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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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Proxy ARP

Proxy ARP

If an ARP Request packet is sent to a host on a different network, the device that connects the two networks can reply to this packet. This function is called proxy ARP.

Proxy ARP has the following characteristics:

  • Proxy ARP is implemented on the ARP subnet gateway without any modifications on any hosts.
  • Proxy ARP can shield topologies of physical networks so that hosts on different physical networks can use the same network ID to communicate. Proxy ARP enables hosts that are on the same network segment but on different physical networks to communicate.
  • Proxy ARP affects only the ARP caches on hosts but does not affect the ARP cache or routing table on the gateway.
  • After proxy ARP is enabled, the aging time of ARP entries on hosts should be shortened so that invalid ARP entries can be deleted as soon as possible. Then IP packet forwarding failures decrease on the router.

Proxy ARP Type

Resolved Issue

Routed proxy ARP

Allows hosts on the same network segment but on different physical networks to communicate.

Intra-VLAN proxy ARP

Allows isolated hosts in a VLAN to communicate.

Inter-VLAN proxy ARP

Allows hosts in different VLANs or hosts in different sub-VLANs of the same VLAN to communicate at Layer 3.

Routed Proxy ARP

Routed proxy ARP enables network devices on the same network segment but on different physical networks to communicate.

In practice, if a host connected to a router is not configured with a default gateway address (that is, the host does not know how to reach the intermediate system of the network), the host cannot transmit packets.

As shown in Figure 2-4, RouterA is connected to two networks through VLAN10 and VLAN20. The IP addresses of VLANIF10 and VLANIF20 are on different network segments. However, the masks make HOSTA and VLANIF10 on the same network segment, HOSTB and VLANIF20 on the same network segment, and HOSTA and HOSTB on the same network segment.

Figure 2-4  Application of routed proxy ARP

The IP addresses of HOSTA and HOSTB are on the same network segment. When HOSTA needs to communicate with HOSTB, HOSTA broadcasts an ARP Request packet, requesting the MAC address of HOSTB. However, HOSTA and HOSTB are on different physical networks (in different broadcast domains). Therefore, HOSTB cannot receive the ARP Request packet sent from HOSTA and does not respond with an ARP Reply packet.

To solve this problem, enable proxy ARP on RouterA. After receiving an ARP Request packet, RouterA enabled with proxy ARP searches for the routing table corresponding to HOSTB. If the router corresponding to HOSTB exists, RouterA responds to the ARP Request packet with its own MAC address. HOSTA forwards data based on the MAC address of RouterA. RouterA functions as the proxy of HOSTB.

Intra-VLAN Proxy ARP

If two hosts belong to the same VLAN but are isolated, enable intra-VLAN proxy ARP on an interface associated with the VLAN to allow the hosts to communicate.

As shown in Figure 2-5, HOSTA and HOSTB are connected to RouterA. The two interfaces connected to HOSTA and HOSTB belong to VLAN10.

Figure 2-5  Application of intra-VLAN proxy ARP

HOSTA and HOSTB cannot communicate at Layer 2 because interface isolation in a VLAN is configured on RouterA.

To solve this problem, enable intra-VLAN proxy ARP on the interfaces of RouterA. After RouterA's interface connected to HOSTA receives an ARP Request packet whose destination address is not its own address, RouterA does not discard the packet but searches for the ARP entry corresponding to HOSTB. If the ARP entry corresponding to HOSTB exists, RouterA sends its MAC address to HOSTA and forwards packets sent from HOSTA to HOSTB. RouterA functions as the proxy of HOSTB.

Inter-VLAN Proxy ARP

If two hosts belong to different VLANs, enable inter-VLAN proxy ARP on interfaces associated with the VLANs to implement Layer 3 communication between the two hosts.

As shown in Figure 2-6, HOSTA and HOSTB are connected to RouterA. The interface connected to HOSTA belongs to VLAN10, and the interface connected to HOSTB belongs to VLAN20.

Figure 2-6  Application of inter-VLAN proxy ARP

The interfaces connected to HOSTA and HOSTB belong to different VLANs. Therefore, HOST A and HOSTB cannot communicate at Layer 2.

To solve this problem, enable inter-VLAN proxy ARP on the interfaces of RouterA. After RouterA's interface connected to HOSTA receives an ARP Request packet whose destination address is not its own address, RouterA does not discard the packet but searches for the ARP entry corresponding to HOSTB. If the ARP entry corresponding to HOSTB exists, RouterA sends its MAC address to HOSTA and forwards packets sent from HOSTA to HOSTB. RouterA functions as the proxy of HOSTB.

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Updated: 2019-05-20

Document ID: EDOC1100034231

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