By the nature of this logic, you can ensure a user only resolves the IP s in the region you specify. But what if you connect a pool with multiple members for a region? You would then have to user WIP level persistence in conjunction with topology.
A really good post. Thanks for posting this. I have a small question. Suppose Techglaze. The answer could vary depending on your DNS setup. What is authoritative for techglaze. Are they DNS Servers? Or Are they the GTMs in question? The GTM normally does not log wideip requests, so you would have to turn it on to catch some logs — careful if you have a busy system..
Enable wideip logging on v This post is very helpful to understand the GTM. I would like to thanks for the same. There is so much you can base your responses on, you can even write iRules and attach them to your objects to get even a more intelligent response.
For example, lets say I have a name server running BIND that is authoritative for the domain worldtechit. I could deletegate wideip. I could then create a wideip on my gtms called saas. The alternative would be to make the GTM authoritative for my whole domain worldtechit. Thanks Austin for this detailed explanation. It helped a lot. Is F5 a good career move.
No Problem Sachin! Personally I would never make a career move because of pay, make a career move because you have a passion for something, otherwise it may prove to be a poor decision.. Good Luck! Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. We typically respond same business day, but guarantee a response by the next business day.
Toggle navigation. This field is for validation purposes and should be left unchanged. Comments Good summary. Hey Austin, Thanks for the article, great comparisson. To learn more on load balancing, visit DevCentral.
Skip to main content Skip to footer Skip to search. Some industry standard algorithms are: Round robin Weighted round robin Least connections Least response time. Let's also assume that the ADC is already configured with a virtual server that points to a cluster consisting of two service points. In this deployment scenario, it is common for the hosts to have a return route that points back to the load balancer so that return traffic will be processed through it on its way back to the client.
This very simple example is relatively straightforward, but there are a couple of key elements to note. First, as far as the client knows, it sends packets to the virtual server and the virtual server responds—simple. Second, the NAT takes place. This is where the ADC replaces the destination IP sent by the client of the virtual server with the destination IP of the host to which it has chosen to load balance the request.
Third is the part of this process that makes the NAT "bi-directional". The source IP of the return packet from the host will be the IP of the host; if this address were not changed and the packet was simply forwarded to the client, the client would be receiving a packet from someone it didn't request one from, and would simply drop it.
Instead, the load balancer, remembering the connection, rewrites the packet so that the source IP is that of the virtual server, thus solving this problem. Usually at this point, two questions arise: How does the load balancing ADC decide which host to send the connection to?
And what happens if the selected host isn't working? Let's discuss the second question first. What happens if the selected host isn't working? The simple answer is that it doesn't respond to the client request and the connection attempt eventually times out and fails. This is obviously not a preferred circumstance, as it doesn't ensure high availability.
That's why most load balancing technology includes some level of health monitoring to determine whether a host is actually available before attempting to send connections to it. There are multiple levels of health monitoring, each with increasing granularity and focus. A basic monitor would simply ping the host itself.
If the host does not respond to the ping, it is a good assumption that any services defined on the host are probably down and should be removed from the cluster of available services. Unfortunately, even if the host responds to the ping, it doesn't necessarily mean the service itself is working. Therefore, most devices can do "service pings" of some kind, ranging from simple TCP connections all the way to interacting with the application via a scripted or intelligent interaction.
These higher-level health monitors not only provide greater confidence in the availability of the actual services as opposed to the host , but they also allow the load balancer to differentiate between multiple services on a single host. The load balancer understands that while one service might be unavailable, other services on the same host might be working just fine and should still be considered as valid destinations for user traffic.
This brings us back to the first question: How does the ADC decide which host to send a connection request to? Each virtual server has a specific dedicated cluster of services listing the hosts that offer that service that makes up the list of possibilities.
Additionally, the health monitoring modifies that list to make a list of "currently available" hosts that provide the indicated service. It is this modified list from which the ADC chooses the host that will receive a new connection. Deciding on the exact host depends on the load balancing algorithm associated with that particular cluster. Some of these algorithms include least connections, dynamic ratio and a simple round robin where the load balancer simply goes down the list starting at the top and allocates each new connection to the next host; when it reaches the bottom of the list, it simply starts again at the top.
While this is simple and very predictable, it assumes that all connections will have a similar load and duration on the back-end host, which is not always true. More advanced algorithms use things like current-connection counts, host utilization, and even real-world response times for existing traffic to the host in order to pick the most appropriate host from the available cluster services.
Sufficiently advanced application delivery systems will also be able to synthesize health monitoring information with load balancing algorithms to include an understanding of service dependency.
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