What is the use of SGsAP-UE-UNREACHABLE message and what action can be taken once it is received ?

Question

In case of SGW relocation, MME sends CSRequest to new SGW, now question how does a SGW come to know that CS Request is received as part of attach procedure or handover procedure ?

Answer 1

SGsAP-UE-UNREACHABLE message is used between VLR and MME. For a "MT CS call", VLR sends SGs-AP-PAGING-REQUEST message to MME and MME responds with SGs-UE-UNREACHABLE message to VLR when UE is marked as "unreachable" at MME.
VLR runs a timer Ts5 to guard this paging procedure. Once VLR receives this message, it stops timer and paging procedure for that paging request towards the MME is stopped but at the same time and there is no change in state of SGs-association for that UE in VLR.

Answer 2

On behalf of Bart:

To signal that SGs Application is not reachable.

When it's down then it's very good to check why, unless you don't care if subscribers can be located for use of CS-services.

What I would do is I would check the protocol stack and stars debugging from the top to bottom. There might be wrong config on SGsAP, or problem on SCTP lvl (are cookies exchanged?), then IP, later VLANs or.. Physical connection is down. Maybe even VXLANs if used - layer 2 over layer 3. Basically same procedure as with any other case.

In general problem is in 3 places. MME, connection between or on MSC/VLR pair end.