Configuring Cisco to work with a Windows NLB Cluster

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This page or section provides device configuration instructions Please note that the information on this page has not been checked for accuracy and is not intended as a replacement to documentation. Please ensure you understand your desired objectives before attempting to apply any examples listed.

[edit] Introduction

To ensure high availability of critical network applications, the Operations team uses Microsoft's Network Load Balancer or NLB. NLB allows load balancing between servers without a dedicated stand alone box. To achieve this Microsoft performs some network hacks which require configuration on the network side.

NLB can work in two modes, unicast and multicast. Unicast is the default operation mode due to compatibility. In this mode the switch will have a common unicast mac-address. Since dynamic CAM entries cannot exist for two ports the switch floods all traffic going to a NLB host.

With multicast mode turned on the NLB uses a shared multicast mac-address with a unicast IP address. This mode works better with a Cisco network but has two drawback Due to this inconsistency with a unicast ip address and a multicast mac-address the router will refuse to learn the ARP for the IP address. In addition the switch cannot determine the membership ports in the multicast stream the traffic is still flooded to all hosts., although being a multicast mac-address has a lesser effect on the CPU of flooded hosts.

Fortunately we can create both static ARP entries and static CAM entries to allow the NLB hosts to behave correctly.

[edit] Creating a Static ARP entry

Have the server team configure the NLB for multicast mode and read off the generated mac-address as well as the desired IP address. Log into the default gateway for the NLB and issue the following command:

For IOS

arp 10.1.12.18 03bf.0a01.0cdf ARPA

For PixOS

arp dmz 192.168.0.73 03bf.c0a8.0047

At this point the NLB ip address should be reachable. If not, double check the IP and mac-address with the server team.

[edit] Creating a Static CAM entry

First determine the two or more physical ports that are being used for the NLB. Server team should be able to help you trace these down with the physical mac address. Log into the switch which these ports are plugged into. This may or may not be the same device as above.

For IOS

mac-address-table static 03bf.0a01.0cdf vlan 12 interface fa4/5 fa5/5

Ensure that you still have connectivity to the NLB cluster

[edit] References

• Catalyst Switches for Microsoft Network Load Balancing Configuration Example, Cisco Document ID: 107995