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Manual Chapter: Virtual Servers
Manual Chapter
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Introduction to virtual servers

A virtual server is one of the most important components of any BIG-IP Local Traffic Manager configuration. When you configure a virtual server, you create two Local Traffic Manager (LTM) objects: a virtual server and a virtual address.

A virtual server is a traffic-management object on the BIG-IP system that is represented by an IP address and a service. Clients on an external network can send application traffic to a virtual server, which then directs the traffic according to your configuration instructions. A primary purpose of a virtual server is to load balance traffic across a pool of servers on an internal network. Virtual servers increase the availability of resources for processing client requests.

Not only do virtual servers distribute traffic across multiple servers, they also treat varying types of traffic differently, depending on your traffic-management needs. For example, 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, a virtual server can apply an entire group of settings, to affect the way that Local Traffic Manager manages that traffic type. You can use various types of virtual servers:

Standard
A Standard virtual server (also known as a load balancing virtual server) directs client traffic to a load balancing pool and is the most basic type of virtual server. When you first create the virtual server, you assign an existing default pool to it. From then on, the virtual server automatically directs traffic to that default pool.
Forwarding (Layer 2)
You can set up a Forwarding (Layer 2) virtual server to share the same IP address as a node in an associated VLAN. To do this, you must perform some additional configuration tasks. These tasks consist of: creating a VLAN group that includes the VLAN in which the node resides, assigning a self-IP address to the VLAN group, and disabling the virtual server on the relevant VLAN.
Forwarding (IP)
A Forwarding (IP) virtual server is just like other virtual servers, except that a forwarding virtual server has no pool members to load balance. The virtual server simply forwards the packet directly to the destination IP address specified in the client request. When you use a forwarding virtual server to direct a request to its originally-specified destination IP address, Local Traffic Manager adds, tracks, and reaps these connections just as with other virtual servers. You can also view statistics for a forwarding virtual servers.
Performance (HTTP)
A Performance (HTTP) virtual server is a virtual server with which you associate a Fast HTTP profile. Together, the virtual server and profile increase the speed at which the virtual server processes HTTP requests.
Performance (Layer 4)
A Performance (Layer 4) virtual server is a virtual server with which you associate a Fast L4 profile. Together, the virtual server and profile increase the speed at which the virtual server processes Layer 4 requests.
Stateless
A stateless virtual server prevents the BIG-IP system from putting connections into the connection table for wildcard and forwarding destination IP addresses. When creating a stateless virtual server, you cannot configure SNAT Automap, iRules, or port translation, and you must configure a default load balancing pool. Note that this type of virtual server applies to UDP traffic only.
DHCP Relay
A DHCP Relay virtual server relays Dynamic Host Control Protocol (DHCP) messages between clients and servers residing on different IP networks. Known as a DHCP relay agent, a BIG-IP system with a DHCP Relay type of virtual server listens for DHCP client messages being broadcast on the subnet and then relays those messages to the DHCP server. The DHCP server then uses the BIG-IP system to send the responses back to the DHCP client. Configuring a DHCP Relay virtual server on the BIG-IP system relieves you of the tasks of installing and running a separate DHCP server on each subnet.
Internal
An Internal virtual server is one that can send traffic to an intermediary server for specialized processing before the standard virtual server sends the traffic to its final destination. For example, if you want the BIG-IP system to perform content adaptation on HTTP requests or responses, you can create an internal virtual server that load balances those requests or responses to a pool of ICAP servers before sending the traffic back to the standard virtual server.

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

About virtual addresses

A virtual address is the IP address with which you associate a virtual server. For example, if a virtual server’s IP address and service are 10.10.10.2:80, then the IP address 10.10.10.2 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 10.10.10.2:80, 10.10.10.2:443, and 10.10.10.2:161 for the same virtual address, 10.10.10.2.

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.

You create a virtual address indirectly when you create a virtual server. When this happens, Local Traffic Manager 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.

Note: To ensure that a server response returns through the BIG-IP system, you must configure the default route on the server to be an internal VLAN’s self IP address. If you cannot do this because the server is on a different network than the BIG-IP system, you can create a SNAT instead.

Types of virtual servers

There are several different types of virtual servers that you can create.

Table 1. Types of virtual servers
Type Description
Standard A Standard virtual server (also known as a load balancing virtual server) directs client traffic to a load balancing pool and is the most basic type of virtual server. When you first create the virtual server, you assign an existing default pool to it. From then on, the virtual server automatically directs traffic to that default pool.
Forwarding (Layer 2) You can set up a Forwarding (Layer 2) virtual server to share the same IP address as a node in an associated VLAN. To do this, you must perform some additional configuration tasks. These tasks consist of: creating a VLAN group that includes the VLAN in which the node resides, assigning a self-IP address to the VLAN group, and disabling the virtual server on the relevant VLAN.
Forwarding (IP) A Forwarding (IP) virtual server is just like other virtual servers, except that a forwarding virtual server has no pool members to load balance. The virtual server simply forwards the packet directly to the destination IP address specified in the client request. When you use a forwarding virtual server to direct a request to its originally-specified destination IP address, Local Traffic Manager adds, tracks, and reaps these connections just as with other virtual servers. You can also view statistics for a forwarding virtual servers.
Performance (HTTP) A Performance (HTTP) virtual server is a virtual server with which you associate a Fast HTTP profile. Together, the virtual server and profile increase the speed at which the virtual server processes HTTP requests.
Performance (Layer 4) A Performance (Layer 4) virtual server is a virtual server with which you associate a Fast L4 profile. Together, the virtual server and profile increase the speed at which the virtual server processes Layer 4 requests.
Stateless A stateless virtual server prevents the BIG-IP system from putting connections into the connection table for wildcard and forwarding destination IP addresses. When creating a stateless virtual server, you cannot configure SNAT automap, iRules, or port translation, and you must configure a default load balancing pool. Note that this type of virtual server applies to UDP traffic only.
Reject A Reject virtual server specifies that the BIG-IP system rejects any traffic destined for the virtual server IP address.
DHCP Relay A DHCP Relay virtual server relays Dynamic Host Control Protocol (DHCP) messages between clients and servers residing on different IP networks. Known as a DHCP relay agent, a BIG-IP system with a DHCP Relay type of virtual server listens for DHCP client messages being broadcast on the subnet and then relays those messages to the DHCP server. The DHCP server then uses the BIG-IP system to send the responses back to the DHCP client. Configuring a DHCP Relay virtual server on the BIG-IP system relieves you of the tasks of installing and running a separate DHCP server on each subnet.
Internal An internal virtual server is one that can send traffic to an intermediary server for specialized processing before the standard virtual server sends the traffic to its final destination. For example, if you want the BIG-IP system to perform content adaptation on HTTP requests or responses, you can create an internal virtual server that load balances those requests or responses to a pool of ICAP servers before sending the traffic back to the standard virtual server.

Destination types for virtual servers

There are two distinct types of virtual servers that you can create: host virtual servers and network virtual servers.

Host destinations

A host virtual server represents a specific site, such as an Internet web site or an FTP site, and it load balances traffic targeted to content servers that are members of a pool.

The IP address that you assign to a host virtual server should match the IP address that Domain Name System (DNS) associates with the site’s domain name. When the BIG-IP system receives a connection request for that site, Local Traffic Manager recognizes that the client’s destination IP address matches the IP address of the virtual server, and subsequently forwards the client request to one of the content servers that the virtual server load balances.

Network destinations

A network virtual server is a virtual server whose IP address has no bits set in the host portion of the IP address (that is, the host portion of its IP address is 0). There are two kinds of network virtual servers: those that direct client traffic based on a range of destination IP addresses, and those that direct client traffic based on specific destination IP addresses that the BIG-IP system does not recognize.

When you have a range of destination IP addresses

With an IP 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 (as is the case for a host virtual server). Thus, when any client connection targets a destination IP address that is in the network specified by the virtual server IP address, Local Traffic Manager (LTM) can direct that connection to one or more pools associated with the network virtual server.

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 load balancing pool such as ingress-firewalls. Or, a virtual server could direct a web connection destined to any address within the subnet 192.168.1.0/24, to the pool default_webservers.

When you have transparent devices (wildcard virtual servers)

Besides directing client connections that are destined for a specific network or subnet, a network 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 network virtual server is known as a wildcard virtual server.

Wildcard virtual servers are a special type of network virtual server designed to manage network traffic that is targeted to transparent network devices. Examples of transparent devices are firewalls, routers, proxy servers, and cache servers. A wildcard virtual server manages network traffic that has a destination IP address unknown to the BIG-IP system.

Unrecognized client IP addresses

A host-type of virtual server typically manages traffic for a specific site. When receiving a connection request for that site, Local Traffic Manager forwards the client to one of the content servers that the virtual server load balances.

However, when load balancing transparent nodes, the BIG-IP system might not recognize a client’s destination IP address. The client might be connecting to an IP address on the other side of the firewall, router, or proxy server. In this situation, Local Traffic Manager cannot match the client’s destination IP address to a virtual server IP address.

Wildcard network virtual servers solve this problem by not translating the incoming IP address at the virtual server level on the BIG-IP system. For example, when Local Traffic Manager does not find a specific virtual server match for a client’s destination IP address, LTM matches the client’s destination IP address to a wildcard virtual server, designated by an IP address of 0.0.0.0. Local Traffic Manager then forwards the client’s packet to one of the firewalls or routers that the wildcard virtual server load balances, which in turn forwards the client’s packet to the actual destination IP address.

Default and port-specific wildcard servers

There are two kinds of wildcard virtual servers that you can create:

Default wildcard virtual servers
A default wildcard virtual server is a wildcard virtual server that uses port 0 and handles traffic for all services. A wildcard virtual server is enabled for all VLANs by default. However, you can specifically disable any VLANs that you do not want the default wildcard virtual server to support. Disabling VLANs for the default wildcard virtual server is done by creating a VLAN disabled list. Note that a VLAN disabled list applies to default wildcard virtual servers only. You cannot create a VLAN disabled list for a wildcard virtual server that is associated with one VLAN only.
Port-specific wildcard virtual servers
A port-specific wildcard virtual server handles traffic only for a particular service, and you define it using a service name or a port number. You can use port-specific wildcard virtual servers for tracking statistics for a particular type of network traffic, or for routing outgoing traffic, such as HTTP traffic, directly to a cache server rather than a firewall or router.

If you use both a default wildcard virtual server and port-specific wildcard virtual servers, any traffic that does not match either a standard virtual server or one of the port-specific wildcard virtual servers is handled by the default wildcard virtual server.

We recommend that when you define transparent nodes that need to handle more than one type of service, such as a firewall or a router, you specify an actual port for the node and turn off port translation for the virtual server.

Multiple wildcard servers

You can define multiple wildcard virtual servers that run simultaneously. Each wildcard virtual server must be assigned to an individual VLAN, and therefore can handle packets for that VLAN only.

In some configurations, you need to set up a wildcard virtual server on one side of the BIG-IP system to load balance connections across transparent devices. You can create another wildcard virtual server on the other side of the BIG-IP system to forward packets to virtual servers receiving connections from the transparent devices and forwarding them to their destination.

About virtual server settings

A virtual server has a number of properties and settings that you can configure to affect the way that a virtual server manages traffic. You can also assign certain resources to a virtual server, such as a load balancing pool and various policies. Together, these properties, settings, and resources represent the definition of a virtual server, and most have default values. When you create a virtual server, you can either retain the default values or adjust them to suit your needs.

If you have created a virtual server that is a load balancing type of virtual server, one of the resources you must assign to the virtual server is a default load balancing pool. A default pool is the pool to which Local Traffic Manager sends traffic if no iRule or policy exists that specifies a different pool. Note that if you plan on using an iRule or policy to direct traffic to a pool, you must assign the iRule or policy as a resource to the virtual server.

About clone pools

You use a clone pool when you want to configure the BIG-IP system to send traffic to a pool of intrusion detection systems (IDSs). An intrusion detection system (IDS) is a device that monitors inbound and outbound network traffic and identifies suspicious patterns that might indicate malicious activities or a network attack. You can use the clone pool feature of a BIG-IP system to copy traffic to a dedicated IDS or a sniffer device.

Important: A clone pool receives all of the same traffic that the load-balancing pool receives.

To configure a clone pool, you first create the clone pool of IDS or sniffer devices and then assign the clone pool to a virtual server. The clone pool feature is the recommended method for copying production traffic to IDS systems or sniffer devices. Note that when you create the clone pool, the service port that you assign to each node is irrelevant; you can choose any service port. Also, when you add a clone pool to a virtual server, the system copies only new connections; existing connections are not copied.

You can configure a virtual server to copy client-side traffic, server-side traffic, or both:

  • A client-side clone pool causes the virtual server to replicate client-side traffic (prior to address translation) to the specified clone pool.
  • A server-side clone pool causes the virtual server to replicate server-side traffic (after address translation) to the specified clone pool.

You can configure an unlimited number of clone pools on the BIG-IP system.

Virtual server and virtual address status

At any time, you can determine the status of a virtual server or virtual address, using the 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 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.

Clustered multiprocessing

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 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.
Note: F5 recommends that you disable the CMP feature if you set a small connection limit on pool members (for example, a connection limit of 2 for the 8400 platform or 4 for the 8800 platform).

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

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