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Cost vs Priority in DNS

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Last Modified: 2008-01-09
Hi Experts,


I've been trying to understand the real difference between Cost and Priority in regards to DNS.

Example, if you have Round Robin setup, and you have 2 A records pointing to 2 different IP addresses, it'll split the load between them.

IF you have 2 different MX records, you can use cost or priority to have it utilize one server more than the other.

In the second case, what would the be difference between configuring cost or priority. Why would you configure one over the other?

Thanks.
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Jeremy WeisingerSenior Network Consultant / Engineer
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Commented:
When dealing with MX records I believe cost and priority are referring to the same thing.
LegendZMPrincipal Security Architect
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What about other things, generally speaking?  when dealing with mx specifically which would be recommended? Cost or Priority?
Brian PiercePhotographer
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Jeremy WeisingerSenior Network Consultant / Engineer
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Commented:
What I'm trying to say is that "cost" and "priority" are the same thing in an MX record. You can call it what you want but I believe most people refer to it as the priority.

Where did you hear/read the term "cost" referring to an MX record? Maybe I can help explain what they mean (or maybe my thinking will get adjusted)...
Brian PiercePhotographer
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Commented:
Thats my thinking also -
DNS Priorites OK -
DNS Costs ??

Now if we were taking about SRV records they have a Priority and a Weighting which can be confusing - but again there is no "cost"
LegendZMPrincipal Security Architect
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Commented:
That's what I meant, sorry for the confusion, Dns Priority and Weight.
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Commented:
Priority is used to determine preferred servers for the service (lowest number is highest priority).

Weighting is used for load balancing the service.

Normally, you don't have to change any of these numbers (with maybe the exception of the MX record priority) since AD Sites control much of the way lookups are referred.  A client will always be directed to a server in his/her own site for the service being requested IF that service is running on a local resource.

Brian PiercePhotographer
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OK glad we got that sorted as Netman66 says - if on doubt leave them alone !!
LegendZMPrincipal Security Architect
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Commented:
What does the weight do in the load balance situation?

So if you have 2 IIS servers, one priority at 100 the other at 50, more requests will go to the lower server? and if it's down, then it'll be referred to the other srever? Or will it split the load, in a 2:1 ratio, or is this where weight comes in?

Can you please give me some examples as to when you would enter weight specifically instead of Priority? Is it only when you have the 2 servers configured as a cluster/load balance? the one with the lower weight will handle more?
Brian PiercePhotographer
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Yes weight is in effect  a ratio and only comes into play when two records have the same priority and load balancing is being used.
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Commented:
This explains things pretty nicely.

http://en.wikipedia.org/wiki/SRV_record

Weighting only becomes a factor against two servers given the same priority.  A value of 0 disables weighting and speeds up SRV record enumeration because you're not forcing DNS to check two or more sources for availability.

LegendZMPrincipal Security Architect
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Commented:
Thank you both Netman and KCT.

Kct, so weight basically creates a ratio? ex: if one record is 50 and another is 100, the ratio would be 1:2 (or 2:1) The higher the weight the better or lower, like priority?
Do you have to enable the load balancing feature in windows? or by adjusting weight are you effectively load balancing?
LegendZMPrincipal Security Architect
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Commented:
Netman, so if you have 2 servers with the priority the same, but weight different, it'll then spilt a ratio between the 2 servers,  based on what their weight is regardless of round robin?
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Commented:
In theory, yes.  However we are speaking only about SRV records - this isn't meant for IIS-type load balancing (that's what Load Balancing on the NIC is for).

Two servers with the same priority will round-robin *if* they are in the same site as the client.

Honestly, there really is little need to change any of those numbers unless you have a very, very specific reason.

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