Anycast support in Citrix ADC 编辑

Anycast is a type of network where a set of servers shares an IP address. The client request is directed to the topographically closest server based on their routing tables. This routing reduces latency issues, ensures high availability, and minimizes downtime.

Citrix ADC supports anycast network with Global Server Load Balancing (GSLB) and DNS features.

The following diagram illustrates a topology diagram of Anycast in Citrix ADC.

Anycast GSLB

The Citrix ADC GSLB feature provides load balancing across globally distributed sites along with disaster recovery and ensures continuous availability of applications.

During an outage, GSLB provides immediate disaster recovery by routing traffic to the closest or the best performing data center. However, GSLB cannot control the following:

• How the DNS traffic is routed to GSLB nodes in different geographical locations.
• How much latency is getting added while DNS queries get routed to GSLB nodes.

In a typical GSLB setup, each data center has a GSLB node configured with the site-specific Authoritative Domain Name Server (ADNS) to receive DNS queries. Each site’s ADNS is configured as the nameserver in the DNS resolver. As the number of GSLB nodes increases, the number of nameserver records also increase. In such cases, if there is a failure of a data center, LDNS has to retry resolution with a different nameserver. This retry adds to the latency in DNS resolution. Also, every time a GSLB node is added, the nameserver records must be updated.

To overcome these drawbacks, you can use Anycast ADNS. In Anycast ADNS, a single ADNS IP address is used for all GSLB nodes and the DNS traffic is routed to GSLB nodes using dynamic routing.

For example, if a GSLB site is DOWN, the routing table is updated and route to this site is removed. As a result, The DNS queries are not sent to the sites that are DOWN. As a result, there are no retries.

If a new GSLB node is added, the new node is assigned the same ADNS IP address. The dynamic routing automatically updates the routing tables with routes to new sites based on the routing algorithms. Hence, you do not have to update the DNS name server records. The rollout of new GSLB sites is made simpler and faster with Anycast.

How to configure an ADNS IP address in an anycast mode

Enable host routing on the ADNS IP in a Citrix ADC appliance, and set the appropriate Route Health Injection (RHI) level. Mostly, there would not be any virtual servers on the ADNS IP and therefore RHI level must be selected as NONE. Enabling host route on the ADNS IP makes it a kernel route. You can then enable the dynamic routing of choice and configure the routing protocol to redistribute the kernel routes.

ADNS IP configuration – Example

At the command prompt, type;

add service adns_public 5.5.5.5 ADNS 53

set ip 5.5.5.5 -hostRoute ENABLED -vserverRHILevel ALL_VSERVERS
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