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Configuration Guide - IP Service

S7700 and S9700 V200R013C00

This document describes the configurations of IP service, including IP address, ARP, DHCP, DNS, mDNS gateway, mDNS relay, UDP Helper, IP performance optimization, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel and IPv4 over IPv6 tunnel.
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Multi-Interface ARP

Multi-Interface ARP

Background

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.

Implementation

An NLB server can work in unicast, multicast, or IGMP multicast mode. Currently, a device can connect to NLB servers working in only unicast or 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.

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.

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.

Upon receiving a packet destined for the cluster IP address, the device performs the following actions:
  1. Searches the ARP table for the cluster MAC address and VLAN based on the cluster IP address.
  2. Searches the matching MAC address table for multiple outbound interfaces based on the cluster MAC address and VLAN.
  3. Sends the packet through these outbound interfaces to set up connections with the NLB servers.
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Updated: 2019-04-20

Document ID: EDOC1100065743

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