7.3. Networking scenarios

This chapter describes two networking scenarios and how the Open vSwitch plug-in and the Linux bridging plug-in implement these scenarios.

7.3.1. Open vSwitch

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

7.3.1.1. Configuration

This example uses VLAN isolation 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 = physnet2:100:110
integration_bridge = br-int
bridge_mappings = physnet2:br-eth1

7.3.1.2. Scenario 1: one tenant, two networks, and 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).
Open vSwitch: Scenario 1: one tenant,two networks, and one router

Figure 7.1. Open vSwitch: Scenario 1: one tenant,two networks, and one router

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
7.3.1.2.1. Scenario 1: Compute host configuration
The following figure shows how to configure various Linux networking devices on the compute host:
Open vSwitch: Scenario 1: Compute host configuration

Figure 7.2. Open vSwitch: Scenario 1: Compute host configuration

Types of network devices

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 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 virtual network interfaces correctly directly together. An ethernet frame sent to one end of a veth pair is received by the other end of a veth pair. OpenStack networking makes use of veth pairs as virtual patch cables in order to make connections between virtual bridges.
A Linux bridge behaves like a hub: you can connect multiple (physical or virtual) network interfaces devices to a Linux bridge. Any ethernet frames that come in from one interface attached to the bridge is transmitted to all of the other devices.
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 OpenvSwitch bridge is the integration bridge: all of the guests running on the compute host connect to this bridge. OpenStack 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.
OpenStack 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.
7.3.1.2.2. Scenario 1: Network host configuration
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 = 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:
Open vSwitch: Network host: network devices

Figure 7.3. Open vSwitch: Network host: network devices

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

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, 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 OpenStack Networking DHCP agent uses a program called dnsmasq to provide DHCP services to guests. OpenStack 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 interface tapXXX is on subnet net01_subnet01, and the interface tapWWW is on net02_subnet01.
L3 agent (routing)
The OpenStack Networking L3 agent implements routing through the use of Open vSwitch internal ports and relies on the network host to route the packets across the interfaces. In this example: interfaceqr-YYY, which is on subnet net01_subnet01, has an IP address of 192.168.101.1/24, interface qr-ZZZ, which is on subnet net02_subnet01, has an IP address of 192.168.102.1/24, and interface qg-VVV, which has an IP address of 10.64.201.254/24. Because of each of these interfaces is visible to the network host operating system, it will route the packets appropriately 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 there is a chance that one of the OpenStack Networking subnets might overlap with one of the physical networks that the host uses. For example, if the management network is implemented on eth2 (not shown in the previous example) and also on the 192.168.101.0/24 subnet, this will cause routing problems. It is impossible to determine whether a packet on the subnet should be sent to qr-YYY or eth2. In general, if end-users are permitted to create their own logical networks and subnets, then the system must be designed to avoid the possibility of such collisions.
OpenStack 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 you will see in the next scenario.
A network namespace can be thought of as an isolated environment that has its own networking stack. A network namespace has its own network interfaces, routes, and iptables rules. You can think of like a chroot jail, except for networking instead of a file system. As an aside, LXC (Linux containers) use network namespaces to implement networking virtualization.
OpenStack Networking creates network namespaces on the network host in order to avoid subnet collisions.
Tn this example, there are three network namespaces, as depicted in the following figure.
  • 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.
Open vSwitch: Network namespaces

Figure 7.4. Open vSwitch: Network namespaces

7.3.1.3. Scenario 2: two tenants, two networks, and 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.
Open vSwitch: Scenario 2: two tenants, two networks, and two routers

Figure 7.5. Open vSwitch: Scenario 2: two tenants, two networks, and two routers

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 102 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 net01 192.168.101.0/24
$ neutron router-interface-add router02 net02_subnet01
7.3.1.3.1. Scenario 2: Compute host configuration
The following figure shows how to configure Linux networking devices on the Compute host:
Open vSwitch: Scenario 2: Compute host configuration

Figure 7.6. Open vSwitch: Scenario 2: Compute host configuration

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.
7.3.1.3.2. Scenario 2: Network host configuration
The following figure shows the network devices on the network host for the second scenario.
Open vSwitch: Scenario 2: Network host configuration

Figure 7.7. Open vSwitch: Scenario 2: Network host configuration

In this configuration, the network namespaces are organized to isolate the two subnets from each other as shown in the following figure.
Open vSwitch: Scenario 2: Isolating subnets

Figure 7.8. Open vSwitch: Scenario 2: Isolating subnets

In this scenario, there are four network namespaces (qhdcp-aaa, qrouter-bbbb, qrouter-cccc, and qhdcp-dddd), instead of three. Since there is no connectivity between the two networks, and so each router is implemented by a separate namespace.