Case Study: Layer 3 Roaming Fails Because a STA Fails to Obtain an IP Address

Symptom

As shown in Figure 6-5, an S5731 functions as an AC to manage APs. Service data is forwarded in direct mode, and different service VLANs are configured for different buildings (for example, A1 and A2). When a STA moves from building A1 to building A2, the STA cannot obtain an IP address and fails to roam.

Figure 6-5 Layer 3 roaming diagram

Relevant Alarms and Logs

None

Cause Analysis

Switches in the buildings fail to transparently transmit packets in VLANs correctly. When a switch functions as an AC and forwards service data in direct mode, the upper-layer network of the FAP needs to transparently transmit the packets in the service VLAN of the HAP.

Procedure

1. Check the roaming process of the STA using the station-trace function.

   [AC-diagnose] station-trace sta-mac sta-mac

   The key information is as follows:

   • The roaming record is displayed.

     A1_AP1 WSRV/7/BTRACE:(BTRACE)(WLAN_AP)(xxxx-xxxx-xxxx):station disassoc process:
     ap id      :0
     radio id   :0
     wlan Id    :0
     offReason  :The STA roams out of the AP (0x2000004)  //The STA roamed out from A1_AP1.
     A2_AP2 WSRV/7/BTRACE:(BTRACE)(WLAN_AP)(xxxx-xxxx-xxxx):Association request new proc:
     Ap id      :0
     Radio id   :1
     Assoc type :reassociation   //The STA reassociated with A2_AP2.
     A2_AP2 WSRV/7/BTRACE:(BTRACE)(WLAN_AP)(xxxx-xxxx-xxxx):reassociation response from ac:
     radio id    :1
     wlan id     :1
     status code :STA roam[65534]
     A2_AP2 WSRV/7/BTRACE:(BTRACE)(WLAN_AP)(xxxx-xxxx-xxxx):4-way handshake switch = 0
     quick roam between ap falg = 0
     Force hand shake = 0
     A2_AP2 WSRV/7/BTRACE:(BTRACE)(WLAN_AP)(xxxx-xxxx-xxxx):AC set the user vlan:
     radio id :1
     vlan     :331

   • After roaming to A2_AP2, the STA keeps sending ARP and DHCP requests but does not receive any response.

     A2_AP2 WIFI/7/BTRACE:[BTRACE][WLAN_WIFI][xxxx-xxxx-xxxx]:SeqNo[539] [ARP] ARP request : who has 10.10.10.1 ? tell 10.10.10.218 elapsed[0 ms] Success to send pkt to software switch
     A2_AP2 WIFI/7/BTRACE:[BTRACE][WLAN_WIFI][xxxx-xxxx-xxxx]:SeqNo[540]  [DHCP] DHCP request elapsed[0 ms] Success to send pkt to software switch

2. According to the preceding analysis, after the STA roams to A2_AP2, the upper-layer network of A2_AP2 does not allow packets from the corresponding service VLAN to pass through. As a result, the STA cannot obtain an IP address after roaming and fails to roam. The problem is resolved after the switch and gateway connected to the FAP (A2_AP2) allow packets from the service VLAN of the HAP to pass through.