Contents

Introduction

Prerequisites

Requirements

Components Used

Problem

Analysis Performed

Solution

Introduction

This document describes the collision observed between Modify Bearer Request (MBReq) and Create Bearer Request (CBReq) in the SGW during N26 handover.

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

- 5G SA

- LTE

- CUPS

Components Used

The information in this document is based on these software and hardware versions:

- Cisco StarOS® 21.28.m5

- Ultra Packet Core CUPS

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Problem

During an N26 inter-RAT 5G SA -> 4G LTE handover for an EPS-Fallback VoLTE voice call, the QCI1 Dedicated Bearer creation and the Access Modify Bearer Request procedures occur independently of each other and can collide. When this collision occurs, the SGW fails to correctly set up the QCI5 ims bearer leading to voice calls and IMS traffic failing on LTE until the UE returns to 5G SA.

Analysis Performed

1. Handover from 5G to 4G completes successfully (TAU Request/Authentication/Encryption/Create Session Requests to move both bearers):

Frame 76        S1AP/NAS-EPS                                         InitialUEMessage, Tracking area update request

Frame 98        GTPv2                 EBI= 5,5  EUTRAN   QCI= 5      Create Session Request

Frame 99        GTPv2                 EBI= 6,6  EUTRAN   QCI= 8      Create Session Request

Frame 109       S1AP/NAS-EPS                                         InitialContextSetupRequest, Tracking area update accept

2. Then, there is the Initial-Context-Setup Response Success and TAU Complete from the eNB with the eNB S1-U IPs. At this point, the MME has not provided these to the SGW and the SGW does not know where to send down-link UP packets:

Frame 111        S1AP                 InitialContextSetupResponse

Frame 113        S1AP/NAS-EPS         UplinkNASTransport, Tracking area update complete

3. MME sends a Modify Bearer Request on all EBIs (5 & 6) to provide the eNB S1-U IP to the SGW (frames 114 and 115). However, at the same time, the SGW gets a Create Bearer Request on EBI= 5 (QCI-5) to create the QCI-1 bearer (frame 117). When this collision happens, the Modify Bearer Request on EBI= 5 does not get processed; therefore, the SGW never learns the eNB S1-U IP on the ims default bearer so down link packets cannot be delivered:

Frame 114        GTPv2                 EBI= 6        EUTRAN                    Modify Bearer Request

Frame 115        GTPv2                 EBI= 5        EUTRAN                    Modify Bearer Request

Frame 116        GTPv2                 EBI= 6                                  Modify Bearer Response

Frame 117        GTPv2                 EBI= 5,0                  QCI= 1        Create Bearer Request

Frame 118        S1AP                                                          UECapabilityInfoIndication, UECapabilityInformation

Frame 121        GTPv2                 EBI= 5                                  Downlink Data Notification

Frame 122        GTPv2                                                         Downlink Data Notification Acknowledgement

Frame 129        GTPv2                 EBI= 5        EUTRAN                    Modify Bearer Request

Frame 130        GTPv2                 EBI= 5                                  Modify Bearer Response

Frame 131        S1AP/NAS-EPS                                                  E-RABSetupRequest, Activate dedicated EPS bearer context request

Frame 132        S1AP                                                          E-RABSetupResponse

Based on 3GPP specification 29.274, SGW must be handling Bearer Context mismatch (for example, the collision between CBReq and MBReq):

14.3 Handling of Bearer Context Mismatch

14.3.2 Exceptional scenarios

During a dedicated bearer creation procedure, temporary Bearer Context mismatch may occur at the SGW, (for example) due to the collision between Create Bearer Request and Modify (Access) Bearer Request messages.

[...]

The SGW should handle such Bearer Context mismatch in an implementation specific way, but in such a way to accept the Modify (Access) Bearer Request message and to not locally delete the missing Bearer Contex.

Solution

This issue is caused by Cisco bug ID CSCwf63318. To handle this collision at SGW as per standards, the Modify Bearer Request takes a higher precedence over the Create Bearer Request. SGW aborts the Create Bearer Request temporarily, then proceeds with the Modify Bearer Request and forwards it to PGW.