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Hi, all:

While resolving ovs restart rebuild br-tun flows[1], we have found
several l2pop problems:

1. L2pop is depending on agent_boot_time to decide whether send all
port information or not, but the agent_boot_time is unreliable, for
example if the service receives port up message before agent status
report, the agent won't receive any port on other agents forever.

2. If the openvswitch restarted, all flows will be lost, including all
l2pop flows, the agent is unable to fetch or recreate the l2pop flows.

To resolve the problems, I'm suggesting some changes:

1. Because the agent_boot_time is unreliable, the service can't decide
whether to send flooding entry or not. But the agent can build up the
flooding entries from unicast entries, it has already been
implemented[2]

2. Create a rpc from agent to service which fetch all fdb entries, the
agent calls the rpc in `provision_local_vlan`, before setting up any
port.[3]

After these changes, the l2pop service part becomes simpler and more
robust, mainly 2 function: first, returns all fdb entries at once when
requested; second, broadcast fdb single entry when a port is up/down.

[1] https://bugs.launchpad.net/neutron/+bug/1332450
[2] https://review.openstack.org/#/c/101581/
[3] https://review.openstack.org/#/c/107409/