why MME will query for PGW prior to SGW

Question

Hi Friends,
I saw somewhere that MME will also do a DNS query for P-GW. As i know S-Gw will do a query for P-Gw address. Is there any situation that MME will do a query for P-GW address using APN? and after this query how MME will use the P-Gw Address.

and is there any possibility of Doing DNS query by P-Gw to get any other nodes Ips and under what conditions P-Gw will do a DNS query?

Answer 1

MME only knows for which APN, UE is trying to connect. MME will get the PDN information(based on APN - PDN Address/ PDN host name) from HSS in diameter ULA(Update Location Answer) message. if MME receives PDN host name, then MME will resolve this host name from DNS server and will get PDN IP address. this PDN IP adress will share with SGW in create session request to which PGW, SGW will contact for this UE. Based on tracking Area(TA) SGW will be selected by MME. So, if TA changed, then SGW may change for this UE. but for each APN there will be fixed set of PDN's(IPv4, IPv6, IPv4v6) mapped at HSS.

Answer 2

Selection of the PGW is done by the MME based on the APN not by the SGW, process is something like this -

1. During UE attach the MME query the DNS Server to select the PGW where a requested (subscribed) PDN connectivity (APN) is located. Selection can be based on the information provided to the MME, when the UE attaches to the network.
2. Following the PGW selection, the MME query the DNS Server to select an available SGW to serve the UE using the TAC, which in most cases is based on network topology and the location of the UE within the network, so that the best SGW is selected.