1.1. Multicast mode


1.1.1. Advantage
· Mutual communication between the NLB members is possible without an extra NIC (compared to Unicast mode)
1.1.2. Disadvantages
· It does not work automatically on all networkequiment (ex. Cisco)
· Traffic to the NLB cluster and to the individual members is received by all servers in the server vlan
1.1.3. Causes
· on layer 2: When choosing multicast as NLB operation mode, the original MAC-adresses of the NLB members are not changed. The switch registers the NLB members in its MAC table with their original MAC-addresses.
· on layer 3: an ARP-request to the NLB cluster is always answered with MAC-address: 03BF.xxxx.xxxx. xxxx.xxxx is the hex representation of the NLB-cluster IP-address. These MAC-addresses are multicast MAC addresses because the broadcast/multicast bit (0100.0000.0000) is set.example: 89.17.67.197 has 03bf.5911.43c5 in its ARP response. This MAC-address does not get registered automatically in a Cisco router or L3-switch because Cisco does not accept an ARP response which connects an Unicast IP adres with a multicast MAC-address.An ARP-request to a NLB member is answered with the original MAC-address of that member. The ARP-responses are registered automatically in the ARP-table.
1.1.4. Impact
· Cluster not reachable from behind routerThe cluster ip-address cannot be reached from behind a Cisco router or L3-switch
· FloodingAll switches wants to deliver packets for the NLB member or the NLB cluster to the 03BF-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 members (other NLB’s or standalone servers) on the same VLAN also get spammed with this traffic.
1.1.5. Possible solutions:
· Cluster not reachable from behind Cisco router Adding a static entry in the ARP-table of the router makes the cluster available from behind the routerSWCOGD01(config)# arp 89.17.67.197 03bf.5911.43c5 ARPA

· FloodingFor each NLB, create a vlan to limit the flooding. This is very labour-intensive with regard to setup and maintenance