-<value> or x-3gpp-pgw: x-s2b-gtp+nc-smf+ue-<value> shall be considered by ePDG. The PGW or SMF+PGW-C IP provided by the DNS server shall be considered only when the ue-usage-type matches with the UE-Usage-Type AVP received from AAA/CPAR in the DEA message.

In case of local/static SMF+PGW-C selection mechanism, the UE-Usage-Type configured under apn-profile should match with the UE-Usage-Type AVP from AAA/CPAR in DEA message.

Below are the sample configurations:

pgw-address 1.1.1.1 ue-usage-type 130 smf-combined

pgw-address 1.1.1.1 ue-usage-type 130

With both Interworking-5G and UE-Usage-Type features enabled, based on the decision matrix if the selected gateway is SMF+PGW-C, ePDG uses the existing fallback mechanisms, priority, and preference order of selection and the same is applicable to UUT associated SMF+PGW-Cs as well. The existing fallback selection order is as follows:

1. error-terminate configured (pgw-selection agent-info error-terminate)

   1. PDN-GW-Allocation-Type from AAA: Dynamic

      • Enabled pgw-selection prefer aaa-pgw-id

        AAA->DNS->Local

      • Enabled pgw-selection local-configuration-preferred and pgw-selection prefer aaa-pgw-id

        AAA->Local->DNS

      • Enabled pgw-selection local-configuration-preferred

        Local->DNS

      • Disabled both pgw-selection local-configuration-preferred and pgw-selection prefer aaa-pgw-id

        DNS->Local

   2. PDN-GW-Allocation-Type from AAA: Static or Absent

      • Enabled pgw-selection prefer aaa-pgw-id

        AAA->DNS->Local

      • Enabled pgw-selection local-configuration-preferred and pgw-selection prefer aaa-pgw-id

        AAA->Local->DNS

      • Enabled pgw-selection local-configuration-preferred

        Local->AAA

      • Disabled both pgw-selection local-configuration-preferred and pgw-selection prefer aaa-pgw-id

        AAA->Local

2. error-terminate unconfigured (no pgw-selection agent-info error-terminate): If error-terminate is unconfigured, all the aforementioned cases, 1.a. and 1.b., remain the same except the below mentioned cases:

   1. PDN-GW-Allocation-Type from AAA: Static or Absent

      • Enabled pgw-selection local-configuration-preferred

        Local->AAA->DNS

      • Disabled both pgw-selection local-configuration-preferred and pgw-selection prefer aaa-pgw-id

        AAA->Local->DNS

DNS Selection

From the DNS responses, ePDG selects SMF+PGW-C with UUT based on the service parameter received from DNS server, i.e., the records with the service parameter x-s2b-gtp+nc-smf+ue-<value1>. <value2>. <value3>….., where one of the UUT value in the service parameter from the received DNS records should match with the UE-Usage-Type value received from AAA for the subscriber. That specific SMF+PGW-C IP with UUT given by DNS shall be selected.

• If DNS response has records for the given UUT with SMF+PGW-C capability, which matches with the UUT received from AAA, the corresponding SMF+PGW-C will be selected. If the selected SMF+PGW-Cs IPs are not reachable, the ePDG shall fallback to the alternate SMF+PGW-C address with matching UUT from the DNS records if available. Else, it shall fallback to the locally configured SMF+PGW-C with matching UUT from AAA based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons.

• If the DNS response has no matching UUT records, but has PGW records with matching UUT or both PGW and SMF+PGW-C records without UUT, in this case, ePDG shall ignore the PGW and SMF+PGW-C records and falls back to locally configured SMF+PGW-C with matching UUT from AAA, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons and the corresponding DNS-related failure statistics would be updated.

• If the DNS query fails or no records received from DNS or DNS is not reachable, ePDG shall fallback to locally configured SMF+PGW-C with matching UUT from AAA, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons and the corresponding DNS-related failure statistics would be updated.

Local Static Selection

If SMF+PGW-Cs configured with UUT matches with the UUT received from AAA, then the selection of GW IP shall be considered based on:

• If Weight is Defined: Existing Weight algorithms for PGW selection would be applied for UUT based SMF+PGW-C selection as well.

• If No Weights Configured: UUT based SMF+PGW-C shall be selected in a round-robin method.

• If SMF+PGW-C is not configured with the matching UUT from AAA or configured SMF+PGW-Cs without UUT or different UUTs: ePDG ignores the other locally configured SMF+PGW-C. ePDG shall fallback to other PGW selection mechanism like DNS or AAA provided IP, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons.

• If the selected SMF+PGW-Cs with UUT is not reachable, then ePDG shall fallback to alternate locally configured SMF+PGW-C IP with matching UUT if available, else ePDG shall fallback to other PGW selection mechanism like DNS or AAA provided IP, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons.

• If no local entries defined: ePDG shall fallback to other PGW selection mechanism like DNS or AAA provided IP, based on the fallback selection order if applicable, else the call would be dropped with appropriate disconnect reasons.

• In both primary and secondary mode configuration under local apn-profile, if the SMF+PGW-C configured with UUT does not matches with the UUT received from AAA, or there is no SMF+PGW-C or SMF+PGW-C configured without UUT, then ePDG shall fallback to the AAA provided IP if available, when the PDN-GW-Allocation-Type received from AAA is static, else the call would be dropped with appropriate disconnect reasons.

Note	All the above mentioned behaviour under DNS selection and local static selection are applicable for PGW selection based on UUT as well. In a handover scenario, ePDG shall consider AAA provided PGW ID (IP address/FQDN) for PGW/SMF+PGW-C selection as per the existing implementation.

Configuring ePDG to select SMF+PGW-C based on UE-Usage-Type involves enabling Interworking-5G and UE-Usage-Type features.

To enable Interworking-5G, see the topic Configuring ePDG to Enable 5G Interworking.

To enable UE-Usage-Type, see the topic Configuring ePDG to Select P-GW Based on UE-Usage-Type .

Use the following configuration command to configure the SMF+PGW-C selection with a specific UE-Usage-Type:

configure 
     apn-profile apn_name 
        pgw-address ip_address ue-usage-type value smf-combined 
        end 

NOTES:

• smf-combined —Specifies if the PGW and SMF+PGW-C are combined.

This section provides information to monitor and troubleshoot this feature using show commands.