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New Member

Network Load balancing

We are trying to configure two Microsoft 2000 servers using network load balancing.

Both servers are connected to a C4000 (6.1 software). When configured, the servers will share a mac address. This appears to be causing problems, as we lose connectivity to one or both servers when Load Balancing is enabled.

Is there a configuration that is recommended to enable Microsoft load balancing in a Cisco switched environment.

New Member

Re: Network Load balancing

I'm confused: when you say the servers will share a mac address., I read that to mean they will share 1 NIC card?? How is that possible?? Or do you mean they share 1 IP address?

New Member

Re: Network Load balancing

SOrry for the confusion. The servers are configured to share an IP Address across multiple NIC cards.


Re: Network Load balancing

New Member

Re: Network Load balancing

Hi David,

Your problem is normal. Normal when trying to load balanced with the Microsoft arrangement. Microsoft NLB is a really bad feature for Ethernet networks, i will explain:

You probably agree that Network Load Balance is a network function, as the name indicated. But to be able to do that (NLB), Microsoft develops a work around with some secondary effects. The work around is:

1- Microsoft is hiding the server's real MAC address of the servers when they communicates.

2. Microsoft revealed only the server's real address to an ARP request done by the upstream router, then upstream router is able to send packets to the servers.

The bad effects is:

1- Since switches are never able to learn the server's real MAC address, when they receive packets from the upstream router, they must retransmit the packets to each port in this Ethernet domain. It's not a broadcast, it's like a manual unicast to each ports you are asking to your switches to do since, servers never revealed their MAC address. Just look to your switch, all lights will flashed (passing packets) to each ports continously.

Conclusion: Microsoft NLB should never, NEVER, NEEVEERR be use except for testing purpose in lab or at home. Never in production.

Now, you can apply some workaround.

- As Microsoft mentioned in one paper i read about that, you can use Ethernet hub instead of switch. Yes, you read correctly, Microsoft say that, a hub for servers connection.

- They said also, what it's a little more intelligent, use VLAN to limit traffic to your servers only.

But the only way to do a good job, use a hardware load balancer.



New Member

Re: Network Load balancing

speaking from experence; I couldn't agree with you more.

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