Applies To:
Show VersionsBIG-IP DNS
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About Global Server Load Balancing
Introducing BIG-IP DNS
BIG-IP® DNS (formerly GTM™) is a system that monitors the availability and performance of global resources and uses that information to manage network traffic patterns. BIG-IP DNS uses load balancing algorithms, topology-based routing, and iRules® to control and distribute traffic according to specific policies.
About global server load balancing
BIG-IP® DNS provides tiered global server load balancing (GSLB). BIG-IP DNS distributes DNS name resolution requests, first to the best available pool in a wide IP, and then to the best available virtual server within that pool. BIG-IP DNS selects the best available resource using either a static or a dynamic load balancing method. Using a static load balancing method, BIG-IP DNS selects a resource based on a pre-defined pattern. Using a dynamic load balancing method, BIG-IP DNS selects a resource based on current performance metrics collected by the big3d agents running in each data center.
Static load balancing methods
This table describes the static load balancing methods available in BIG-IP® DNS.
Name | Description | Recommended Use | Wide IP Load Balancing | Preferred Method | Alternate Method | Fallback Method |
---|---|---|---|---|---|---|
Drop Packet | BIG-IP DNS drops the DNS request. | Use Drop Packet for the Alternate load balancing method when you want to ensure that BIG-IP DNS does not offer in a response a virtual server that is potentially unavailable. | No | Yes | Yes | Yes |
Fallback IP | BIG-IP DNS distributes DNS name resolution requests to a virtual server that you specify. This virtual server is not monitored for availability. | Use Fallback IP for the fallback load balancing method when you want BIG-IP DNS to return a disaster recovery site when the preferred and alternate load balancing methods do not return an available virtual server. | No | No | No | Yes |
Global Availability | BIG-IP DNS distributes DNS name resolution requests to the first available virtual server in a pool. BIG-IP DNS starts at the top of a manually configured list of virtual servers and sends requests to the first available virtual server in the list. Only when the virtual server becomes unavailable does BIG-IP DNS send requests to the next virtual server in the list. Over time, the first virtual server in the list receives the most requests and the last virtual server in the list receives the least requests. | Use Global Availability when you have specific virtual servers that you want to handle most of the requests. | Yes | Yes | Yes | Yes |
None | BIG-IP DNS distributes DNS name resolution requests skipping either the next available pool in a multiple pool configuration or the current load balancing method. If all pools are unavailable, BIG-IP DNS returns an aggregate of the IP addresses of all the virtual servers in the pool using BIND. | Use None for the alternate and fallback methods when you want to limit each pool to a single load balancing method. If the preferred load balancing method fails, BIG-IP DNS offers the next pool in a load balancing response. | No | No | Yes | Yes |
Ratio | BIG-IP DNS distributes DNS name resolution requests among the virtual servers in a pool or among pools in a multiple pool configuration using weighted round robin, a load balancing pattern in which requests are distributed among several resources based on a priority level or weight assigned to each resource. | Use Ratio when you want to send twice as many connections to a fast server and half as many connections to a slow server. | Yes | Yes | Yes | Yes |
Return to DNS | BIG-IP DNS immediately distributes DNS name resolution requests to an LDNS for resolution. | Use Return to DNS when you want to temporarily remove a pool from service. You can also use Return to DNS when you want to limit a pool in a single pool configuration to only one or two load balancing attempts. | No | Yes | Yes | Yes |
Round Robin | BIG-IP DNS distributes DNS name resolution requests in a circular and sequential pattern among the virtual servers in a pool. Over time each virtual server receives an equal number of requests. | Use Round Robin when you want to distribute requests equally among all virtual servers in a pool. | Yes | Yes | Yes | Yes |
Static Persist | BIG-IP DNS distributes DNS name resolution requests to the first available virtual server in a pool using the persist mask with the source IP address of the LDNS and a hash algorithm to determine the order of the virtual servers in the list. This hash algorithm orders the virtual servers in the list differently for each LDNS that is passing traffic to the system taking into account the specified CIDR of the LDNS. Each LDNS (and thus each client) generally resolves to the same virtual server; however, when the selected virtual server becomes unavailable, BIG-IP DNS sends requests to another virtual server until the original virtual server becomes available. Then BIG-IP DNS again resolves requests to that virtual server. | Use Static Persist when you want requests from a specific LDNS to resolve to a specific virtual server. | No | Yes | Yes | Yes |
Topology | BIG-IP DNS distributes DNS name resolution requests using proximity-based load balancing. BIG-IP DNS determines the proximity of the resource by comparing location information derived from the DNS message to the topology records in a topology statement you have configured. | Use Topology when you want to send requests from a client in a particular geographic region to a data center or server located in that region. | Yes | Yes | Yes | Yes |
Dynamic load balancing methods
This table describes the dynamic load balancing methods available in BIG-IP® DNS.
Name | Description | Wide IP load balancing | Preferred method | Alternate method | Fallback method |
---|---|---|---|---|---|
Completion Rate | BIG-IP® DNS distributes DNS name resolution requests to 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's LDNS. | No | Yes | No | Yes |
CPU | BIG-IP DNS distributes DNS name resolution requests to the virtual server that currently has the most CPU processing time available. | No | Yes | No | Yes |
Hops | BIG-IP DNS distributes DNS name resolution requests to a virtual server in the data center that has the fewest router hops from the client's LDNS. BIG-IP DNS uses the traceroute utility to track the number of router hops between a client's LDNS and each data center. | No | Yes | No | Yes |
Kilobytes/Second | BIG-IP DNS distributes DNS name resolution requests to the virtual server that is currently processing the fewest number of kilobytes per second. Use Kilobytes/Second only with virtual servers for which BIG-IP DNS can collect the kilobytes per second metric. | No | Yes | No | Yes |
Least Connections | BIG-IP DNS distributes DNS name resolution requests to virtual servers on BIG-IP® Local Traffic Manager™ (LTM®) that currently hosts the fewest connections. Use Least Connections only with LTM servers. | No | Yes | No | Yes |
Packet Rate | BIG-IP DNS distributes DNS name resolution requests to the virtual server that is currently processing the fewest number of packets per second. | No | Yes | Yes | Yes |
Quality of Service | BIG-IP DNS distributes DNS name resolution requests to virtual servers based on a score assigned to each virtual server that is calculated from current performance metrics. Use Quality of Service only when you have configured BIG-IP DNS to calculate an overall score for each virtual server based on performance metrics. | No | Yes | No | Yes |
Round Trip Time | BIG-IP DNS distributes DNS name resolution requests to the virtual server with the fastest measured round trip time between a data center and a client's LDNS. | No | Yes | No | Yes |
Virtual Server Score | BIG-IP DNS distributes DNS name resolution requests to virtual servers on LTM based on a user-defined ranking. Use Virtual Server Score only with LTM systems on which you have assigned scores to each virtual server. | No | Yes | Yes | Yes |
Virtual Server Capacity | BIG-IP DNS distributes DNS name resolution requests to virtual servers in a list that are weighted by the number of available virtual servers in the pool. Use Virtual Server Capacity for load balancing virtual servers managed by LTM Systems. BIG-IP DNS selects a virtual server that has the most available (UP) members. When selecting a virtual server from a wide IP pool and two or more virtual servers result in equal scores, BIG-IP DNS will return one of the equal scored virtual servers randomly. | No | Yes | Yes | Yes |
About load balancing and resource availability
BIG-IP® DNS load balances DNS name resolution requests to resources based on availability. A resource is available when it meets one or more pre-defined requirements. BIG-IP DNS uses three methods to determine resource availability: a dependency on another resource, limit settings, or a set of values returned by a monitor. When BIG-IP DNS considers a resource unavailable, BIG-IP DNS attempts to select the next resource based on the current load balancing method.
About virtual server dependency
Within BIG-IP® DNS, you can configure a virtual server to be available based on the availability of other virtual servers.
Configuring virtual server availability to be dependent on the status of other virtual servers
Limit settings for resource availability
This table describes the limit settings BIG-IP® DNS uses to determine resource availability. A limit setting is a threshold for a statistic associated with a system.
Limit setting | Server-level | Pool-level | Virtual Server-level | BIG-IP Systems | Other Load Balancers | Hosts |
---|---|---|---|---|---|---|
Maximum allowable throughput in bits per second | Y | Y | Y | Y | Y | Y |
Packets | Y | Y | Y | Y | Y | Y |
Current connections | Y | Y | Y | Y | Y | Y |
Connection | N | N | Y | Y | N | N |
CPU | Y | N | N | N | Y | Y |
Memory | Y | N | N | N | Y | Y |
About wide IP-level load balancing
BIG-IP® DNS selects pools based on the order in which they are listed in a wide IP. When you organize pools in conjunction with the Global Availability, Ratio, Round Robin, and Topology load balancing methods, consider the order in which the pools are listed in the Pool List.
About the Global Availability load balancing method
The Global Availability load balancing method distributes DNS name resolution requests based on the order of resources in a list. Using global availability, BIG-IP® DNS sends a request to the first available resource in a list. Only when a resource becomes unavailable does BIG-IP DNS send requests to the next resource in the list. Over time, the first resource in the list receives the most requests and the last resource in the list receives the least requests.
Testing global server load balancing without verifying availability of virtual servers
About the Ratio load balancing method
The Ratio load balancing method distributes DNS name resolution requests among the virtual servers in a pool or among pools in a multiple pool configuration using weighted round robin, a load balancing pattern in which requests are distributed among several resources based on a priority level or weight assigned to each resource.
About wide IPs and weighting pools for the Ratio load balancing method
When you configure a wide IP to use the Ratio load balancing method, BIG-IP®DNS load balances DNS name resolution requests across the pools in the wide IP based on the weight assigned to each pool. BIG-IP DNS uses pool weight as a percentage of the total of the weights of all the pools in the wide IP to determine the frequency at which a pool receives connection requests.
- Pool 1: weight 50
- Pool 2: weight 25
- Pool 3: weight 25
- Pool 1: selected 50 percent of the time
- Pool 2: selected 25 percent of the time
- Pool 3: selected 25 percent of the time
About pools and weighting pool members for the Ratio load balancing method
When you configure a pool to use the Ratio load balancing method, BIG-IP® DNS, formerly Global Traffic Manager™ (GTM™), load balances requests across the pool members based on the weight assigned to each pool member (virtual server). The system uses pool member weight as a percentage of the total of the weights of all the members assigned to the pool to determine the frequency at which a pool member receives connection requests.
- Virtual Server 1: weight 50
- Virtual Server 2: weight 25
- Virtual Server 3: weight 25
- Virtual Server 1: selected 50 percent of the time
- Virtual Server 2: selected 25 percent of the time
- Virtual Server 3: selected 25 percent of the time
About the Round Robin load balancing method
The Round Robin load balancing method distributes DNS name resolution requests in a circular and sequential pattern among the virtual servers in a pool. Over time, each virtual server receives an equal number of connections.
About Topology load balancing
Topology load balancing distributes DNS name resolution requests based on the proximity of the client to the data center housing the resource that responds to the request. When Topology load balancing is enabled, the BIG-IP® system uses topology records to make load balancing decisions.
About pool-level load balancing
BIG-IP® DNS provides three tiers of pool-level load balancing to identify a virtual server to handle a DNS name resolution request.
- Preferred Load Balancing Method
- The first load balancing method BIG-IP DNS uses to return the IP address of a virtual server in response to a DNS name resolution request. The preferred method can be either static or dynamic.
- Alternate Load Balancing Method
- If the preferred load balancing method fails to return a valid resource in response to a DNS name resolution request, it is likely that BIG-IP DNS was unable to acquire the proper metrics to perform load balancing. The alternate method can be only static.
- Fallback Load Balancing Method
- If the alternate load balancing method fails to return a valid resource in response to a DNS name resolution request, BIG-IP DNS uses the fallback method. To ensure that BIG-IP DNS returns a response to a request, the fallback method ignores the availability status of a resource. The fallback method can be either static or dynamic.
If all of the configured load balancing methods fail to provide a valid resource in response to a DNS name resolution request, either the request fails or BIG-IP DNS uses the local BIND to resolve the request.
About the Drop Packet load balancing method
The Drop Packet load balancing method indicates that BIG-IP® DNS drops a DNS name resolution request. This load balancing method is most often selected for the Alternate load balancing method to ensure that BIG-IP DNS does note return an IP address for an unavailable resource.
About the Virtual Server Score load balancing method
The Virtual Server Score load balancing method distributes DNS name resolution requests to pool members (virtual servers) based on a user-defined ranking system.
About the Virtual Server Capacity load balancing method
Use Virtual Server Capacity for load balancing virtual servers managed by LTM Systems. BIG-IP DNS selects a virtual server that has the most available (UP) members. When selecting a virtual server from a wide IP pool and two or more virtual servers result in equal scores, BIG-IP DNS will return one of the equal scored virtual servers randomly.
About the Round Trip Times load balancing method
The Round Trip Times load balancing method distributes DNS name resolution requests to the pool member (virtual server) with the fastest measured round trip time between a data center and a client's LDNS.
About the Packet Rate load balancing method
The Packet Rate load balancing method distributes DNS name resolution requests to the pool member (virtual server) that is currently processing the fewest number of packets per second.
About the Least Connections load balancing method
The Least Connections load balancing method distributes DNS name resolution requests to pool members (virtual servers) that are managed by load balancing servers, such as BIG-IP® Local Traffic Manager™ (LTM™). BIG-IP DNS selects a pool member that currently hosts the fewest connections.
About the Kilobyte/Second load balancing method
The Kilobyte/Second load balancing method distributes DNS name resolution requests to the pool member (virtual server) that is currently processing the fewest number of kilobytes per second.
About the Hops load balancing method
The Hops load balancing method distributes DNS name resolution requests based on the traceroute utility and tracks the number of intermediate system transitions (router hops) between a client's LDNS and each data center. BIG-IP DNS distributes requests to a pool member in the data center that is the fewest router hops from the LDNS.
About the Completion Rate load balancing method
The Completion Rate load balancing method distributes DNS name resolution requests to the pool member (virtual server) that currently maintains the least number of dropped or timed-out packets during a transaction between a pool member in a data center and the client's LDNS.
About the CPU load balancing method
The CPU load balancing method distributes DNS name resolution requests to the pool member (virtual server) that currently has the most CPU processing time available.
About the Return to DNS load balancing method
The Return to DNS load balancing method immediately returns DNS name resolution requests to the LDNS for resolution. When you use this load balancing method, for client queries, the BIG-iP system increments the Return to DNS statistics; otherwise, the system increments the Return from DNS statistics.
About Static Persist load balancing
The Static Persist load balancing method uses the persist mask, with the source IP address of the LDNS, in a deterministic algorithm to send requests to a specific pool member (virtual server). Using this method, BIG-IP® DNS sends DNS name resolution requests to the first available pool member based on a hash algorithm that determines the order of the pool members. This algorithm orders the pool members differently for each LDNS that is sending requests to BIG-IP DNS, taking into account the Classless Inter-Domain Routing (CIDR) of the LDNS. As BIG-IP DNS distributes requests across all pool members, requests from each LDNS (and thus, each client) are generally sent to the same pool member. When the selected pool member becomes unavailable, BIG-IP DNS sends requests to another pool member. When the original pool member becomes available again, BIG-IP DNS sends requests to that pool member.
About the Fallback IP load balancing method
The Fallback IP load balancing method distributes DNS name resolution requests to a specific user-specified IP address. This IP address is not monitored for availability. Use this load balancing method only for the Fallback IP method and specifically to provide a disaster recovery site.
Verifying the availability of virtual servers when using the fallback load balancing method
About the None load balancing method
The None load balancing method skips the current load balancing method, distributes DNS name resolution requests to the next available pool in a multi-pool configuration.
About the QoS load balancing method
The Quality of Service (QoS) dynamic load balancing method uses current performance metrics to calculate an overall QoS score for each pool member (virtual server). When load balancing DNS name resolution requests, BIG-IP DNS selects a virtual server with the best overall QoS score. If virtual servers have identical scores, BIG-IP® DNS load balances connections to those virtual servers using the round robin method. If QoS scores cannot be determined, BIG-IP DNS load balances connections across all pool members using the round robin method.
Understanding the QoS equation
POOL_CONFIG->rtt * (GLOBALS->rtt / path->rtt) * 10 + POOL_CONFIG->hops * (GLOBALS->hops / path->hops) + POOL_CONFIG->hit_ratio * (path->hit_ratio / GLOBALS->hit_ration+ POOL_CONFIG->packet_rate * (GLOBALS->packet_rate / vs->packet_rate) * 100 + POOL_CONFIG->bps * (GLOBALS->bps / vs->bps) + POOL_CONFIG->topology * (topology_match->score / GLOBALS->topology) + POOL_CONFIG->vs_capacity * vs->cur_serv_cnt + POOL_CONFIG->vs_score * vs->cur_vs_score + POOL_CONFIG->lcs * vs->link->lcs * 10
Pool members (virtual servers) inherit the QoS settings from the pool. In the equation, the value of POOL_CONFIG->"setting name" can be found in the properties of a pool, the value of GLOBALS->"setting name" in the global BIG-IP DNS setting, and the value of path->"setting name" These are measured values that come from path metrics. If there are no path metrics, the system does not perform path metric calculations and computes the QoS score using the other calculations. vs->"field" These are measured values that come from measurements the system makes on virtual servers. If there are no measurements, the system does not perform these calculations and computes the QoS score using the other calculations. Each QoS coefficient, its scale, default value, upper limit, and whether a higher or lower value is more efficient are defined in the table.
Coefficient | Scale | Default value | Upper limit | Is higher or lower value more efficient? |
---|---|---|---|---|
Round trip time (rtt) | Microseconds | 50 | 2,000,000 | L |
Completion rate (hit ratio) | Percentage of successfully transferred packets (0-100%) | 5 | 100% | H |
Hops | Number of intermediate systems transitions | 0 | 64 | L |
Packet rate | Packets per second | 1 | 700 | L |
bits/second | Bits per second throughput | 3 | 15000 | L |
Topology | Score that defines network proximity by comparing server and LDNS IP addresses (0-232) | 0 | 100 | H |
Virtual server capacity (vs capacity) | Number of nodes up | 0 | 20 | H |
Virtual server score (vs score) | User-defined ranking of virtual servers | 0 | 100 | H |
Link capacity (lcs) | Based on the target dynamic ratio | 30 | 2,000,000 | H |
About customizing the QoS equation
When you customize the QoS equation, consider these three concepts:
- Scale
- The raw metrics for the coefficients in the QoS equation are on different scales. For example, completion rate is measured in percentages, while packet rate is measured in packets per second.
- Normalization
- BIG-IP DNS normalizes the raw metrics to values in the range of 0 - 10.
- Emphasis
- You can adjust coefficients to emphasize one normalized metric over another.
Coefficient | Value |
---|---|
Round Trip Time | 50 |
Hops | 0 |
Topology | 0 |
Completion Rate | 5 |
Packet Rate | 10 |
VS Capacity | 0 |
Bits/second | 35 |
Link Capacity | 30 |
Virtual Server Score | 10 |
Kilobytes/Second (KBPS) | 3 |
Customizing the QoS equation for load balancing global traffic
About dynamic ratio load balancing
When you use dynamic ratio load balancing, BIG-IP DNS treats dynamic load balancing values as ratios, and distributes DNS name resolution requests to the virtual servers in the pool in proportion to these ratios.