Multicast option in nlb cluster

p1xels

Hi all,
When should one choose the "Unicast" or "Multicast" option in NLB(Network Load Balancing) clustering? What should one choose and when & why?
Please advice.
Thanks & Regards

gespenstern

Did that several times, so AFAIR and not looking to docs, and assuming that you are talking about windows NLB (or similar) multicast means layer 2 multicast, i.e. multicast ethernet MAC address. This is a special type of address with a certain bit (or two bits) set to specific value. (IIRC it is 2E or 3E in second octet from left to right). Needs a switch ports to be configured to support this configuration. In general it is preferable because it doesn't flood the switch.

Unicast means that NICs of machines participating in NLB cluster receive additional MAC address and this MAC address is the same for all machines. Therefore when switch receives a frame with this address in destination field, the switch can't decide which port to pass this frame to because in its local table several ports are marked as having this MAC. Therefore switch passes this frame to all (or several?) ports, which means flood situation plus rises security concerns.

Anyways, NLB itself will work both ways just fine with almost all types of hardware (provided that if you choose multicast you configure the switch for multicast).

Hope someone with more fresh experience will correct me if I forgot something or omitted something critical...

p1xels

Hi
Thanks for answering. That was helpful but can you say which one is preferred -unicast or multicast in Windows NLB Clustering? Yes I am speaking about Windows 2008 clustering covered in MCITP.

gespenstern

Multicast is preferred. Pros: no flood, better security. Cons: needs hardware support from switch.

p1xels

Thanks gespenstern and a rep for you