MA5600T V800R006C02 and connecting a router to ONT connected to OLT,
when this router is trying to establish BGP session with another router at uplink, the session cannot go up, when connecting the 2 routers back to back, the session can be established, so not a BGP configuration problem.
Although BGP packets are unicast, however MA5600T considers them as control packets which are not forwarded directly, but directed to CPU which will not handle them.
If the attribute of the vlan carrying packets is changed to QinQ, BGP packets will not be captured by CPU and will be transmitted transparently.
After using below command problem is solved.
huawei(config)#vlan attrib vlan-list q-in-q
For multicast & Control packets, Access devices don't forward them transparently , so make sure when using any of these packets what special configuration you need to pass them transparently through Access device.