The Link Controller system uses different configuration modes to manage
traffic across multiple links. As requests come across the Link Controller, the system identifies the best available virtual server in the Link Controller system configuration and sends the traffic to that virtual server. This process is known as inbound load balancing.
The Link Controller system has external IP addresses configured for each
network or ISP link that it manages. The external IP addresses are configured as virtual servers on the Link Controller system, and these virtual servers belong to a wide IP on the Link Controller system. The translations for the virtual servers direct the incoming requests to the appropriate content servers (for example, a web server or a database server) on the internal network. This process is known as inbound load balancing
When the Link Controller system receives a name resolution request from a
local DNS server, the system uses a load balancing mode to select the best available virtual server in a wide IP. Once the Link Controller system selects the best virtual server, it constructs the DNS answer and sends the answer back to the requesting clients local DNS server, using the best available link. The DNS answer, or resource record
, is an A
record that contains one or more virtual server IP addresses.
The Link Controller system chooses a virtual server from a wide IP using
either a static load balancing mode or a dynamic load balancing mode. A static load balancing mode
selects a virtual server based on a pre-defined pattern. A dynamic load balancing mode
selects a virtual server based on current performance metrics.
Within each wide IP, you can specify up to three load balancing modes that
the system uses in sequential order: the preferred method, the alternate method, and the fallback method. The preferred
method is the first load balancing mode that the Link Controller system uses for load balancing. If the preferred method fails to select a virtual server, the system then uses the alternate method for load balancing. If the alternate load balancing mode fails to select a virtual server, the system uses the fallback load balancing mode. If the fallback method fails, then the Link Controller system randomly picks an available virtual server.
The Link Controller system supports several modes for inbound load
balancing. Table 9.1
shows a complete list of the load balancing modes, indicates where you can use each mode, and indicates whether the mode is static or dynamic. The sections following the table describe how each load balancing mode works.
Static load balancing modes
distribute requests according to predefined patterns, and take virtual server availability into account. Dynamic load balancing modes
distribute requests to links that show the best current performance. The performance metrics taken into account depend on the particular dynamic mode you are using.
Static load balancing modes
distribute connections according to predefined patterns, and take server availability into account. The Link Controller system supports the following static load balancing modes:
The None load balancing mode is a special mode that you can use to skip
load balancing under certain conditions. The other static load balancing modes perform true load balancing as described in the following sections.
When you specify the Drop Packet load balancing mode, the Link
Controller system does nothing with the packet, and simply drops the request.
We recommend that you use the Drop Packet load balancing mode only for
the fallback method. The Link Controller system uses the fallback method when the preferred and alternate load balancing modes do not provide at least one virtual server to return as an answer to a query.
When you specify the Fallback IP mode, the Link Controller system returns
the IP address that you specify as the fallback IP as an answer to the query. Note that the IP address that you specify is not monitored for availability before being returned as an answer. When you use the Fallback IP mode, you can specify a disaster recovery site to return when no load balancing mode returns an available virtual server. We recommend that you use the Fallback IP load balancing mode only for the fallback method. The Link Controller system uses the fallback method when the preferred and alternate load balancing modes do not provide at least one virtual server to return as an answer to a query.
The Global Availability load balancing mode uses the virtual servers
included in the pool in the order in which they are listed. For each connection request, this mode starts at the top of the list and sends the connection to the first available virtual server in the list. Only when the current virtual server is full or otherwise unavailable does Global Availability mode move to the next virtual server in the list. Over time, the first virtual server in the list receives the most connections and the last virtual server in the list receives the least number of connections.
The Ratio load balancing mode distributes connections among a pool of
virtual servers as a weighted Round Robin. For example, you can configure the Ratio mode to send twice as many connections to a fast, new server, and only half as many connections to an older, slower server.
The Ratio load balancing mode requires that you define a ratio weight for
each virtual server in a pool, or for each pool if you are load balancing requests among multiple pools. The default ratio weight for a server or a pool is set to 1
The Round Robin load balancing mode distributes connections in a circular
and sequential pattern among the virtual servers in a pool. Over time, each virtual server receives an equal number of connections.
The Static Persist load balancing mode provides static persistence of local
DNS servers to virtual servers; it consistently maps an LDNS IP address to the same available virtual server for the duration of the session. This mode guarantees that certain transactions are routed through a single transaction manager (for example, a Local Traffic Manager system or other server array manager); this is beneficial for transaction-oriented traffic, such as e-commerce shopping carts, online trading, and online banking.
The Topology load balancing mode allows you to direct or restrict traffic
flow by adding topology records to a topology statement in the configuration file. When you use the Topology load balancing mode, you can develop proximity-based load balancing. For example, a client request from a particular IP subnet can be directed to a specific IP subnet on your network. The Link Controller system determines the proximity of servers by comparing location information derived from the DNS message to the topology records.
This load balancing mode requires you to do some advanced configuration
planning, such as gathering the information you need to define the topology records. See Chapter 10, Working with Topologies
, for detailed information about working with this and other topology features.
Dynamic load balancing modes
distribute connections to links that show the best current performance. The performance metrics taken into account depend on the particular dynamic mode you are using.
The Completion Rate load balancing mode selects the virtual server that
currently maintains the least number of dropped or timed-out packets during a transaction between a data center and the client LDNS.
The Hops load balancing mode is based on the traceroute
utility, and tracks the number of intermediate system transitions (router hops) between a client LDNS and each data center. Hops mode selects a virtual server in the data center that has the fewest router hops from the LDNS server.
The Least Connections load balancing mode is used for load balancing to
virtual servers managed by a load balancing server, such as a Local Traffic Manager. The Least Connections mode simply selects a virtual server on the Local Traffic Manager system that currently hosts the fewest connections.
The Round Trip Times (RTT) load balancing mode selects the virtual server
with the fastest measured round trip time between a data center and a client LDNS.
The Quality of Service load balancing mode uses current performance
information to calculate an overall score for each virtual server, and then distributes connections based on each virtual servers score. The performance factors that the Link Controller system takes into account include:
The Quality of Service load balancing mode is a customizable load
balancing mode. For simple configurations, you can easily use this load balancing mode with its default settings. For more advanced configurations, you can specify different weights for each performance factor in the equation.
You can also configure the Quality of Service load balancing mode to use
the dynamic ratio feature. With the dynamic ratio feature turned on, the Quality of Service mode becomes similar to the Ratio mode, where the connections are distributed in proportion to ratio weights assigned to each virtual server. The ratio weights are based on the QOS scores: the better the score, the higher percentage of connections the virtual server receives.
The VS Capacity load balancing mode creates a list of the virtual servers,
weighted by capacity, then picks one of the virtual servers from the list. The virtual servers with the greatest capacity are picked most often, but over time all virtual servers are returned. If more than one virtual server has the same capacity, then the Link Controller system load balances using the Round Robin mode among those virtual servers.
The Virtual Server Score load balancing mode instructs the Global Traffic
Manager system to assign connection requests to virtual servers based on a user-defined ranking system. This load balancing mode is only available for managing connections between virtual servers controlled through BIG-IP®
Local Traffic Manager systems.
Unlike other settings that affect load balancing operations, you cannot
assign a virtual server score to a virtual server through the Global Traffic Manager system. Instead, you assign this setting through the Local Traffic Manager responsible for the virtual server. See the Configuration Guide for BIG-IP® Local Traffic Manager
for more information.
The Quality of Service mode is a dynamic load balancing mode that
includes a configurable combination of the Round Trip Time (RTT), Completion Rate, Packet Rate, Topology, Hops, Link Capacity, VS Capacity, and Kilobytes/Second (KBPS) modes. The Quality of Service mode is based on an equation that takes each of these performance factors into account. When the Link Controller system selects a virtual server, it chooses the server with the best overall score.
The Quality of Service mode has default settings that make it easy to use:
simply specify Quality of Service as your preferred load balancing mode. There is no need to configure Quality of Service, but if you want to change the settings, you can customize the equation to put more or less weight on each individual factor. The following topics explain how to use and adjust the various settings.
lists each Quality of Service (QOS) coefficient, its scale, a likely upper limit for each, and whether a higher or lower value is more efficient.
| || |Scale
The raw metrics for each coefficient are not on the same scale. For example, completion rate is measured in percentages, while the packet rate is measured in packets per second.
| || |Normalization
The Link Controller system normalizes the raw metrics to values in the range of 0 to10. As the QOS value is calculated, a high measurement for completion rate is good, because a high percentage of completed connections are being made, but a high value for packet rate is not desirable because the packet rate load balancing mode attempts to find a virtual server that is not overly taxed at the moment.
| || |Emphasis
You can adjust coefficients to emphasize one normalized metric over another. For example, consider the following QOS configuration:
In this configuration, if the completion rates for two virtual servers are
close, the virtual server with the best packet rate is chosen. If both the completion rates and the packet rates are close, the round trip time (RTT) breaks the tie. In this example, the metrics for Topology, Hops, Link Capacity, VS Capacity, and Kilobytes/Second modes are not used in determining how to distribute connections.
Note: You cannot set a value for both the Round Trip Time
settings simultaneously. In situations where the Link Controller system has a value for both settings, the Round Trip Time
value is incorporated, while the value for the Hops
setting is reset to 0
If you want to establish your own custom settings for the Quality of Service
load balancing method, you can do so at any time. You can only customize the Quality of Service equation at the pool level.
| |Click the Update
button to save your changes.
The dynamic load balancing modes also support the Dynamic Ratio
option. When you activate this option, the Link Controller system treats dynamic load balancing values as ratios, and it uses each server in proportion to the ratio determined by this option. When the Dynamic Ratio
option is off, the Link Controller system uses only the server with the best result based on the dynamic load balancing mode you implemented (in which case it is a winner-takes-all situation), until the metrics information is refreshed.
To illustrate how the Dynamic Ratio
option works, consider a pool, primaryOne
, that contains several pool members. This pool is configured so that the Link Controller system load balances name resolution requests based on the Round Trip Time load balancing mode. The primaryOne
pool contains two pool members: memberOne
. For this example, the Link Controller system determines that the round trip time for memberOne
is 50 microseconds, while the round trip time for memberTwo
is 100 microseconds.
If the primaryOne
pool has the Dynamic Ratio
option disabled (the default setting), the Link Controller system always load balances to the pool with the best value. In this case, this results in requests going to memberOne
, because it has the lowest round trip time value.
If the primaryOne
pool has the Dynamic Ratio
option enabled, however, the Link Controller system treats the round trip time values as ratios and divide requests among pool members based on these ratios. In this case, this results in memberOne
getting twice as many connections as memberTwo
, because the round trip time for memberOne
is twice as fast as the round trip time for memberTwo
. Note that, with the Dynamic Ratio
option enabled, both pool members are employed to handle connections, while if the option is disabled, only one pool member receives connections.
| |From the Configuration
list, select Advanced
| |Click the Update
button to save your changes.
Once you identify which load balancing methods are best for your network,
you can configure the Link Controller system to implement those methods. This process ensures that traffic flows through your network as efficiently as possible. You configure inbound load balancing at the wide IP level.
When you define a wide IP, you specify the preferred and alternate load
balancing methods to use in selecting a virtual server within the wide IP.
| |In the Load Balancing Method
section, select the appropriate preferred, alternate and fallback load balancing methods.
| |Click the Update
button to save your changes.
By default, the preferred load balancing method is set to Quality of Service,
and the alternate and fallback load balancing methods are set to Round Robin. For details on these load balancing modes as well as the additional load balancing modes, see Introducing inbound load balancing
Round Robin LDNS is an attribute that you can use in conjunction with any
load balancing mode. The round robin LDNS attribute allows the Link Controller system to return a list of available virtual servers, instead of a single virtual server. Certain browsers keep the answer returned by DNS servers. By enabling this attribute, the Link Controller system returns a maximum of 16 virtual servers as the answer to a DNS resolution request. This provides browsers with alternate answers if a virtual server becomes unavailable.
You can adjust the QOS coefficients to best match your load balancing
requirements. By default, only the Link Capacity coefficient has a value. For more information on working with the QOS coefficients, see Understanding QOS coefficients