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 Open vSwitch

This section describes how the Open vSwitch plug-in implements the Networking abstractions.

 Configuration

This example uses VLAN segmentation on the switches to isolate tenant networks. This configuration labels the physical network associated with the public network as physnet1, and the physical network associated with the data network as physnet2, which leads to the following configuration options in ovs_neutron_plugin.ini:

[ovs]
tenant_network_type = vlan
network_vlan_ranges = physnet1,physnet2:100:110
integration_bridge = br-int
bridge_mappings = physnet2:br-eth1
 Scenario 1: one tenant, two networks, one router

The first scenario has two private networks (net01, and net02), each with one subnet (net01_subnet01: 192.168.101.0/24, net02_subnet01, 192.168.102.0/24). Both private networks are attached to a router that connects them to the public network (10.64.201.0/24).

Under the service tenant, create the shared router, define the public network, and set it as the default gateway of the router

$ tenant=$(keystone tenant-list | awk '/service/ {print $2}')
$ neutron router-create router01
$ neutron net-create --tenant-id $tenant public01 \
          --provider:network_type flat \
          --provider:physical_network physnet1 \
          --router:external True
$ neutron subnet-create --tenant-id $tenant --name public01_subnet01 \
          --gateway 10.64.201.254 public01 10.64.201.0/24 --disable-dhcp
$ neutron router-gateway-set router01 public01

Under the demo user tenant, create the private network net01 and corresponding subnet, and connect it to the router01 router. Configure it to use VLAN ID 101 on the physical switch.

$ tenant=$(keystone tenant-list|awk '/demo/ {print $2}'
$ neutron net-create --tenant-id $tenant net01 \
          --provider:network_type vlan \
          --provider:physical_network physnet2 \
          --provider:segmentation_id 101
$ neutron subnet-create --tenant-id $tenant --name net01_subnet01 net01 192.168.101.0/24
$ neutron router-interface-add router01 net01_subnet01

Similarly, for net02, using VLAN ID 102 on the physical switch:

$ neutron net-create --tenant-id $tenant net02 \
          --provider:network_type vlan \
          --provider:physical_network physnet2 \
          --provider:segmentation_id 102
$ neutron subnet-create --tenant-id $tenant --name net02_subnet01 net02 192.168.102.0/24
$ neutron router-interface-add router01 net02_subnet01
 Scenario 1: Compute host config

The following figure shows how to configure various Linux networking devices on the compute host:

 Types of network devices
[Note]Note

There are four distinct type of virtual networking devices: TAP devices, veth pairs, Linux bridges, and Open vSwitch bridges. For an Ethernet frame to travel from eth0 of virtual machine vm01 to the physical network, it must pass through nine devices inside of the host: TAP vnet0, Linux bridge qbrNNN, veth pair (qvbNNN, qvoNNN), Open vSwitch bridge br-int, veth pair (int-br-eth1, phy-br-eth1), and, finally, the physical network interface card eth1.

A TAP device, such as vnet0 is how hypervisors such as KVM and Xen implement a virtual network interface card (typically called a VIF or vNIC). An Ethernet frame sent to a TAP device is received by the guest operating system.

A veth pair is a pair of directly connected virtual network interfaces. An Ethernet frame sent to one end of a veth pair is received by the other end of a veth pair. Networking uses veth pairs as virtual patch cables to make connections between virtual bridges.

A Linux bridge behaves like a simple MAC learning switch: you can connect multiple (physical or virtual) network interfaces devices to a Linux bridge. The Linux bridge uses a MAC caching table to record which interface on the bridge is used to communicate with a host on the link. For any Ethernet frames that come in from one interface attached to the bridge, the host MAC address and port on which the frame was received is recorded in the MAC caching table for a limited time. When the bridge needs to forward a frame, it will check to see if the frame's destination MAC address is recorded in the table. If so, the Linux bridge forwards the frame through only that port. If not, the frame is flooded to all network ports in the bridge, with the exception of the port where the frame was received.

An Open vSwitch bridge behaves like a virtual switch: network interface devices connect to Open vSwitch bridge's ports, and the ports can be configured much like a physical switch's ports, including VLAN configurations.

 Integration bridge

The br-int Open vSwitch bridge is the integration bridge: all guests running on the compute host connect to this bridge. Networking implements isolation across these guests by configuring the br-int ports.

 Physical connectivity bridge

The br-eth1 bridge provides connectivity to the physical network interface card, eth1. It connects to the integration bridge by a veth pair: (int-br-eth1, phy-br-eth1).

 VLAN translation

In this example, net01 and net02 have VLAN ids of 1 and 2, respectively. However, the physical network in our example only supports VLAN IDs in the range 101 through 110. The Open vSwitch agent is responsible for configuring flow rules on br-int and br-eth1 to do VLAN translation. When br-eth1 receives a frame marked with VLAN ID 1 on the port associated with phy-br-eth1, it modifies the VLAN ID in the frame to 101. Similarly, when br-int receives a frame marked with VLAN ID 101 on the port associated with int-br-eth1, it modifies the VLAN ID in the frame to 1.

 Security groups: iptables and Linux bridges

Ideally, the TAP device vnet0 would be connected directly to the integration bridge, br-int. Unfortunately, this isn't possible because of how OpenStack security groups are currently implemented. OpenStack uses iptables rules on the TAP devices such as vnet0 to implement security groups, and Open vSwitch is not compatible with iptables rules that are applied directly on TAP devices that are connected to an Open vSwitch port.

Networking uses an extra Linux bridge and a veth pair as a workaround for this issue. Instead of connecting vnet0 to an Open vSwitch bridge, it is connected to a Linux bridge, qbrXXX. This bridge is connected to the integration bridge, br-int, through the (qvbXXX, qvoXXX) veth pair.

 Scenario 1: Network host config

The network host runs the neutron-openvswitch-plugin-agent, the neutron-dhcp-agent, neutron-l3-agent, and neutron-metadata-agent services.

On the network host, assume that eth0 is connected to the external network, and eth1 is connected to the data network, which leads to the following configuration in the ovs_neutron_plugin.ini file:

[ovs]
tenant_network_type = vlan
network_vlan_ranges = physnet1,physnet2:101:110
integration_bridge = br-int
bridge_mappings = physnet1:br-ex,physnet2:br-eth1

The following figure shows the network devices on the network host:

As on the compute host, there is an Open vSwitch integration bridge (br-int) and an Open vSwitch bridge connected to the data network (br-eth1), and the two are connected by a veth pair, and the neutron-openvswitch-plugin-agent configures the ports on both switches to do VLAN translation.

An additional Open vSwitch bridge, br-ex, connects to the physical interface that is connected to the external network. In this example, that physical interface is eth0.

[Note]Note

While the integration bridge and the external bridge are connected by a veth pair (int-br-ex, phy-br-ex), this example uses layer 3 connectivity to route packets from the internal networks to the public network: no packets traverse that veth pair in this example.

 Open vSwitch internal ports

The network host uses Open vSwitch internal ports. Internal ports enable you to assign one or more IP addresses to an Open vSwitch bridge. In previous example, the br-int bridge has four internal ports: tapXXX, qr-YYY, qr-ZZZ, and tapWWW. Each internal port has a separate IP address associated with it. An internal port, qg-VVV, is on the br-ex bridge.

 DHCP agent

By default, the Networking DHCP agent uses a process called dnsmasq to provide DHCP services to guests. Networking must create an internal port for each network that requires DHCP services and attach a dnsmasq process to that port. In the previous example, the tapXXX interface is on net01_subnet01, and the tapWWW interface is on net02_subnet01.

 L3 agent (routing)

The Networking L3 agent uses Open vSwitch internal ports to implement routing and relies on the network host to route the packets across the interfaces. In this example, the qr-YYY interface is on net01_subnet01 and has the IP address 192.168.101.1/24. The qr-ZZZ, interface is on net02_subnet01 and has the IP address 192.168.102.1/24. The qg-VVV interface has the IP address 10.64.201.254/24. Because each of these interfaces is visible to the network host operating system, the network host routes the packets across the interfaces, as long as an administrator has enabled IP forwarding.

The L3 agent uses iptables to implement floating IPs to do the network address translation (NAT).

 Overlapping subnets and network namespaces

One problem with using the host to implement routing is that one of the Networking subnets might overlap with one of the physical networks that the host uses. For example, if the management network is implemented on eth2 and also happens to be on the 192.168.101.0/24 subnet, routing problems will occur because the host can't determine whether to send a packet on this subnet to qr-YYY or eth2. If end users are permitted to create their own logical networks and subnets, you must design the system so that such collisions do not occur.

Networking uses Linux network namespaces to prevent collisions between the physical networks on the network host, and the logical networks used by the virtual machines. It also prevents collisions across different logical networks that are not routed to each other, as the following scenario shows.

A network namespace is an isolated environment with its own networking stack. A network namespace has its own network interfaces, routes, and iptables rules. Consider it a chroot jail, except for networking instead of for a file system. LXC (Linux containers) use network namespaces to implement networking virtualization.

Networking creates network namespaces on the network host to avoid subnet collisions.

In this example, there are three network namespaces, as shown in the figure above:

  • qdhcp-AAA: contains the tapXXX interface and the dnsmasq process that listens on that interface to provide DHCP services for net01_subnet01. This allows overlapping IPs between net01_subnet01 and any other subnets on the network host.

  • qrouter-BBBB: contains the qr-YYY, qr-ZZZ, and qg-VVV interfaces, and the corresponding routes. This namespace implements router01 in our example.

  • qdhcp-CCC: contains the tapWWW interface and the dnsmasq process that listens on that interface, to provide DHCP services for net02_subnet01. This allows overlapping IPs between net02_subnet01 and any other subnets on the network host.

 Scenario 2: two tenants, two networks, two routers

In this scenario, tenant A and tenant B each have a network with one subnet and one router that connects the tenants to the public Internet.

Under the service tenant, define the public network:

$ tenant=$(keystone tenant-list | awk '/service/ {print $2}')
$ neutron net-create --tenant-id $tenant public01 \
          --provider:network_type flat \
          --provider:physical_network physnet1 \
          --router:external True
$ neutron subnet-create --tenant-id $tenant --name public01_subnet01 \
          --gateway 10.64.201.254 public01 10.64.201.0/24 --disable-dhcp

Under the tenantA user tenant, create the tenant router and set its gateway for the public network.

$ tenant=$(keystone tenant-list|awk '/tenantA/ {print $2}')
$ neutron router-create --tenant-id $tenant router01
$ neutron router-gateway-set router01 public01

Then, define private network net01 using VLAN ID 101 on the physical switch, along with its subnet, and connect it to the router.

$ neutron net-create --tenant-id $tenant net01 \
          --provider:network_type vlan \
          --provider:physical_network physnet2 \
          --provider:segmentation_id 101
$ neutron subnet-create --tenant-id $tenant --name net01_subnet01 net01 192.168.101.0/24
$ neutron router-interface-add router01 net01_subnet01

Similarly, for tenantB, create a router and another network, using VLAN ID 102 on the physical switch:

$ tenant=$(keystone tenant-list|awk '/tenantB/ {print $2}')
$ neutron router-create --tenant-id $tenant router02
$ neutron router-gateway-set router02 public01
$ neutron net-create --tenant-id $tenant net02 \
          --provider:network_type vlan \
          --provider:physical_network physnet2 \
          --provider:segmentation_id 102
$ neutron subnet-create --tenant-id $tenant --name net02_subnet01 net02 192.168.102.0/24
$ neutron router-interface-add router02 net02_subnet01
 Scenario 2: Compute host config

The following figure shows how to configure Linux networking devices on the compute host:

[Note]Note

The compute host configuration resembles the configuration in scenario 1. However, in scenario 1, a guest connects to two subnets while in this scenario, the subnets belong to different tenants.

 Scenario 2: Network host config

The following figure shows the network devices on the network host for the second scenario.

In this configuration, the network namespaces are organized to isolate the two subnets from each other as shown in the following figure.

In this scenario, there are four network namespaces (qhdcp-AAA, qrouter-BBBB, qrouter-CCCC, and qhdcp-DDDD), instead of three. Because there is no connectivity between the two networks, each router is implemented by a separate namespace.

 Configure Open vSwitch tunneling

Tunneling encapsulates network traffic between physical Networking hosts and allows VLANs to span multiple physical hosts. Instances communicate as if they share the same layer 2 network. Open vSwitch supports tunneling with the VXLAN and GRE encapsulation protocols.

 

Figure 5.1. Example VXLAN tunnel

Example VXLAN tunnel

This diagram shows two instances running on separate hosts connected by a VXLAN tunnel. The required physical and virtual components are also illustrated. The following procedure creates a VXLAN or GRE tunnel between two Open vSwitches running on separate Networking hosts:

 

Example tunnel configuration

  1. Create a virtual bridge named OVS-BR0 on each participating host:

    ovs-vsctl add-br OVS-BR0
  2. Create a tunnel to link the OVS-BR0 virtual bridges. Run the ovs-vsctl command on HOST1 to create the tunnel and link it to the bridge on HOST2:

    GRE tunnel command:

    ovs-vsctl add-port OVS-BR0 gre1 -- set Interface gre1 type=gre options:remote_ip=192.168.1.11

    VXLAN tunnel command:

    ovs-vsctl add-port OVS-BR0 vxlan1 -- set Interface vxlan1 type=vxlan options:remote_ip=192.168.1.11
  3. Run the ovs-vsctl command on HOST2 to create the tunnel and link it to the bridge on HOST1.

    GRE tunnel command:

    ovs-vsctl add-port OVS-BR0 gre1 -- set Interface gre1 type=gre options:remote_ip=192.168.1.10
                    

    VXLAN tunnel command:

    ovs-vsctl add-port OVS-BR0 vxlan1 -- set Interface vxlan1 type=vxlan options:remote_ip=192.168.1.10
                    

Successful completion of these steps results in the two instances sharing a layer 2 network.

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