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Configuration Guide - IP Service

CloudEngine 8800, 7800, 6800, and 5800 V200R002C50

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
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Proxy ARP

Proxy ARP

Possible Causes

ARP only applies to communication among hosts on both the same network segment and the same physical network. After a switch on a network receives an ARP Request packet from a host, the switch checks whether the destination IP address in the packet is its IP address to determine whether its MAC address is requested. If the destination IP address is its IP address, the switch responds with an ARP Reply packet. If the destination IP address is not its IP address, the switch discards the received ARP Request packet.

For hosts on different physical networks but on the same network segment, or hosts on both the same physical network and the same network segment but cannot communicate at Layer 2, you can configure proxy ARP on the switch between the two networks to allow communication among these hosts. If the proxy ARP-enabled switch detects that the destination IP address in a received ARP Request packet is not its IP address, the switch sends an ARP Reply packet containing its MAC address and the IP address of the destination host to the source host. In this process, the switch responds with an ARP Reply packet for the destination host.

Usage Scenario

Proxy ARP is classified into the following types: routed proxy ARP, intra-VLAN proxy ARP, and inter-VLAN Proxy ARP. Table 2-1 describes the usage scenarios.

Table 2-1  Proxy ARP usage scenarios

Proxy ARP Type

Usage Scenario

Routed proxy ARP

Two hosts that need to communicate belong to the same network segment but different physical networks (different broadcast domains). The gateways to which hosts are connected have different IP addresses.

Intra-VLAN proxy ARP

Two hosts that need to communicate belong to the same network segment on the same physical network and are in the same VLAN, but Layer 2 isolation is configured on ports in the VLAN.

Inter-VLAN proxy ARP

Two hosts that need to communicate belong to the same network segment on the same physical network, but are in different VLANs.

NOTE:
In VLAN aggregation scenarios, inter-VLAN proxy ARP can be enabled on the VLANIF interface corresponding to the super-VLAN to implement communication between sub-VLANs.

Implementation

  • Routed proxy ARP

    If hosts on the same network segment but different physical networks need to communicate while the gateways connecting to these hosts have different addresses, you need to enable routed proxy ARP on the switch interface connecting to the hosts.

    As shown in Figure 2-4, the IP address of Host A is 10.10.10.1/16 and that of Host B is 10.10.11.1/16, and Host A and Host B are located on the same network segment. The Switch connects to two networks through VLANIF 10 and VLANIF 20. The IP addresses of VLANIF 10 and VLANIF 20 are located on different network segments. The IP addresses of Host A and Host B are on the same network segment. When Host A needs to communicate with Host B, Host A broadcasts an ARP Request packet, requesting the MAC address of Host B. However, Host A and Host B are on different physical networks (in different broadcast domains). Host B cannot receive the ARP Request packet sent from Host A and does not respond with an ARP Reply packet.
    Figure 2-4  Routed proxy ARP implementation

    You can enable routed proxy ARP on VLANIF 10 and VLANIF 20 of the Switch to solve this problem.

    1. Host A sends an ARP Request packet for the MAC address of Host B.
    2. After the Switch receives the ARP Request packet, the Switch checks the destination IP address of the packet and finds that it is not its own IP address and determines that the requested MAC address is not its MAC address. The Switch then checks whether there are routes to Host B.
      • If no route to Host B is available, the Switch discards the ARP Request packet sent by Host A.
      • If there is a route to Host B, the Switch checks whether routed proxy ARP is enabled on the packet receiving interface.
        • If routed proxy ARP is enabled, the Switch sends an ARP Replay packet containing the MAC address of VLANIF 10 to Host A.

          After receiving the ARP Replay packet from the Switch, Host A considers the packet as the ARP Replay packet from Host B. Host A learns the MAC address of VLANIF 10 on the Switch and uses this MAC address to send data packets to Host B.

        • If routed proxy ARP is not enabled, the Switch discards the ARP Request packet sent by Host A.
  • Intra-VLAN proxy ARP

    If the hosts belong to the same VLAN with Layer 2 port isolation configured, intra-VLAN proxy ARP can be enabled on the interfaces associated with the VLAN to allow the hosts to communicate at Layer 3.

    As shown in Figure 2-5, Host A and Host B are connected to the Switch. The two interfaces connected to Host A and Host B belong to VLAN 4 on the Switch. Host A and Host B cannot communicate at Layer 2 because Layer 2 port isolation in a VLAN is configured on the Switch.
    Figure 2-5  Intra-VLAN proxy ARP implementation

    You can enable intra-VLAN proxy ARP on VLANIF 4 of the Switch to solve this problem.

    1. Host A sends an ARP Request packet for the MAC address of Host B.
    2. After receiving the ARP Request packet, the Switch detects that the destination IP address is not its IP address and determines that the requested MAC address is not its MAC address. The Switch then checks whether there is an ARP entry of Host B.
      • If there is an ARP entry that matches Host B and VLAN information in this entry is the same as that in the receiving port, the Switch checks whether intra-VLAN proxy ARP is enabled on the corresponding VLANIF interface.
        • If intra-VLAN proxy ARP is enabled, the Switch sends the MAC address of VLANIF 4 to Host A.

          After receiving the ARP Replay packet from the Switch, Host A considers the packet as the ARP Replay packet from Host B. Host A learns the MAC address of VLANIF 4 on the Switch and uses this MAC address to send data packets to Host B.

        • If intra-VLAN proxy ARP is not enabled, the Switch discards the ARP Request packet sent by Host A.
      • If there is no ARP entry of Host B, the Switch discards the ARP Request packet sent by Host A, and checks whether intra-VLAN proxy ARP is enabled on the packet receiving interface.
        • If intra-VLAN proxy ARP is enabled, the Switch broadcasts the ARP Request packet with the IP address of Host B as the destination IP address within VLAN 4. After the Switch receives an ARP Reply packet from Host B, the Switch generates an ARP entry indicating the mapping between the IP and MAC addresses of Host B.
        • If intra-VLAN proxy ARP is not enabled, the Switch does not perform any operations.
  • Inter-VLAN proxy ARP

    If hosts on the same network segment of the same physical network but in different VLANs need to communicate at Layer 3, you need to enable inter-VLAN proxy ARP on the corresponding VLANIF interfaces.

    As shown in Figure 2-6, Host A and Host B on the same network segment are connected to the Switch, Host A belongs to VLAN 3, and Host B belongs to VLAN 2. Host A and Host B belong to different sub-VLANs, so they cannot communicate at Layer 2.
    Figure 2-6  Inter-VLAN proxy ARP implementation

    You can enable inter-VLAN proxy ARP on VLANIF 4 of the Switch to solve this problem.

    1. Host A sends an ARP Request packet for the MAC address of Host B.
    2. After receiving the ARP Request packet, the Switch detects that the destination IP address is not its IP address and determines that the requested MAC address is not its MAC address. The Switch then checks whether there is an ARP entry of Host B.
      • If there is an ARP entry that matches Host B and VLAN information in this entry is different from that in the receiving port, the Switch checks whether inter-VLAN proxy ARP is enabled on the corresponding VLANIF interface.
        • If inter-VLAN proxy ARP is enabled, the Switch sends the MAC address of VLANIF 4 to Host A.

          After receiving the ARP Replay packet from the Switch, Host A considers the packet as the ARP Replay packet from Host B. Host A learns the MAC address of VLANIF 4 on the Switch and uses this MAC address to send data packets to Host B.

        • If inter-VLAN proxy ARP is not enabled, the Switch discards the ARP Request packet sent by Host A.
      • If there is no ARP entry of Host B, the Switch discards the ARP Request packet sent by Host A, and checks whether inter-VLAN proxy ARP is enabled on the corresponding VLANIF interface.
        • If inter-VLAN proxy ARP is enabled, the Switch broadcasts the ARP Request packet with the IP address of Host B as the destination IP address within VLAN 2. After the Switch receives an ARP Reply packet from Host B, the Switch generates an ARP entry indicating the mapping between the IP and MAC addresses of Host B.
        • If inter-VLAN proxy ARP is not enabled, the Switch does not perform any operations.
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Updated: 2019-03-21

Document ID: EDOC1000166635

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