Typical Neutron architecture

Compute host: instance networking (A,B,C)

A: virtual NIC on the VM
B: the tap device on the VM
qbr is the linux bridge device to connect VM's NIC with br-int (workaround to support openstack security group)
C: attach the firewall bridge to the integration-bridge (br-int)

Compute host: integration bridge (D,E): 

The integration bridge, br-int, performs VLAN tagging and un-tagging for traffic coming from and to your instances
D: attach the firewall bridge to the integration-bridge (br-int)
E: attach the integration-bridge to the bridge-tunnel

Compute host: tunnel bridge (F,G)  Network host: tunnel bridge (H,I)

The tunnel bridge translates VLAN-tagged traffic from the integration bridge into GRE tunnels.

Network host: integration bridge

The integration bridge on the network controller serves to connect instances to network services, such as routers and DHCP servers. It is like the network switch.

Network host: DHCP server (O,P):

DHCP is used to allocate/manage the IP address.

Network host: Router (M,N):

works as a router to the external network. A Neutron router is a network namespace with a set of routing tables and iptables rules that performs the routing between subnets.