With Releases 18.6, 19.4, 20.0 and higher, the operator can configure the MME to enable limiting the number of SGWs tried when the MME is attempting to find an available SGW during Attach or Handover procedures. If the feature-specific sgw-retry-max command is configured, as described in the Configuring a Limit to the Number of SGWs Tried section (see below), then:
the MME’s default retry behavior is ignored, and
the MME limits the retries with different SGWs from the DNS pool to only retry a maximum of the configured number of times.
If this feature is not enabled or is disabled, the MME uses or falls back to the default behavior which is in compliance with 3GPP TS 29.274, Section 7.6. The MME sends Create-Session-Request message to one SGW in the pool. If the SGW node is not available, the MME picks the next SGW from the pool and again sends a Create-Session-Request message. The MME repeats this process. For an Attach procedure, the MME tries up to five (1 + 4 retries) different SGWs from the pool. In the case of a HO procedure, the MME will try every SGW in the entire pool of SGWs sent by the DNS. If there are no further SGW nodes available in the DNS pool or if the guard timer expires, then MME stops trying and sends a Reject with cause "Network-Failure" towards the UE and the UE must restart the Attach/Handover procedure.
For statically configured SGW nodes (i.e. when no DNS is used) SGW selection takes place only once. If Create-Session-Request times out or SGW responds with a reject Cause value in Create-Session-Response, no subsequent SGW selection is attempted by MME.
The amount of signaling at Attach or Handover can be reduced.
The amount of time to find an available SGW can be reduced.