OpenStack Neutron Network Notes

1. When a network and its subnet get created with DHCP enabled (Linuxbridge used as example):

    A Linux name space will be created, the namespace should start with qdhcp-<network-id>, the namespace should have a tap device, the name starts with ns-<tap device name>, that device will bear an IP address of x.x.x.2 (by default), it is where the dhcp server will run on.  The other end of the tap device will be in the default name space, the name simply starts as tap<tap device name>

    A Linux bridge will be created. The bridge will be named brq< first 11 characters of the network id >. The bridge will have the tap<tap device name> for the dhcp server and the vxlan device which connects this bridge with bridges on other compute node when VM gets created on the network.

2. When a router gets created without external gateway or connect to a tenant network, there is nothing happening other than a record in neutron database. No actual network construct gets created.

3. When a router gets external gateway set:

    A Linux name space will be created, the namespace name will be qroute-<router-id>.
    A tap device will be created in that namespace, the tap device name starts qg-<first 11 characters of the port id>, this is the port that bears floating IP address (so here we consume one floating IP), qg means quantum gateway. The other end of this tap device will be in the default namespace and the name is tap<first 11 characters of the port id>, this device bears no IP address since it is hooked up in the bridge which take all the traffic to the public network which gets created for the public (provider) network.

4. When a router gets hooked up with a tenant network (add interface from a tenant sub network to router):

     A pair of tap device will be created. One end will be placed in the qrouter name space created in step 3. The name of that tap device will be qr-< first 11 characters of the port id >, qr means quantum router. This device will normally take the  first IP of that subnet which mostly is .1 IP address. The other end of the tap device is named as tap< first 11 characters of the port id >, this tap device will be in default name space and placed in the bridge which represents the network created in step 1.

Use the following command to show tap device pairs:

ip -d link show