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Enabling the DHCP in the MPLS L3VPN PE side causes the 2G BTS can’t build connection with the BSC after it has reset

Publication Date:  2019-07-24 Views:  136 Downloads:  0

Issue Description

In a project of full network relocation of the T operator in the S country, the 2G, 3G base transceiver stations have accessed through the microwave RTN 900, and aggregating to the S93 switch of the cluster, terminate the QinQ via the VLAN sub-interface, and send to the BSC/RNC of the core node connected with the master/standby switch. After the wireless 2G base transceiver stations have upgraded, there are multiple sites is resetting repeatedly, the former two upgrading haven’t this kind of problem. The base transceiver stations which are resetting repeatedly distribute in different S93 switches and the transmission devices composed by the microwave RTN900, the 2G BTS can ping through the BSC after it has completed resetting, but can’t build service with the BSC normally, it will enter in to the resetting status in 6 minutes later.

Alarm Information

The BTS is resetting repeatedly, there aren’t alarm in the S93 switch and RTN.

Handling Process

1. Analyze the distribution of the resetting BTS, the false base transceiver stations are distributed in different switches and microwave devices; check the configuration of the aggregating switches, we haven’t found the DHCP configuration information.
2. Mirroring image and getting packet in the microwave RTN connecting with the 2G BTS directly, we find there are DHCP packets sending from the BTS normally.
3. Implement the flow classing statistic analysis on the inbound port of the BTS aggregating switch peer, we find there are DHCP packets sending from the BTS.
4. From the outbound to the MPLS Tunnel, we can’t validate whether the DHCP packets have been sent via the outbound flow classing statistic.
5. Query the configuration and operation logs, we find the client has enabled the DHCP in the core peer switch.
6. Delete the DHCP configuration information, the services of the BTS have revived fully.

Root Cause

The 2G BTS initiates statically and resets are both need using the DHCP and BSC to create connection and download the configuration. Due to the clients don’t know the work theory of our company’s 2G BTS, they enable the DHCP in the core peer switch to test the fixed network, so the DHCP packet produced by the following resetting BTS interacting with the BSC can’t send to the BSC normally (the switch between the BTS and the BSC has deployed the MPLS L3VPN), finally causes the BTS is resetting continually.
In the application scene of the S9300V1R3 version MPLS L3VPN, if enabling the DHCP replay in the DHCP server peer switch, because the hardware chip confines the time, the DHCP packet will be discarded after quitting of the Tunnel; however, enabling the DHCP Relay in the DHCP client peer’s S9300, the DHCP packet will not be discarded.