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14.1 - Current Release 13.1 13.0 12.1
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Anycast support in Citrix ADC

August 16, 2023
Contributed by: 
C S

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 Topology

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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BGP configuration in GSLB site – Example 

Site1#sh run
!
hostname Site1
!
log syslog
log record-priority
!
ns route-install bgp
!
interface lo0
 ip address 127.0.0.1/8
 ipv6 address fe80::1/64
 ipv6 address ::1/128
!
interface vlan0
 ip address 10.102.148.94/25
 ipv6 address fe80::e84c:f4ff:fe74:4588/64
!
interface vlan2
 ip address 172.18.30.15/24
!
router bgp 5
 redistribute kernel -----> redistributing the kernel routes
 neighbor 172.18.30.30 remote-as 4
 neighbor 172.18.30.30 advertisement-interval 1
 neighbor 172.18.30.30 timers 4 16
!
End

Site1#
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GSLB site routing table - Example 

Site1#sh ip route
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, I - Intranet
       * - candidate default

K       5.5.5.5/32 via 0.0.0.0 ---------------------------------------> Kernel Route for ADNS
C       10.102.148.0/25 is directly connected, vlan0
C       127.0.0.0/8 is directly connected, lo0
B       172.18.10.0/24 [20/0] via 172.18.30.30, vlan2, 01w5d22h
B       172.18.20.0/24 [20/0] via 172.18.30.30, vlan2, 01w5d22h
C       172.18.30.0/24 is directly connected, vlan2
B       192.168.3.0/24 [20/0] via 172.18.30.30, vlan2, 01w5d22h
B       192.168.5.0/24 [20/0] via 172.18.30.30, vlan2, 01w5d22h
B       192.168.10.0/24 [20/0] via 172.18.30.30, vlan2, 01w5d22h

Gateway of last resort is not set
Site1#
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Anycast DNS

You can use Anycast DNS for DNS proxy virtual servers on Citrix ADC. When there are multiple DNS name servers configured, the DNS resolver responds based on round robin method. For example, if the resolver does not receive any response from the first server, it switches to the second server after the configured timeout value expires. The switching from first server to second server adds to the latency in DNS resolution. If the DNS resolvers are configured with Anycast, then this latency can be eliminated.

DNS configuration – Example

At the command prompt, type;

add lb vserver dns DNS 5.5.5.50 53

set ip 5.5.5.50 -hostRoute ENABLED -vserverRHILevel ALL_VSERVERS
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Anycast support in Citrix ADC
August 16, 2023
Contributed by: 
C S
August 16, 2023
Contributed by: 
C S

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