I would like to know how MME select for the following scenario.
Two different P-GW,
Two different S-GW, one is standalone, another is collocated for P-GW for APN-2.
1. S-GW and P-GW, which is not collocated, are already selected for APN-1.
The same UE tries to set up a different PDN connectivity using different APN-2.
For PDN with APN-2, collocated S/P-GW are responded from DNS.
I saw in TS 29.303 that collocation takes a precedence over other criteria like topology and order.
Can MME select different S-GW for APN-2 instead of the current S-GW for APN-1 in this case based on collocation ?
2. if MME MUST select the currently selected S-GW for APN-2,
what is the reason for it ?
I think you would select the SGW based on the TA, so irrespective of the APN, you would go to the same SGW, and then you decide whether the PGW selection is based on colocation or topological closeness or NAPTR priority. So in the cases below, you would still be served by the same SGW (this is also desirable to prevent tromboning of traffic across the network)
A UE is connected to only one S-GW at a time, regardless of the number of APNs and P-GWs in use. If a bearer to a new APN is needed, the MME will determine which P-GWs support that APN; if the UE already has a bearer to one of those P-GWs, it will use that P-GW to set up the new bearer; otherwise it will pick another P-GW, preferably one close to the S-GW in use.