Windows Network Load Balancing NLB and Cisco Routers/Switches

Problem: Windows NLB IPs are are not reachable through and from Cisco routers and switches. NLB services could be IIS arrays, Exchange CAS arrays, etc.

Solution: NLB mode was set to Multicast. In this mode Windows incorrectly uses multicast mac addresses. Set the NLB mode to Unicast and configure static mac address table entries on your switch to prevent broadcast flooding.

Discussion: Windows NLB does only work if all members of an array get all packets for a balanced service. To achieve this, Windows knows 3 modes of load balancing: Unicast, Multicast, and Multicast IGMP. And all of them have problems:

Unicast: Windows uses a normal mac address for his virtual IP address, but never sends any packet from this mac. The switch never learns a mac address entry and has to broadcast all packets for this mac. (Broadcast Flooding)

Multicast (IGMP): Windows uses mutlicast macs for the virtual IP. This seams correct, because this way the switch could learn which ports are part of the array and which are not (IGMP), but the problem is: an arp request for an unicast address (virtual IP) must not resolve to an multicast mac address. Cisco switches simply ignore this arp responses. Multicast mac addresses start with the lsb bit of the first bit set, typically 01:XX:XX:XX:XX:XX or 03:XX:XX:XX:XX:XX.

My solution was to use Unicast mode and don’t use IGMP. The other solution would be to statically set arp and the mac-address-table on the cisco switch, and force it to use the incorrect mac address.

BTW: A Network Load Balancing mechanism where every array member receives all traffic, in no real “Network” load balancing, because you dont’t reduce the traffic per server, it just adds additional computers and no additional network capacity.

More info:

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