Global Server Load Balancing
(GSLB) Concepts
Section
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Section Objectives
GSLB Overview
GSLB Configuration Options
GSLB Components
Server Mode Configuration
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Global Server Load Balancing (GSLB)
Key ACOS GSLB benefits
Provides data center failover and
continuity
Optimizes multi-site deployments
Ensures users' Web experience is
the fastest
DNS Proxy Technology
Continue to use existing DNS infrastructure without changing DNS server configuration
No need to create or delegate sub domains, existing DNS maintains control
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Types of Global Server Load Balancing
DNS-Based
Global Server Load Balancing (GSLB)Global Server Load Balancing enables AX to add intelligence
to authoritative Domain Name System (DNS) servers
The GSLB controller evaluates the DNS replies and based on the results of that evaluation it
directs traffic to the 'best' site by replacing the IP address in the DNS reply
IP-Based - Route Health Injection (RHI)
Routing based global server load balancing
RHI allows the ACOS to advertise the availability of a VIP throughout the network.
Inject static route for VIP and redistribute to routing protocol, support RIP, OSPF, IS-IS, BGP, RIPng,
OSPFv3, IS-ISv6, BGP4+
Typical topology includes primary and backup site, with backup monitoring primary’s health, and
inject VIP route in case of primary failure
Also supports 'IP Anycast'
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AX DNS-based GSLB Overview
DNS-based GSLB uses Domain Name Service (DNS) technology to extend load balancing
to a global scale
Provides dynamic and flexible policies for selecting fairness and distribution to multiple
sites
Operates in two main modes
Proxy mode
The ACOS device acts as a proxy for an external DNS server. In proxy mode, the ACOS device can update the
A and AAAA records in its response to client requests, but it forwards requests for all other record types to
the external DNS server.
Server mode
The ACOS device directly responds to queries for specific service IP addresses in the GSLB zone. In server
mode, the ACOS device can reply with A, AAAA, MX, NS, PTR, SRV and SOA records. For all other records, the
ACOS device will attempt proxy mode unless configured as fully authoritative.
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ACOS GSLB Proxy Mode
Advantages
Can be implemented without impacting current DNS traffic
Does not require change in DNS server IP address
Customer can be using external DNS service
Disadvantages
Requires changes to DNS server configuration
Add Sub-domain to existing DNS for ACOS
Add ACOS “proxy ip” as NS records
Add ACOS “proxy ip” as A records
CNAME existing records to sub-domain
Requires second DNS request by client
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ACOS GSLB Server Mode (Authoritative)
Advantages
Does not require changes to current DNS server configuration
Single client request for domain resolution services
Can be implemented with DNS firewall, and provide SLB services to DNS servers
Disadvantages
Requires changes to DNS server IP address, or change in registered NS server IP address
Can not be implemented without downtime
Customer has to own and run their own DNS servers
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GSLB Components
Controller
Sites
Receives client DNS requests, maintains
GSLB configuration and health status among
site devices. Can have multiple controllers
for redundancy
Policy
Configurable parameters evaluated against a
client request to select the best site to send
the request to
Zones
A DNS domain for GSLB. A device can be
configured with one or more GSLB zones.
Each zone can contain one or more GSLB
sites. “xyz.com” is a domain.
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A server farm locally managed by an ACOS
device that performs ADC services for the
site
Services
An application such as HTTP or FTP. Each
zone can be configured with one or more
services. “www.xyz.com” is a service where
“www” is the http service or an application
in the “xyz.com” zone
Service IP
The virtual servers defined under service-ip
are used for GSLB
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GSLB Server Mode Configuration
Configuration steps
Configure SLB (if not already configured)
Create DNS Server VIP
Configure Service IPs for VIPs
Create (or modify existing Default) GSLB Policy
Create Sites, add SLB Devices and VIPs for the Site
Create Zone and configure service
Enable the GSLB protocol for site device function (Controller or Device)
Note – To configure Proxy mode, follow standard SLB procedures (Servers, Service
Groups, VIP, etc.) that utilize “external” DNS servers and enable it for GSLB when
configuring the virtual port
Note 2 – GSLB Policies will be covered in another module
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Configuring the DNS VIP
For Server Mode configurations
Create the Virtual Server
slb virtual-server dns1 100.0.0.53
Add the UDP port (usually 53)
port 53 dns-udp
Enable GSLB on the port
gslb-enable
To configure Proxy Mode, create Servers for the actual (external) DNS servers, place
them in a Service Group and apply to the Virtual Port
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Configuring Service IPs
The Service IPs are the addresses of Virtual Servers that will be part of the GSLB solution
in a given zone
Add the name and ip, then the port hosting the service
gslb service-ip vip3 100.0.0.66
port 80 tcp
The Service IP can also have health checks assigned and, if needed, an External IP
allowing a service IP that has an internal IP address to be reached from outside the
internal network
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GSLB Site configuration
Sites represent the server farm that is locally managed by the device that performing
server load balancing for the site
Create the site, define the IP of the ACOS device for the site, then add the VIP servers
configured earlier
gslb site newyork
slb-dev A3 60.0.0.1
vip-server vip2
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Configuring GSLB Zones and Services P 1
A zone is a DNS domain used by GSLB and acts as the start of authority for the name
space and, when combined with the service name, creates the FQDN for client DNS
queries
A service is an application such as HTTP or FTP and can be the well-known name of the
application or by port number
gslb zone a10class.com
service http www
In the above example, the zone name is “a10class.com”, the service is HTTP with the
name “www.” Clients would then query www.a10class.com when connecting to the VIP
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Configuring GSLB Zones and Services P 2
The dns-a-record command is used to create the A records for the zone, binding the
service/zone name to the service IPs (VIPs) within the zone
gslb zone a10training.com
service http www
dns-a-record vip2 static
dns-a-record vip1 static
At the Service level of the configuration, additional dns records such as C-NAME, mx,
and NS can be created
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The GSLB Protocol
Uses TCP port 4149
AX devices use the GSLB protocol for GSLB management traffic (between GSLB controller and
sites)
The GSLB controller collects following information from the site AX load balancers
Virtual IP addresses & active servers
aRDT (active-Round Delay Time)
Site session capacity statistics
Connection load
Number of active sessions
Update interval default is 30 seconds (ranges from 1 to 300 seconds)
VIP information is sent asynchronously
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Enabling the GSLB Protocol
AX devices use the GSLB protocol for GSLB management traffic. The protocol must be
enabled on the GSLB controller
gslb protocol enable controller
For redundancy, multiple controllers can be enabled and placed in a controller group
which can automatically synchronize GSLB configurations and service IP status among
multiple GSLB controllers for a GSLB zone
Enabling the protocol on devices in other sites in the GSLB configuration is optional, but
is required for in order to take advantage of certain policy options and default health
checks. A10 recommends enabling the GSLB protocol on all devices
gslb protocol enable device
Note - For more information on Controller Groups see the GSLB configuration guide
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GSLB Configuration Best Practices
For redundancy, use Controller Groups with Controllers configured in multiple sites
Use Controllers for both GSLB and SLB
Server Mode (authoritative) configurations can also have the customers existing DNS
servers in a service group under the DNS VIP. These servers hold records or name space
for which the Controller is not authoritative. Non-authoritative queries are
automatically forwarded to those servers .
Enable the GSLB protocol on all devices
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GSLB Policy
Section
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Section Objectives
Policy Overview
Policy Metrics
Policy Settings
Policy Configuration
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GSLB Policy
A list of metrics used to determine the best site to use for a given client’s request
Health Check, Round Robin and Geographic enabled by default but can be disabled
All other metrics must be enabled to be used
Applied to the zone or service level within a zone
Features a “Default” policy which is used for all GSLB zones and services unless an
Admin created policy is applied to a zone or service
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GSLB Policy – Metric Evaluation
Each Site metric is evaluated in a (configurable) order and is marked when a match
occurs
Evaluations continue only on marked sites until all configured parameters are checked
Once each Site is evaluated, the user request is sent to the Site with the most matches
In the event of a tie, requests are fulfilled in round robin
Four Site Example: Site A, Site B, Site C and Site D all could potentially handle a client request
Site B fails Health Check, leaving A, C and D for the next metric
Site A and D match on Geographic, eliminating C
Site A has an assigned higher weight than D, eliminating D
Request will be sent to Site A
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GSLB Policy Settings 1 of 2
Health Check (1)
Weighted Site (3)
Services that pass health checks are
preferred *
Sites with higher assigned weights are used
more often
Round Robin (14)
Session Capacity (4)
Sites are selected in sequential order *
Sites with more available sessions based on
respective maximum Session-Capacity are
preferred
Geographic (7)
Services located within the client’s
geographic region are preferred *
Active-Servers (5)
Sites with most currently active servers are
preferred
Weighted-IP (2)
Service IP with higher assigned weight are
used more often than the service-IP with
lower weights
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NOTE - Numbers in parentheses represent default
metric order number which can be modified
* Enabled by default but can be disabled
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GSLB Policy Settings 2 of 2
Active Round Delay Time (6)
Admin Preference (10)
Sites with faster round delay times for DNS
queries and replies between a site and local
GSLB are preferred
The site with the highest admin set
preference is selected
BW-Cost (11)
Connection Load (8)
Sites that are not exceeding their thresholds
for new connections are preferred
Num-Session (9)
Sites that are not exceeding available session
capacity threshold compared to other sites
are treated as having the same preference
Selects sites based on bandwidth utilization
on the site AX links
Least Response (12)
Service IP addresses with the fewest hits are
preferred
Admin-IP (13)
IP addresses are preferred based on
administratively assigned weight
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Policy Configuration
To create a Policy use the following
gslb policy [name]
Once in the policy’s context, enable and configure policy entries. Some metrics are
enabled by entering the name of the metric
(config-gslb policy)#least-response
Other metrics are first configured at the site or zone level and then enabled by adding
them to the policy
(config-gslb site-slb dev)#admin-preference ?
<0-255>
Specify admin-preference value, default is 100
In the above example, a priority is set at the device level of a site, the metric will then be
evaluated once enabled on the policy
(config-gslb policy)#admin-preference
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Modifying Metric Order – CLI
Use the “metric order” command under the context of the policy followed by the
metrics you wish to use:
(config-gslb policy)#metric-order least-response admin-preference
Using the above example, least-response and admin-preference are now 1 and 2 in the
evaluation order. Heath-check, being the previous number 1 drops to 3
#show gslb policy pol1
-----------------------------------least-response
| 1 |
| yes
admin-preference | 2 |
| yes
health-check
| yes
| 3 |
The above example is only partial output for the command
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Modifying Metric Order – GUI
Config> GSLB> Policy. Select Policy, Drag and drop to modify metric order
Dragging metrics to the left will automatically enable them. Once enabled, these metrics
can also be dragged up or down to put them in desired order.
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Applying GSLB Policies – CLI
At the zone level
(config)#gslb zone a10training.com
(config-gslb zone)#policy pol-1
At the service level
(config)#gslb zone a10training.com
(config-gslb zone)#service http www
(config-gslb zone-gslb service)#policy pol-2
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Applying GSLB Policies – GUI
Config> GSLB> Zone
Click zone name and choose a policy from dropdown for Zone level.
To apply at Service level, from the Zone page, select the service name and click edit.
Choose policy from the dropdown.
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GSLB Policy Best Practices
For “Active Standby” data centers use Admin IP policy to always send traffic to primary
site, unless it is unreachable
For “Active Active” scenarios, take advantage of geo-location, weighting, or RTT to
determine best site to send client request
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