Multi-Interface ARP

Multi-interface ARP allows devices to connect to network load balance (NLB) servers. NLB is developed by Microsoft for a cluster set up by multiple Windows servers. NLB supports the load balancing and redundancy backup of cluster servers. If a server in the cluster fails, service is switched to another server. To ensure a quick switchover, NLB requires that network devices send service traffic to each server in the cluster. Upon receipt, the servers use the same NLB algorithm to determine whether to process the service traffic.

An NLB server can work in unicast, multicast, or IGMP multicast mode.

• When NLB servers work in unicast mode, the cluster MAC address starts with 02bf.
• When NLB servers work in multicast mode, the cluster MAC address starts with 03bf.
• When NLB servers work in IGMP multicast mode, the cluster MAC address starts with 0100-5e.

In Figure 2-6, each server in the NLB cluster has its own IP and MAC addresses, and these servers also share a cluster IP address and MAC address. When the switch functioning as the access gateway connects to the NLB cluster, the switch needs to send the data packets destined for the cluster IP address to each server in the cluster.

When the Switch sends to the cluster an ARP Request packet to request the cluster MAC address, all the cluster servers send an ARP Reply packet to the Switch. In the ARP Reply packet, the source IP and MAC addresses are the cluster IP and MAC addresses. If multi-interface ARP is not configured on the Switch, the following problems occur when the Switch is connected to the NLB servers, depending on whether the servers are working in unicast or multicast mode:

• Unicast mode

  The Switch can learn only one outbound interface matching the cluster IP address in the ARP entry. Therefore, the Switch sends packets destined for the cluster IP address to only one server in the cluster.

• Multicast mode

  The Switch receives an ARP Reply packet and does not learn the ARP entry because the source MAC address is a multicast MAC address. If static ARP entries are configured on the Switch, only one outbound interface can be specified for binding the cluster IP and MAC addresses. If the Switch is enabled to dynamically learn ARP entries with multicast MAC addresses, the Switch can learn only one outbound interface. Therefore, the Switch sends packets destined for the cluster IP address to only one server in the cluster.

• IGMP multicast mode

  If an NLB server can send IGMP packets, the Switch can learn the outbound interface of the server through the packets. Only the NLB server that sends IGMP packets can receive packets destined for the cluster IP address.

Multi-interface ARP allows the Switch to send the packet destined for the cluster IP address to all the servers in the cluster.

Figure 2-6 Network diagram of multi-interface ARP

If multi-interface ARP is configured, the cluster IP and MAC addresses are bound in the static ARP table. The cluster MAC address and multiple outbound interfaces connecting to NLB servers are bound in the MAC address table.