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Strange problem with NLB on Exchange FE servers

Posted on 2007-11-14
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Last Modified: 2013-11-30
Hi,
I have recently set up our 2 front-end and SMTP servers in an MS NLB. I gave the NLB cluster the same IP as our original mail server to avoid changing any MX records or A records for the OWA. Everything works fine except for a strange problem for which I can find no defined errors.

To test the cluster I am using telnet on the port for the SMTP service (25). This connects fine every time from outside our organisation, but for some reason the ability to connect in this method internally is distinctly sporadic.

Can anyone shed any light as to why this might be happening, or at least somewhere that I can look to find out the errors?

Additionally, I have run Ethereal on a number of clients whilst attempting the connection tothe cluster and it seems that the cluster simply does not respond to some clients internally, despite the fact that the same client can ping the cluster and connections from outside the organsiation work fine. Also, clients that can't connect at one time seem to just start working with no changes being made.
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Question by:accidentexchange
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seeimred earned 1500 total points
ID: 20286007
I think in our testing we found that Windows NLB would not work well for FE & SMTP farms. We use DNS RoundRobin entries for them instead.
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by:accidentexchange
ID: 20287962
Thanks for the response. What I find strangest about this problem is that from outside our network, the load balance works perfectly, it is only when we try to connect from internal that the problem occurs.

I've increased the points available on this question as it is an urgent problem.
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by:Andrew_Wallbank
ID: 20610390
Do clients on your internal network pass through a router to get to the Mail server?  or is it on the same subnet?

Presumably the external device pass through a router/firewall?

it could be that the Internal devices are having trouble resolving the MAC address for the NLB (depending how it has been setup).  With traffic passing through a router/firewall these devices may be resolving them correctly.
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