1.1. Unicast mode


1.1.1. Advantage
· It always “works”. No special actions on the network level are needed. All registrations on network devices work automatically.

1.1.2. Disadvantages
· Traffic to the NLB cluster and to the individual NLB members is received by all servers in the server vlan
· Mutual communication between the NLB members is not possible without an extra NIC
1.1.3. Causes
· on layer 2: The switch registers the NLB members in its MAC table with this MAC-address: 02pp.xxxx.xxxx. “pp” is the priority of the NLB member. example: an NLB member with priority 1 and cluster ip address 89.17.67.197 gets MAC-adres 0201.5911.43c5. The second NLB member with priority 2 gets MAC-adres 0202.5911.43c5
· on layer 3: an arp-request to the NLB cluster or to an NLB member is always answered with MAC-address: 02BF.xxxx.xxxx where xxxx.xxxx is the hex representation of the NLB-cluster IP-address. example: 89.17.67.197 gets dynamically registered in the ARP-table with 02bf.5911.43c5

1.1.4. Impact
· Mutual communication impossibleWhen member1 wants to communicate with member2 of the same NLB it first sends an ARP-request. Member2 answers with 02BF-MAC-adres which also belongs to member1. As a result member1 is unable to sent out a packet.
· FloodingAll switches wants to deliver packets for the NLB member or the NLB cluster to the 02BF-MAC address. This MAC-address is not registered on in the switches MAC-table. Therefore these packets are sent out on every port of every switch. This behaviour is called “flooding”.As a result every NLB member receives the trafic destined for the NLB cluster. This is how the cluster gets his work done.The downside of this behaviour is that all other servers (other NLB’s or standalone servers) on the same VLAN also get spammed with this traffic.
1.1.5. Possible solutions:
· Mutual communication This can be solved by using an extra NIC on each NLB member. These non-NLB NICs register in the MAC-tables of the switches with their real MAC-adres. When the NLB member gets ARP’ed on this new NIC, it answers with its real MAC-adres. Because of this NLB members of the same cluster can now talk to each other.
· FloodingFor each NLB, create a vlan to limit the flooding to that vlan. This is very labour-intensive with regard to setup and maintenance