Application Delivery (ADX)

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Contributor
Posts: 49
Registered: ‎04-24-2009

Why should I use GSLB instead of DNS RR?

I am confronted with an internal discussion here about the advantages of GSLB compared to DNS round robin... I thought GSLB is for sure better but I have problems to find good arguments. Anyone?

Super Contributor
Posts: 316
Registered: ‎05-01-2009

Re: Why should I use GSLB instead of DNS RR?

Complex question. I could start to write tons of stuff related to this Q even if it sounds simple.

DNS round robin is a pretty simple way to distribute the load in between two datacenters/locations A client is going to send a DNS request out in case it wants to connect to www.brocade.com - the client gets a list of IP addresses back. Most of the clients are going to try to connect to the first address in the list. The DNS server is changing the order of the IPs in the list to achieve some kind of load balancing in between multiple IPs. It is going to change the order all the time. The first reply might contain the following three IPs in the following order (please ignore the fact that I am using private addresses):


Reply 1: 192.168.5.102 192.168.8.10 192.168.100.2

Reply 2: 192.168.8.10 192.168.100.2 192.168.5.102

Reply 3: 192.168.100.2 192.168.5.102 192.168.8.10

This is going to distribute the traffic - client 1 is most probably hitting IP .102, client 2 IP .10 and client 3 IP .2.

What is the problem with that? The DNS server is not really looking at things like availibility, proximity and performance. Most of the DNS servers are going to send the same set of IPs out all the time. The site with IP 192.168.100.2 might be down and 1/3rd of the client still do get this IP back as the first on in the list.

GSLB tries to solve these problems. I am going to write some more tomorrow but I hope this is already somehow useful.

Super Contributor
Posts: 316
Registered: ‎05-01-2009

Re: Why should I use GSLB instead of DNS RR?

DNS-based Global Server Load Balancing tries to improve the situation with DNS RR. A GSLB controller is going to act as ADNS server and it is actually influencing the DNS traffic. ServerIrons do act as some kind of DNS proxy - client queries do arrive at the SI and the SI is going to forward the DNS queries to real DNS servers in the backend. The replies are getting changed on the fly.

The GSLB controller is going to do health checks for all sites in regular intervals. The GSBL controller is collecting various statistics to determine the load of every site. It is going to remove IP addresses out of the replies which are not healthy right now and it is going to change the order based on the statistics collected. It is as well possible to send clients to the closest location from the clients point of view based on IP addresses.

There is a lot more but I would suggest to have a look at the documentation.

Is GSLB a perfect solution? No - all GSLB implementations are somehow similar and there are problems - of course. Have you ever seen something perfect? I have not.

GSLB is trying to make things a bit better and it is working in most of the scenarios and in thousands of installations.

Contributor
Posts: 49
Registered: ‎04-24-2009

Re: Why should I use GSLB instead of DNS RR?

Thx!

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