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CLI-based Configuration Guide - IP Service

AR500, AR510, AR531, AR550, AR1500, and AR2500 V200R010

This document describes the concepts and configuration procedures of IP Service features on the device, and provides the configuration examples.
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Multi-Interface ARP

Multi-Interface ARP

Network Load Balance (NLB) is a Microsoft implementation of clustering and load balancing on Windows servers. Servers in an NLB group support load balancing and redundancy. When a server in the group fails, data can be quickly switched to other servers. To implement quick data switching, devices on the network must be able to forward service traffic to each server in the NLB group. Each server in the NLB group uses the same NLB algorithm to determine whether to process the service traffic.

NLB servers can work in unicast, multicast, and IGMP multicast mode. Currently, the device can only be connected to NLB servers working in unicast or multicast mode.

  • When NLB servers work in unicast mode, the virtual MAC address starts with 02BF.
  • When NLB servers work in multicast mode, the virtual MAC address starts with 03BF.

As shown in Figure 2-7, each server in the NLB group has its own IP address and MAC address. In addition, the servers share a virtual IP address and a virtual MAC address. Router functions as the access gateway and directly connects to the NLB group. Router needs to forward packets destined for the virtual IP address to all servers in the NLB group.

After servers in the NLB group receive ARP Request packets for the virtual MAC address from Router, all the servers return ARP Reply packets. In the ARP Reply packets, the protocol source IP address is the virtual IP address and the protocol source MAC address is the virtual MAC address.

  • When NLB servers work in unicast mode, Router learns only one outbound interface (interface connecting Router to an NLB server) from the ARP entry matching the NLB group virtual IP address in the ARP table. Therefore, Router can forward packets destined for the virtual IP address to only one server in the NLB group.
  • When NLB servers work in multicast mode, Router does not learn ARP entries after receiving ARP Reply packets from NLB servers because the protocol source MAC address is a multicast MAC address. If you configure a static ARP entry by binding the virtual IP address to the virtual MAC address, only one outbound interface is specified. Therefore, Router can forward packets destined for the virtual IP address to only one server in the NLB group.

To resolve the preceding problems, you can deploy multi-interface ARP to allow Router to forward packets destined for the virtual IP address to all servers in the NLB group.

Figure 2-7  Typical networking diagram for multi-interface ARP

In multi-interface ARP, a static ARP entry is configured by binding the virtual IP address to the virtual MAC address, and MAC address entries are configured by binding the virtual MAC address to multiple outbound interfaces. When Router forwards packets destined for the virtual IP address, Router first searches for the ARP entry matching the virtual IP address to determine the virtual MAC address and the VLAN to which the NLB servers belong. Router then searches the MAC address table for multiple outbound interfaces based on the virtual MAC address and VLAN. Router forwards packets to each connected NLB server through the outbound interfaces.

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Updated: 2019-05-20

Document ID: EDOC1100034231

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