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Configuration Guide - IP Service

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DHCP policy VLAN, 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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Configuring Egress ARP Inspection

Configuring Egress ARP Inspection

Pre-configuration Task

Before configuring Egress ARP Inspection (EAI), configure DHCP snooping. For details, see DHCP Snooping Configuration in the S1720, S2700, S5700, and S6720 V200R012(C00&C20) Configuration Guide - Security.

Context

Figure 2-19 shows an example of a network using egress ARP inspection.

Figure 2-19  EAI networking diagram

In Figure 2-20, if all or most users connected to Switch_1 obtain IP addresses through DHCP and belong to the same VLAN, EAI can be enabled to prevent broadcast of ARP packets. EAI allows the device to look up the DHCP snooping table for the outbound interface matching the destination IP address. The device then sends the ARP Request packets directly from the outbound interface. This avoids needless broadcast of ARP Request packets, reduces the impact on the network, ensures normal service running, and prevents ARP attacks.

For example, if Switch_1 detects that the outbound interface IF3 is isolated from the inbound interface IF1, it discards the ARP Request packet sent from Host_1. As a result, Host_1 and Host_3 cannot communicate.

To solve this problem, Switch_1 is enabled to forward ARP packets to a trusted interface when the inbound and outbound interfaces are isolated, and intra-VLAN proxy ARP is configured on Switch_2.

When detecting that the outbound interface IF3 is isolated from the inbound interface IF1, Switch_1 directly forwards the received ARP Request packet to the trusted interface IF4 (in DHCP snooping, the interface connected to a DHCP server is configured as a trusted interface). As Switch_2 is configured with intra-VLAN proxy ARP, it sends an ARP Reply packet after receiving the ARP Request packet. Host_1 and Host_3 can now communicate with each other.

Figure 2-20  EAI networking diagram

Procedure

  1. Run system-view

    The system view is displayed.

  2. Run vlan vlan-id

    The VLAN view is displayed.

  3. Run dhcp snooping arp security enable

    EAI is enabled.

    By default, EAI is disabled.

    NOTE:
    • After EAI is enabled, the device sends all the received ARP packets to the CPU for software forwarding, which degrades the ARP packet forwarding performance.

    • EAI and MAC-forced forwarding (MFF) cannot be configured in the same VLAN because the two functions use mutually exclusive ARP mechanisms. MFF uses the proxy ARP mechanism, whereas EAI forwards ARP Request packets. For the MFF implementation, see MFF Configuration in the S1720, S2700, S5700, and S6720 V200R012(C00&C20) Configuration Guide - Security.

    • EAI enabled in a super VLAN does not take effect.

  4. (Optional) Run dhcp snooping arp security isolate-forwarding-trust

    The device is enabled to forward ARP packets to trusted interfaces when port isolation is enabled on the inbound and outbound interfaces.

    By default, the device is disabled from forwarding packets to trusted interfaces when port isolation is enabled on the inbound and outbound interfaces.

    NOTE:

    On the device enabled with EAI, if port isolation is configured on the interfaces connecting to users, you must perform this step and configure intra-VLAN proxy ARP on the uplink device. For example, if port isolation is configured on the interfaces connecting Switch_1 to users in Figure 2-19, perform Step 4 on Switch_1 and configure intra-VLAN proxy ARP on Switch_2. For the configuration of intra-VLAN proxy ARP, see Configuring Intra-VLAN Proxy ARP.

Verifying the Configuration

Run the display current-configuration | include dhcp snooping arp security command to verify the EAI configuration.

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Updated: 2018-12-24

Document ID: EDOC1100038342

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