Cisco Cisco Packet Data Gateway (PDG)
Evolved Packet Data Gateway Overview
▀ Features and Functionality
▄ ePDG Administration Guide, StarOS Release 17
26
Dynamic Selection
For a given APN, when the HSS returns Dynamic Allocation Allowed for the P-GW ID and the selection is not for a
3GPP-to-non-3GPP handover, the ePDG ignores the P-GW ID and instead performs dynamic selection.
3GPP-to-non-3GPP handover, the ePDG ignores the P-GW ID and instead performs dynamic selection.
The figure below shows the message exchange for dynamic selection. The table that follows the figure describes each
step in the flow.
step in the flow.
Figure 4.
P-GW Dynamic Selection
Table 6. P-GW Dynamic Selection
Step
Description
1.
The WLAN UE sends the APN name to the ePDG.
2.
The ePDG constructs the APN FQDN from the received APN name. The ePDG uses this query string to send an S-NAPTR
(Server-Name Authority Pointer) query to the DNS proxy.
(Server-Name Authority Pointer) query to the DNS proxy.
3.
The DNS proxy sends the S-NAPTR query to the DNS.
4.
The DNS may return multiple NAPTR resource records with an ‘S’ flag (for SRV records) with the same or different
service parameters.
service parameters.
5.
The DNS proxy forwards the NAPTR resource records to the ePDG.