About Virtual Servers

A virtual server is one of the most important components of any BIG-IP^® system configuration. A virtual server is a traffic-management object on the BIG-IP system that is represented by a virtual IP address and a service, such as 192.168.20.10:80. When clients on an external network send application traffic to virtual server, the virtual server listens for that traffic and, through destination address translation, directs the traffic according to the way that you configured the settings on the virtual server. A primary purpose of a virtual server is to distribute traffic across a pool of servers that you specify in the virtual server configuration.

o customize the way that the BIG-IP system processes various types of traffic, you can assign profiles to a virtual server. For example, through profile assignment, a virtual server can enable compression on HTTP request data as it passes through the BIG-IP system, or decrypt and re-encrypt SSL connections and verify SSL certificates. For each type of traffic, such as TCP, UDP, HTTP, SSL, SIP, and FTP, you can assign a custom profile to the virtual server or use the default profile.

When you create a virtual server, you specify the pool or pools that you want to use as the destination for any traffic coming from that virtual server. You also configure its general properties, profiles, SNATs, and other resources you want to assign to it, such as iRules or session persistence types.

When creating a virtual server, you must specify a destination address. You can specify either a host address or a network address, in either IPv4 or IPv6 format:

• With a host address, a virtual server can listen for client connections that are destined for the host address and then direct them to a server in a server pool. If you do not append a prefix (in CIDR notation) to a host address, the default prefix is /32.
• With a network address (specifically, an address whose host bit is set to 0), a virtual server can direct client connections that are destined for an entire range of IP addresses, rather than for a single destination IP address. For example, the virtual server can direct client traffic that is destined for any of the nodes on the 192.168.1.0 network to a specific pool such as ingress-firewalls. Or, a virtual server can direct a web connection destined to any address within the subnet 192.168.1.0/24, to the pool default_webservers. If you do not append a prefix (in CIDR notation) to a network address, the default prefix is /24.

Besides directing client connections that are destined for a specific network or subnet, a virtual server can also direct client connections that have a specific destination IP address that the virtual server does not recognize, such as a transparent device. This type of virtual server is known as a wildcard virtual server. Examples of transparent devices are firewalls, routers, proxy servers, and cache servers.

Wildcard virtual servers are a special type of virtual server that have a network IP address as the specified destination address instead of a host IP address.

When the BIG-IP^® system does not find a specific virtual server that matches a client’s destination IP address, the BIG-IP system matches the client’s destination IP address to a wildcard virtual server, designated by an IP address of 0.0.0.0. The BIG-IP system then forwards the client’s packet to one of the firewalls or routers assigned to that virtual server. Wildcardvirtual servers do not translate the destination IP address of the incoming packet.

A virtual address is the specific node or network IP address with which you associate a virtual server. For example, if a virtual server's destination address and service port are 192.168.20.10:80, then the IP address 192.168.20.10 is a virtual address.

You can create a many-to-one relationship between virtual servers and a virtual address. For example, you can create the three virtual servers 192.168.20.10:80, 192.168.20.10:443, and 192.168.20.10:161 for the same virtual address, 192.168.20.10.

You cannot explicitly create a virtual address; the BIG-IP system creates a virtual address whenever you create a virtual server, if the virtual address has not already been created. However, you can modify the properties of a virtual address, and you can enable and disable a virtual address. When you disable a virtual address, none of the virtual servers associated with that address can receive incoming network traffic.

When you create a virtual server, BIG-IP^® internally associates the virtual address with a MAC address. This in turn causes the BIG-IP^® system to respond to Address Resolution Protocol (ARP) requests for the virtual address, and to send gratuitous ARP requests and responses with respect to the virtual address. As an option, you can disable ARP activity for virtual addresses, in the rare case that ARP activity affects system performance. This most likely occurs only when you have a large number of virtual addresses defined on the system.

Whenever you configure the Source Address and Destination Address settings on a virtual server, the BIG-IP system requires that the route domain IDs match, if route domain IDs are specified. To ensure that this requirement is met, the BIG-IP system enforces specific rules, which vary depending on whether you are modifying an existing virtual server or creating a new virtual server.

At any time, you can determine the status of a virtual server or virtual address, using the BIG-IP^® Configuration utility. You can find this information by displaying the list of virtual servers or virtual addresses and viewing the Status column, or by viewing the Availability property of the object.

The BIG-IP Configuration utility indicates status by displaying one of several icons, distinguished by shape and color:

• The shape of the icon indicates the status that the monitor has reported for that node.
• The color of the icon indicates the actual status of the node.

The BIG-IP^® system includes a performance feature known as Clustered Multiprocessing™, or CMP^®. CMP is a traffic acceleration feature that creates a separate instance of the Traffic Management Microkernel (TMM) service for each central processing unit (CPU) on the system. When CMP is enabled, the workload is shared equally among all CPUs.

Whenever you create a virtual server, the BIG-IP system automatically enables the CMP feature. When CMP is enabled, all instances of the TMM service process application traffic.

When you view standard performance graphs using the BIG-IP Configuration utility, you can see multiple instances of the TMM service (tmm0, tmm1, and so on).

When CMP is enabled, be aware that:

• While displaying some statistics individually for each TMM instance, the BIG-IP system displays other statistics as the combined total of all TMM instances.
• Connection limits for a virtual server with CMP enabled are distributed evenly across all instances of the TMM service.

You can enable or disable CMP for a virtual server, or you can enable CMP for a specific CPU.