Load balancing on the MME permits UEs that are entering into an MME pool area to be directed to an appropriate MME in a more efficient manner, spreading the load across a number of MMEs.

The MME load rebalancing functionality permits UEs that are registered on an MME (within an MME pool area) to be moved to another MME in the pool. The rebalancing is triggered using an exec command on the mme-service from which UEs should be offloaded.

When initiated, the MME begins to offload a cross-section of its subscribers with minimal impact on the network and users. The MME avoids offloading only low activity users, and it offloads the UEs gradually (configurable from 1-1000 minutes). The load rebalancing can off-load part of or all the subscribers.

The eNodeBs may have their load balancing parameters adjusted beforehand (e.g., the weight factor is set to zero if all subscribers are to be removed from the MME, which will route new entrants to the pool area into other MMEs).

Offloading of a certain percentage of users can be configured using the mme offload command. The MME sends S1 Release (with cause "load balancing TAU required" for offload) to the configured percentage of UEs attached to the MME. The MME does not distinguish between VoLTE and Non-VoLTE subscribers. Some subscribers with voice bearers are also offloaded as a result calls are dropped. This feature enhancement is targeted to preserve VoLTE voice bearers during MME offloading. A new CLI keyword is added to the mme offload command to preserve VoLTE subscribers (QCI = 1) from offloading until voice calls are terminated.

Note	This feature enhancement is license controlled. Contact your Cisco Account or Support representative for information on how to obtain a license.

MME load balancing can be used in conjunction with congestion control. For more information on congestion control, refer to the Congestion Control section in the Mobility Management Entity Overview chapter of the MME Administration Guide.

Load balancing is achieved by setting a weight factor for each MME so that the probability of the eNodeB selecting an MME is proportional to its weight factor. The weight factor is set by the operator according to the capacity of an MME node relative to other MME nodes. The relative-capacity mme-service level command is used to specify this relative weighting factor.

Once set, the Relative MME Capacity IE is included in the S1AP S1 SETUP RESPONSE message from MME to relay this weight factor. If the relative MME capacity is changed after the S1 interface is already initialized, then the MME CONFIGURATION UPDATE message is used to update this information to the eNodeB.

Call Handling and Other Messaging Considerations

The MME uses the mme offload mme-service exec level command to enable the operator to offload UEs for a particular mme-service for load rebalancing among MMEs in a MME pool area. The command enables the operator to specify a percentage of UEs to offload, and the desired time duration in which to complete the offload.

The operator can also include the keyword option disable-implicit-detach . By default, if the UE context is not transferred to another MME within 5 minutes, the UE will be implicitly detached. This option disables this implicit detach timer.

To offload ECM-CONNECTED mode UEs, the MME initiates the S1 Release procedure with release cause "load balancing TAU required".

To offload UEs which perform TA Updates or Attaches initiated in ECM-IDLE mode, the MME completes that procedure and the procedure ends with the MME releasing S1 with release cause "load balancing TAU required".

To offload UEs in ECM-IDLE state without waiting for the UE to perform a TAU or perform Service request and become ECM CONNECTED, the MME first pages the UE to bring it to ECM-CONNECTED state.

New calls are processed normally (as per the new call policy configuration). The offloading process does not reject INIT UE messages for new subscribers. To prevent new calls from entering this MME, set the relative-capacity on this mme-service to 0.

When Init UE messages are received for an existing offloaded subscriber, the ue-offloading state is set as MARKED and the offload procedure continues until the UE is offloaded.

Once a UE is offloaded, messages such as EGTP events, Create bearer, Update bearer, Idle mode exit, and Paging trigger are be rejected. HSS initiated events also will be rejected for offloaded UEs.

Detach events are processed as usual.

Important

Emergency attached UEs in Connected or Idle mode are not considered for offloading.

The mme offload command is enhanced with the keyword preserve-volte-subscribers , this keyword enables preservation of subscribers with voice bearers (QCI=1) from offloading until the voice bearers are deactivated. In any MME service both VoLTE and Non-VoLTE subscribers are present. The offload command now has options to configure the percentage of total subscribers to be offloaded and to preserve VoLTE subscribers from offloading until voice calls are terminated. With this feature enhancement if VoLTE preservation is not enabled, all subscribers are marked for offloading. But when the keyword preserve-volte-subscribers is enabled, Non-VoLTE subscribers are first marked for offloading based on configured offload-percentage. If the configured offload-percentage is greater than the available Non-VoLTE subscribers, VoLTE subscribers are also marked for offloading but the VoLTE UEs will be offloaded only when voice bearers are deactivated.

Set the relative capacity of an MME service to enable load balancing across a group of MME services within an MME pool.

Use the following example to set the relative capacity of this MME service. The higher the value, the more likely the corresponding MME is to be selected.

config 
   context context_name 
      mme-service  mme_svc  -noconfirm 
         relative-capacity rel_cap_value 
         exit 

Notes:

Enter the show mme-service all causes the MME to generate a display similar to the following to indicate the configured relative capacity:

show mme-service all 
Relative Capacity:      50 

Start Offloading

The following example command rebalances (offloads) 30 percent of all UEs from the specified MME service (to other MME services in the MME pool) over the course of 10 minutes.

mme offload mme-service mme_svc time-duration 10 offload-percentage 30 -noconfirm 

This command can also be entered with the disable-implicit-detach option. By default, if the UE context is not transferred to another MME within 5 minutes, the UE will be implicitly detached. This option disables this implicit detach timer.

mme offload mme-service mme_svc time-duration 10 offload-percentage 30 disable-implicit-detach -noconfirm 

Stop Offloading

To stop the offloading process, issue the command with the stop keyword option.

mme offload mme-service mme_svc stop -noconfirm  

The following command shows the offload configuration as well as the status of the rebalancing.

show mme-service name svc_name offload statistics   
show mme-service name mme1 offload statistics 
Current Offload Status: In Progress 
Implicit Detach Status: Enabled 
Time Duration Requested: 600 secs 
Percentage of Subscribers Requested: 30 
Total Number of Subscribers: 0 
Total Number of Subscribers to be Offloaded: 0 
Total Number of Subscribers Offloaded: 0 
Total Number of Subscribers Received Context Transfer: 0 
Remaining Time:  0 secs 

These counters are reset each time an offload procedure is initiated, or when the following command is entered:

clear mme-service statistics offload 

The following configuration command is used to configure preservation of VoLTE subscribers from offloading during active calls (QCI=1); the offload command is enhanced with the key wordpreserve-volte-subscribers :

mme offload mme-service mme_svc_name { time-duration minutes offload-percentage percent [ disable-implicit-detach | preserve-volte-subscribers  ] ] | stop } [- noconfirm ] 

By default, the subscribers with voice bearer with QCI = 1 will not be preserved during MME offloading. Configuring the keyword preserve-volte-subscribers enables preservation of subscribers with voice bearer.

The following example command re-balances(offloads) 30 percent of Non-VoLTE subscribers from the specified mme-service (to other mme-services in the MME pool) over the course of 30 minutes with VoLTE preservation.

mme offload mme-service mmesvc  time-duration 30 offload-percentage 30 preserve-volte-subscribers 

The following show command display is used to verify if VoLTE preservation is enabled and the number of VoLTE subscribers preserved during offloading:

show mme-service name svc_name offload statistics 
 Current Offload Status : Completed 
 Implicit Detach Status : Disabled 
 Preserve VoLTE subscribers Status : Enabled  
 Time Duration Requested : 60 secs 
 Percentage of Subscribers Requested : 1 
 Total Number of Subscribers : 0 
 Total Number of Subscribers Marked for Offloading: 1 
 Total Number of Subscribers Offloaded : 0 
 Total Cumulative Number of Subscribers Offloaded: 2 
 Total Number of VoLTE Subscribers Preserved : 0  
 Total Cumulative Number of VoLTE Subscribers Preserved:7 
 Total Number of Subscribers Received Context Transfer: 0 
 Remaining Time : 0 secs 

This section provides information regarding show commands and their outputs in support of load rebalancing (UE offload).

The following show command displays current statistics for the Load Rebalancing feature.

show mme-service name mme_svc offload statistics 

The following command also provides information relating to load balancing:

show mme-service session full all 

Only the output field which relates to load rebalancing is shown.