Ah, but there is no difference. In RR, you are not guaranteed which is the first, or subsequent, addess to be returned. Simply that the resolver will return all the addresses in a random order (without repeats).
Sent from my HTC Tilt™ 2, a Windows® phone from AT&T ________________________________ From: Don Andrews <don.andr...@safeway.com> Sent: Monday, February 28, 2011 2:49 PM To: MS-Exchange Admin Issues <exchangelist@lyris.sunbelt-software.com> Subject: RE: Load Balancing Incoming Email Using DNS MX Records I thought DNS would return equally weighted MX results in random or round robin order. ________________________________ From: Michael B. Smith [mailto:mich...@smithcons.com] Sent: Monday, February 28, 2011 9:05 AM To: MS-Exchange Admin Issues Subject: RE: Load Balancing Incoming Email Using DNS MX Records If the sender is implemented properly, there is no difference whatsoever in the OP’s question. Per RFC, both choices are completely random. (One being the DNS RFC, which used to be RFC 1034 for IPv4, I don’t know what it currently is; the second being the SMTP RFC, which is believe is RFC 2821, but it could’ve been updated.) A short excerpt from my June 2008 WindowsITPro article “DNS and Exchange”: Next, there is an MX record, and it points to mail.theessentialexchange.com. The host corresponding to this MX record is defined in this zone file as having an IP address of 64.202.166.12. An MX record for a zone indicates the destination of e-mail for that zone. The number (“10” in this example) that follows the MX is referred to as the “MX weight” and it is used to prioritize which MX record is used when there are multiple MX records. The lower the weight, the higher the priority of the MX record; thus a record having a weight of 10 would be used before a record with a weight of 20. If there are multiple records having the same weight, then a sending server should choose one randomly. Italics mine, of course. :) Regards, Michael B. Smith Consultant and Exchange MVP http://TheEssentialExchange.com From: Campbell, Rob [mailto:rob_campb...@centraltechnology.net] Sent: Monday, February 28, 2011 11:43 AM To: MS-Exchange Admin Issues Subject: RE: Load Balancing Incoming Email Using DNS MX Records I can’t think of any reason why having multiple MX records with the same precedence would cause failover to happen any faster if one is down. You may get less delay in getting incoming email processed with multiple MX records with the same precedence because it will be more of a load balancing arrangement than a failover arrangement. IMHO, the biggest downside is going to be when it comes to message tracking. If you’re load balancing across multiple mail servers then you’re always going to have to check the logs on all the servers. If it’s in a failover arrangement, then as long as the primary is up and responding you usually only have to check the logs on the primary server to find the information you’re looking for. From: Tu, Kevin [mailto:k...@ccscorporation.ca] Sent: Monday, February 28, 2011 10:30 AM To: MS-Exchange Admin Issues Subject: Load Balancing Incoming Email Using DNS MX Records Hello, It seems there are two ways to load balancing incoming email using DNS MX records. Method 1: Multiple MX records with equal priority. Method 2: One MX record with multiple IP address. What’s the pros and cons? Somebody suggested the Method 1, because multiple MX entries with the same priority will cause the mail to be delivered faster to the alternative server if the first one it tries is unavailable. Is that right? Thanks, This email and any files transmitted with it are solely intended for the use of the addressee(s) and may contain information that is confidential and privileged. If you receive this email in error, please advise us by return email immediately. Please also disregard the contents of the email, delete it and destroy any copies immediately. 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