you the man Ron...thanks for that, it was a great help.. I'm still slightly confused...so excuse the rambling in circles... The Microsoft documentation on NLB says something like this.... "In multicast mode, each cluster node makes its own unique MAC address based on the virtual IP address." So, each port on the switch will have a unique MAC address for each machine. If each packet is sent with that unique MAC address when returning packets to the client machine, and subsequent packets from the client will be addressed to that unique MAC address, then how would all cluster nodes receive that packet? My guess is that, the server must register two addresses on the switch, one for its unique MAC address (because the switch can only have unique MAC addresses) and one for the multicast address which each cluster node is listening on. If this is the case, then sureley when the cluster nodes are sending packets back to clients, then the source address given would be the multicast address and not the unique address as per the Microsoft description... I think I need to go and read some more... Thanks for the blurb Big Ron, it has helped... :o) PS. Big Ron is the standard name given to the sterotypical East End of London big gangster dude.. -----Original Message----- From: Ron Oglesby [mailto:roglesby@xxxxxxxxxxxx] Sent: 08 September 2003 17:37 To: thin@xxxxxxxxxxxxx Subject: [THIN] Re: slightly ot : load balancing win2k nfuse servers Not to pound the new book but since I already had it written here is a little blurb: NLB has the ability to work in two different modes: Unicast and Multicast. When activated NLB automatically places itself in unicast mode to allow itself to be more router friendly and kinda dummie proof. When in unicast mode NLB creates a new MAC address to use for the load balancing operation and assigns it to the network card that has NLB enabled on it. This then becomes the address that is used by all the hosts in the cluster. All incoming packets are then received by all cluster hosts, the NLB drivers are responsible for filtering which packets are for that host and which are not. When in multicast mode, NLB uses a multicast MAC address for the cluster adapters. This allows the hosts to retain their existing MACs and participate in the cluster by receiving the multicast packets. Both unicast and multicast modes have their benefits and drawbacks. One benefit of unicast mode is that it will work out of the box with all routers and switches (unlike multicast). The disadvantage is that the hosts in the cluster now all have the same MAC and IP address, due to this fact they do not have the ability to communicate with each other via their NLB network card. A second network card is required for communication between the servers. Multicast mode does not have the intra-server communication problem that unicast operation does. The reason for this is that even though all the hosts share the same IP and a Multicast MAC address they still retain their original IP address and MAC for their NLB enabled network card. The major drawback to multicast mode is that it generally requires some manual configuration on the network infrastructure side. This is generally the case where Cisco routers reject the ARP replies sent by hosts in the cluster. Cisco routers see the response to the ARP request that contains a unicast IP address with a multicast MAC address. The IOS in Cisco routers sees this as invalid and rejects the update to the ARP table. In order to resolve this, a manual ARP entry must be made in the router. Ron Oglesby Senior Technical Architect RapidApp Office 312.372.7188 Mobile 815.325.7618 email roglesby@xxxxxxxxxxxx -----Original Message----- From: Brian Lilley [mailto:Brian.Lilley@xxxxxxxxxxxxx] Sent: Monday, September 08, 2003 11:32 AM To: 'thin@xxxxxxxxxxxxx' Subject: [THIN] slightly ot : load balancing win2k nfuse servers sorry, couldn't remember the list address for win2k bits....besides, you lot will probably know the answer , well you better had or else I will stamp my feet and cry until you do..ok anyway..a question on network load balancing in a switched environment I want to build two load balanced IIS servers on win2k advanced server. I am trying to understand how NLB works in its two modes Unicast and Multicast. In Uni-cast I think that the cluster host responds with a unique source mac address and in multi-cast the cluster host responds with a multicast address... My confusion is this : In Unicast mode, if the switch sees the source MAC address and then remembers this as the MAC address for that port, presumably all subsequent frames for that MAC address will go out of that specific port only?? in this case, how would all cluster hosts receive the frames?? Brian Lilley Systems Integration m +44 (0)7929 002501 t +44 (0)1249 665421 e brian.lilley@xxxxxxxxxxxxxx ********************************************************************** The information contained in this e-mail message is intended only for the individuals named above. If you are not the intended recipient, you should be aware that any dissemination, distribution, forwarding or other duplication of this communication is strictly prohibited. The views expressed in this e-mail are those of the individual author and not necessarily those of Vivista Limited. Prior to taking any action based upon this e-mail message you should seek appropriate confirmation of its authenticity. 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