ARP uses two types of messages: ARP request and ARP reply. Figure 1 shows the format of ARP request/reply messages. Numbers in the figure refer to field lengths.

Figure 1 ARP message format

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· Hardware type—Hardware address type. The value 1 represents Ethernet.

· Protocol type—Type of the protocol address to be mapped. The hexadecimal value 0x0800 represents IP.

· Hardware address length and protocol address length—Length, in bytes, of a hardware address and a protocol address. For an Ethernet address, the value of the hardware address length field is 6. For an IPv4 address, the value of the protocol address length field is 4.

· OP—Operation code, which describes the type of ARP message. The value 1 represents an ARP request, and the value 2 represents an ARP reply.

· Sender hardware address—Hardware address of the device sending the message.

· Sender protocol address—Protocol address of the device sending the message.

· Target hardware address—Hardware address of the device to which the message is being sent.

· Target protocol address—Protocol address of the device to which the message is being sent.

ARP operating mechanism

As shown in Figure 2, Host A and Host B are on the same subnet. Host A sends a packet to Host B as follows:

1. Host A looks through the ARP table for an ARP entry for Host B. If one entry is found, Host A uses the MAC address in the entry to encapsulate the IP packet into a data link layer frame. Then Host A sends the frame to Host B.

2. If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request. The payload of the ARP request contains the following information:

¡ Sender IP address and sender MAC address—Host A's IP address and MAC address.

¡ Target IP address—Host B's IP address.

¡ Target MAC address—An all-zero MAC address.

All hosts on this subnet can receive the broadcast request, but only the requested host (Host B) processes the request.

3. Host B compares its own IP address with the target IP address in the ARP request. If they are the same, Host B operates as follows:

a. Adds the sender IP address and sender MAC address into its ARP table.

b. Encapsulates its MAC address into an ARP reply.

c. Unicasts the ARP reply to Host A.

4. After receiving the ARP reply, Host A operates as follows:

a. Adds the MAC address of Host B into its ARP table.

b. Encapsulates the MAC address into the packet and sends the packet to Host B.

Figure 2 ARP address resolution process

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If Host A and Host B are on different subnets, Host A sends a packet to Host B as follows:

1. Host A broadcasts an ARP request where the target IP address is the IP address of the gateway.

2. The gateway responds with its MAC address in an ARP reply to Host A.

3. Host A uses the gateway's MAC address to encapsulate the packet, and then sends the packet to the gateway.

4. If the gateway has an ARP entry for Host B, it forwards the packet to Host B directly. If not, the gateway broadcasts an ARP request, in which the target IP address is the IP address of Host B.

5. After the gateway gets the MAC address of Host B, it sends the packet to Host B.

ARP entry types

An ARP table stores dynamic ARP entries, OpenFlow ARP entries, Rule ARP entries, and static ARP entries.

Dynamic ARP entry

ARP automatically creates and updates dynamic entries. A dynamic ARP entry is removed when its aging timer expires or the output interface goes down. In addition, a dynamic ARP entry can be overwritten by a static ARP entry.

Static ARP entry

A static ARP entry is manually configured and maintained. It does not age out and cannot be overwritten by any dynamic ARP entry.

Static ARP entries protect communication between devices because attack packets cannot modify the IP-to-MAC mapping in a static ARP entry.

The device supports the following types of static ARP entries:

· Long static ARP entry—It is directly used for forwarding packets. A long static ARP entry contains the IP address, MAC address, and one of the following combinations:

¡ VLAN and output interface.

¡ Input and output interfaces.

· Short static ARP entry—It contains only the IP address and MAC address.

If the output interface is a Layer 3 Ethernet interface, the short ARP entry can be directly used to forward packets.

If the output interface is a VLAN interface, the device sends an ARP request whose target IP address is the IP address in the short entry. If the sender IP and MAC addresses in the received ARP reply match the short static ARP entry, the device performs the following operations:

¡ Adds the interface that received the ARP reply to the short static ARP entry.

¡ Uses the resolved short static ARP entry to forward IP packets.

· Multiport ARP entry—It contains the IP address, MAC address, and VLAN.

The device can use a multiport ARP entry that has the same MAC address and VLAN as a multicast or multiport unicast MAC address entry for packet forwarding. A multiport ARP entry is manually configured. It does not age out and cannot be overwritten by any dynamic ARP entry. For more information about multicast MAC, see IP Multicast Configuration Guide.

To communicate with a host by using a fixed IP-to-MAC mapping, configure a short static ARP entry on the device. To communicate with a host by using a fixed IP-to-MAC mapping through an interface in a VLAN, configure a long static ARP entry on the device.

OpenFlow ARP entry

ARP creates OpenFlow ARP entries by learning from the OpenFlow module. An OpenFlow ARP entry does not age out, and it cannot be updated. An OpenFlow ARP entry can be used directly to forward packets. For more information about OpenFlow, see OpenFlow Configuration Guide.

Rule ARP entry

Rule ARP entries can be directly used for packet forwarding. A Rule ARP entry does not age out, and it cannot be updated. It can be overwritten by a static ARP entry.

ARP creates Rule ARP entries by learning from the following modules:

· VXLAN. For more information about VXLAN, see VXLAN Configuration Guide.

· OVSDB. For more information about OVSDB, see VXLAN configuration in VXLAN Configuration Guide.

ARP tasks at a glance

All ARP tasks are optional.

· Configuring a static ARP entry

¡ Configuring a short static ARP entry

¡ Configuring a long static ARP entry

¡ Configuring a multiport ARP entry

· Configuring features for dynamic ARP entries

¡ Setting the dynamic ARP learning limit for a device

¡ Setting the dynamic ARP learning limit for an interface

¡ Setting the aging timer for dynamic ARP entries

¡ Enabling dynamic ARP entry check

· Synchronizing ARP entries across all cards

· Configuring a customer-side or network-side port

· Enabling interface consistency check for ARP entries and MAC address entries

· Enabling ARP logging

Configuring a static ARP entry

Static ARP entries are effective when the device functions correctly.

Configuring a short static ARP entry

Restrictions and guidelines

A resolved short static ARP entry becomes unresolved upon certain events, for example, when the resolved output interface goes down, or the corresponding VLAN or VLAN interface is deleted.

Procedure

1. Enter system view.

system-view

2. Configure a short static ARP entry.

arp static ip-address mac-address [ vpn-instance vpn-instance-name ]

Configuring a long static ARP entry

About long static ARP entries

Long static ARP entries can be effective or ineffective. Ineffective long static ARP entries cannot be used for packet forwarding. A long static ARP entry is ineffective when any of the following conditions exists:

· The corresponding VLAN interface or output interface is down.

· The IP address in the entry conflicts with a local IP address.

· No local interface has an IP address in the same subnet as the IP address in the ARP entry.

A long static ARP entry in a VLAN is deleted if the VLAN or VLAN interface is deleted.

Procedure

1. Enter system view.

system-view

2. Configure a long static ARP entry.

arp static ip-address mac-address [ vlan-id interface-type interface-number | vsi-interface vsi-interface-id tunnel number vsi vsi-name | vsi-interface vsi-interface-id interface-type interface-number service-instance instance-id vsi vsi-name ] [ vpn-instance vpn-instance-name ]

Configuring a multiport ARP entry

About multiport ARP entries

A multiport ARP entry contains an IP address, MAC address, and VLAN ID. The VLAN and output interfaces are specified by a multiport unicast MAC address entry or a multicast MAC address entry.

A multiport ARP entry can overwrite a dynamic, short static or long static ARP entry. Conversely, a short static or long static ARP entry can overwrite a multiport ARP entry.

Restrictions and guidelines

For a multiport ARP entry to be effective for packet forwarding, make sure the following conditions are met:

· A multiport unicast MAC address entry or a multicast MAC address entry exists.

· The IP address in the multiport ARP entry must reside on the same subnet as the VLAN interface of the specified VLAN.

Procedure

1. Enter system view.

system-view

2. Configure a multiport unicast MAC address entry or a multicast MAC address entry.

¡ Configure a multiport unicast MAC address entry.

mac-address multiport mac-address interface interface-list vlan vlan-id

For more information about multiport unicast MAC address entries, see the mac-address command in Layer 2—LAN Switching Command Reference.

¡ Configure a multicast MAC address entry.

mac-address multicast mac-address interface interface-list vlan vlan-id

For more information about multicast MAC address entries, see the mac-address multicast command in IGMP snooping commands in IP Multicast Command Reference.

3. Configure a multiport ARP entry.

arp multiport ip-address mac-address vlan-id [ vpn-instance vpn-instance-name ]

The specified MAC address and VLAN ID must be the same as those in the multiport unicast MAC address entry or the multicast MAC address entry.

Configuring features for dynamic ARP entries

Setting the dynamic ARP learning limit for a device

About the dynamic ARP learning limit for a device

A device can dynamically learn ARP entries. To prevent a device from holding too many ARP entries, you can set the maximum number of dynamic ARP entries that the device can learn. When the limit is reached, the device stops ARP learning.

If you set a value lower than the number of existing dynamic ARP entries, the device does not delete the existing entries unless they age out. You can use the reset arp dynamic command to clear dynamic ARP entries.

Procedure

1. Enter system view.

system-view

2. Set the dynamic ARP learning limit for the device.

In standalone mode:

arp max-learning-number max-number slot slot-number

In IRF mode:

arp max-learning-number max-number chassis chassis-number slot slot-number

The following matrix shows the default values for the ARP learning limit:

Hardware		ARP learning limit on the device
S12500X-AF switch series	FE cards HF cards	The limit depends on the ARP table capacity set by using the hardware-resource tcam command. For information about this command, see the device management in Fundamentals Command Reference.
	Other F cards	16000
	Other H cards	48000
S12500-X switch series	FE cards	The limit depends on the ARP table capacity set by using the hardware-resource tcam command. For information about this command, see the device management in Fundamentals Command Reference.
S12500-X switch series	Other cards	16000
S9800 switch series		16000

To disable the device from dynamic ARP learning, set the value to 0.

Setting the dynamic ARP learning limit for an interface

About setting the dynamic ARP learning limit for an interface

An interface can dynamically learn ARP entries. To prevent an interface from holding too many ARP entries, you can set the maximum number of dynamic ARP entries that the interface can learn. When the limit is reached, the interface stops ARP learning.

You can set limits for both a Layer 2 interface and the VLAN interface for a permitted VLAN on the Layer 2 interface. The Layer 2 interface learns an ARP entry only when neither limit is reached.

The total dynamic ARP learning limit for all interfaces will not be higher than the dynamic ARP learning limit for the device.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Set the dynamic ARP learning limit for the interface.

arp max-learning-num max-number [ alarm alarm-threshold ]

The following matrix shows the default values for the ARP learning limit:

Hardware		ARP learning limit on an interface
S12500X-AF switch series	FE cards HF cards	The limit depends on the ARP table capacity set by using the hardware-resource tcam command. For information about this command, see the device management in Fundamentals Command Reference.
	Other F cards	16000
	Other H cards	48000
S12500-X switch series	FE cards	The limit depends on the ARP table capacity set by using the hardware-resource tcam command. For information about this command, see the device management in Fundamentals Command Reference.
S12500-X switch series	Other cards	16000
S9800 switch series		16000

To disable the interface from dynamic ARP learning, set the value to 0.

Setting the aging timer for dynamic ARP entries

About the aging timer for dynamic ARP entries

Each dynamic ARP entry in the ARP table has a limited lifetime, called an aging timer. The aging timer of a dynamic ARP entry is reset each time the dynamic ARP entry is updated. A dynamic ARP entry that is not updated before its aging timer expires is deleted from the ARP table.

Procedure

1. Enter system view.

system-view

2. Set the aging timer for dynamic ARP entries.

arp timer aging { aging-minutes | second aging-seconds }

The default setting is 20 minutes.

Enabling dynamic ARP entry check

About dynamic ARP entry check

The dynamic ARP entry check feature disables the device from supporting dynamic ARP entries that contain multicast MAC addresses. The device cannot learn dynamic ARP entries containing multicast MAC addresses. You cannot manually add static ARP entries containing multicast MAC addresses.

When dynamic ARP entry check is disabled, ARP entries containing multicast MAC addresses are supported. The device can learn dynamic ARP entries containing multicast MAC addresses obtained from the ARP packets sourced from a unicast MAC address. You can also manually add static ARP entries containing multicast MAC addresses.

Procedure

1. Enter system view.

system-view

2. Enable dynamic ARP entry check.

arp check enable

By default, dynamic ARP entry check is enabled.

Synchronizing ARP entries across all cards

About ARP entry synchronization

This task ensures that all cards on the device have the same ARP entries.

Restrictions and guidelines

To synchronize ARP entries across all cards in a timely manner, you can schedule the device to automatically execute the arp smooth command. For information about scheduling a task, see the device management in Fundamentals Configuration Guide.

Procedure

To synchronize ARP entries from the active MPU to all other cards, execute the following command in user view:

arp smooth

Configuring a customer-side or network-side port

About the customer-side and network-side port

The device generates a host route when it learns an ARP entry from a network-side port. To save hardware resources, you can specify a port that connects to a user terminal as a customer-side port. The device will not generate a host route for the learned ARP entry of the user terminal.

Restrictions and guidelines

When an interface is configured as a customer-side port, the ARP learning limit for the device is as follows:

· For the F cards of S12500X-AF switch series, the device can learn a maximum of 120000 ARP entries with consecutive MAC addresses or 16000 ARP entries with inconsecutive MAC addresses.

· For the H cards of S12500X-AF switch series, the device can learn a maximum of 350000 ARP entries with consecutive MAC addresses or 48000 ARP entries with inconsecutive MAC addresses.

· For S12500-X and S9800 switch series, the device can learn a maximum of 120000 ARP entries with consecutive MAC addresses or 16000 ARP entries with inconsecutive MAC addresses.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Configure the interface as a customer-side port or a network-side port.

¡ Configure the interface as a customer-side port.

arp mode uni

¡ Configure the interface as a network-side port.

undo arp mode uni

By default, a port operates as a network-side port.

Enabling interface consistency check for ARP entries and MAC address entries

About interface consistency check for ARP entries and MAC address entries

This feature enables the device to periodically check the consistency of the interfaces in the ARP entry and the MAC address entry for a user. If the interfaces are inconsistent, the device sends an ARP request and updates the interface in the ARP entry with the interface that receives the ARP reply.

On an unstable network, the output interface in the ARP entry for a user might not be updated in a timely manner. Using this feature can avoid incorrect packet forwarding.

Execute the display mac-address command on the device to view MAC address entries. For more information about the command, see Layer 2-LAN Switching Command Reference.

Software version and feature compatibility

This feature is supported only in Release 2719 and later.

Procedure

1. Enter system view.

system-view

2. Enable interface consistency check for ARP entries and MAC address entries.

arp mac-interface-consistency check enable

By default, interface consistency check for ARP entries and MAC address entries is disabled.

Enabling ARP logging

About ARP logging

This feature enables a device to log ARP events when ARP cannot resolve IP addresses correctly. The log information helps administrators locate and solve problems. The device can log the following ARP events:

· On a proxy ARP-disabled interface, the target IP address of a received ARP packet is not one of the following IP addresses:

¡ The IP address of the receiving interface.

¡ The virtual IP address of the VRRP group.

¡ The public IP address after NAT.

· The sender IP address of a received ARP reply conflicts with one of the following IP addresses:

¡ The IP address of the receiving interface.

¡ The virtual IP address of the VRRP group.

¡ The public IP address after NAT.

The device sends ARP log messages to the information center. You can use the info-center source command to specify the log output rules for the information center. For more information about information center, see Network Management and Monitoring Configuration Guide.

Procedure

1. Enter system view.

system-view

2. Enable ARP logging.

arp check log enable

By default, ARP logging is disabled.

Display and maintenance commands for ARP

CAUTION:

Clearing ARP entries from the ARP table might cause communication failures. Make sure the entries to be cleared do not affect current communications.

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Execute display commands in any view and reset commands in user view.

Task	Command
Display ARP entries.	In standalone mode: display arp [ [ all \| dynamic \| multiport \| static ] [ slot slot-number ] \| vlan vlan-id \| interface interface-type interface-number ] [ count \| verbose ] In IRF mode: display arp [ [ all \| dynamic \| multiport \| static ] [ chassis chassis-number slot slot-number ] \| vlan vlan-id \| interface interface-type interface-number ] [ count \| verbose ]
Display the maximum number of ARP entries that a device supports.	display arp entry-limit
Display the ARP entry for an IP address.	In standalone mode: display arp ip-address [ slot slot-number ] [ verbose ] In IRF mode: display arp ip-address [ chassis chassis-number slot slot-number ] [ verbose ]
Display the number of OpenFlow ARP entries.	In standalone mode: display arp openflow count [ slot slot-number ] In IRF mode: display arp openflow count [ chassis chassis-number slot slot-number ]
Display the aging timer of dynamic ARP entries.	display arp timer aging
Display the ARP entries for a VPN instance.	display arp vpn-instance vpn-instance-name [ count \| verbose ]
Clear ARP entries from the ARP table.	In standalone mode: reset arp { all \| dynamic \| interface interface-type interface-number \| multiport \| slot slot-number \| static } In IRF mode: reset arp { all \| chassis chassis-number slot slot-number \| dynamic \| interface interface-type interface-number \| multiport \| static }

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ARP configuration examples

Example: Configuring a long static ARP entry

Network configuration

As shown in Figure 3, hosts are connected to Device B. Device B is connected to Device A through interface FortyGigE 1/0/1 in VLAN 10.

To ensure secure communications between Device A and Device B, configure a long static ARP entry for Device A on Device B.

Figure 3 Network diagram

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Procedure

IMPORTANT:

By default, interfaces on the device are disabled (in ADM or Administratively Down state). To have an interface operate, you must use the undo shutdown command to enable that interface.

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# Create VLAN 10.

<DeviceB> system-view

[DeviceB] vlan 10

[DeviceB-vlan10] quit

# Add interface FortyGigE 1/0/1 to VLAN 10.

[DeviceB] interface fortygige 1/0/1

[DeviceB-FortyGigE1/0/1] port access vlan 10

[DeviceB-FortyGigE1/0/1] quit

# Create VLAN-interface 10 and configure its IP address.

[DeviceB] interface vlan-interface 10

[DeviceB-vlan-interface10] ip address 192.168.1.2 8

[DeviceB-vlan-interface10] quit

# Configure a long static ARP entry that has IP address 192.168.1.1, MAC address 00e0-fc01-0000, and output interface FortyGigE 1/0/1 in VLAN 10.

[DeviceB] arp static 192.168.1.1 00e0-fc01-0000 10 fortygige 1/0/1

Verifying the configuration

# Verify that Device B has a long static ARP entry for Device A.

[DeviceB] display arp static

Type: S-Static D-Dynamic O-Openflow R-Rule M-Multiport I-Invalid

IP address MAC address VLAN/VSI Interface/Link ID Aging Type

192.168.1.1 00e0-fc01-0000 10 FGE1/0/1 -- S

Example: Configuring a short static ARP entry

Network configuration

As shown in Figure 4, hosts are connected to Device B. Device B is connected to Device A through interface FortyGigE 1/0/2.

To ensure secure communications between Device A and Device B, configure a short static ARP entry for Device A on Device B.

Figure 4 Network diagram

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Procedure

IMPORTANT:

By default, interfaces on the device are disabled (in ADM or Administratively Down state). To have an interface operate, you must use the undo shutdown command to enable that interface.

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# Configure an IP address for FortyGigE 1/0/2.

<DeviceB> system-view

[DeviceB] interface fortygige 1/0/2

[DeviceB-FortyGigE1/0/2] ip address 192.168.1.2 24

[DeviceB-FortyGigE1/0/2] quit

# Configure a short static ARP entry that has IP address 192.168.1.1 and MAC address 00e0-fc01-001f.

[DeviceB] arp static 192.168.1.1 00e0-fc01-001f

Verifying the configuration

# Verify that Device B has a short static ARP entry for Device A

[DeviceB] display arp static

Type: S-Static D-Dynamic O-Openflow R-Rule M-Multiport I-Invalid

IP address MAC address VLAN/VSI Interface/Link ID Aging Type

192.168.1.1 00e0-fc01-001f -- -- -- S

Example: Configuring a multiport ARP entry

Network configuration

As shown in Figure 5, a device connects to three servers through interfaces FortyGigE 1/0/1, FortyGigE 1/0/2, and FortyGigE 1/0/3 in VLAN 10. The servers share the IP address 192.168.1.1/24 and MAC address 00e0-fc01-0000.

Configure a multiport ARP entry so that the device sends IP packets with the destination IP address 192.168.1.1 to the three servers.

Figure 5 Network diagram

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Procedure

IMPORTANT:

By default, interfaces on the device are disabled (in ADM or Administratively Down state). To have an interface operate, you must use the undo shutdown command to enable that interface.

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# Create VLAN 10.

<Device> system-view

[Device] vlan 10

[Device-vlan10] quit

# Add FortyGigE 1/0/1, FortyGigE 1/0/2, and FortyGigE 1/0/3 to VLAN 10.

[Device] interface fortygige 1/0/1

[Device-FortyGigE1/0/1] port access vlan 10

[Device-FortyGigE1/0/1] quit

[Device] interface fortygige 1/0/2

[Device-FortyGigE1/0/2] port access vlan 10

[Device-FortyGigE1/0/2] quit

[Device] interface fortygige 1/0/3

[Device-FortyGigE1/0/3] port access vlan 10

[Device-FortyGigE1/0/3] quit

# Create VLAN-interface 10 and specify its IP address.

[Device] interface vlan-interface 10

[Device-vlan-interface10] ip address 192.168.1.2 24

[Device-vlan-interface10] quit

# Configure a multiport unicast MAC address entry that has MAC address 00e0-fc01-0000, and output interfaces FortyGigE 1/0/1, FortyGigE 1/0/2, and FortyGigE 1/0/3 in VLAN 10.

[Device] mac-address multiport 00e0-fc01-0000 interface fortygige 1/0/1 to fortygige 1/0/3 vlan 10

# Configure a multiport ARP entry with IP address 192.168.1.1 and MAC address 00e0-fc01-0000.

[Device] arp multiport 192.168.1.1 00e0-fc01-0000 10

Verifying the configuration

# Verify that the device has a multiport ARP entry with IP address 192.168.1.1 and MAC address 00e0-fc01-0000.

[Device] display arp

Type: S-Static D-Dynamic O-Openflow R-Rule M-Multiport I-Invalid

IP address MAC address VLAN/VSI Interface/Link ID Aging Type

192.168.1.1 00e0-fc01-0000 10 -- -- M

Configuring gratuitous ARP

About gratuitous ARP

In a gratuitous ARP packet, the sender IP address and the target IP address are the IP address of the sending device.

A device sends a gratuitous ARP packet for either of the following purposes:

· Determine whether its IP address is already used by another device. If the IP address is already used, the device is informed of the conflict by an ARP reply.

· Inform other devices of a MAC address change.

IP conflict detection

When an interface obtains an IP address, the device broadcasts gratuitous ARP packets in the LAN where the interface resides. If the device receives an ARP reply, its IP address conflicts with the IP address of another device in the LAN. The device displays a log message about the conflict and informs the administrator to change the IP address. The device will not use the conflicting IP address. If no ARP reply is received, the device uses the IP address.

Gratuitous ARP packet learning

This feature enables a device to create or update ARP entries by using the sender IP and MAC addresses in received gratuitous ARP packets.

When this feature is disabled, the device uses received gratuitous ARP packets to update existing ARP entries only. ARP entries are not created based on the received gratuitous ARP packets, which saves ARP table space.

Periodic sending of gratuitous ARP packets

Periodic sending of gratuitous ARP packets helps downstream devices update ARP entries or MAC entries in a timely manner.

This feature can implement the following functions:

· Prevent gateway spoofing.

Gateway spoofing occurs when an attacker uses the gateway address to send gratuitous ARP packets to the hosts on a network. The traffic destined for the gateway from the hosts is sent to the attacker instead. As a result, the hosts cannot access the external network.

To prevent such gateway spoofing attacks, you can enable the gateway to send gratuitous ARP packets at intervals. Gratuitous ARP packets contain the primary IP address and manually configured secondary IP addresses of the gateway, so hosts can learn correct gateway information.

· Prevent ARP entries from aging out.

If network traffic is heavy or if the host CPU usage is high, received ARP packets can be discarded or are not promptly processed. Eventually, the dynamic ARP entries on the receiving host age out. The traffic between the host and the corresponding devices is interrupted until the host re-creates the ARP entries.

To prevent this problem, you can enable the gateway to send gratuitous ARP packets periodically. Gratuitous ARP packets contain the primary IP address and manually configured secondary IP addresses of the gateway, so the receiving hosts can update ARP entries in a timely manner.

· Prevent the virtual IP address of a VRRP group from being used by a host.

The master router of a VRRP group can periodically send gratuitous ARP packets to the hosts on the local network. The hosts can then update local ARP entries and avoid using the virtual IP address of the VRRP group. The sender MAC address in the gratuitous ARP packet is the virtual MAC address of the virtual router. For more information about VRRP, see High Availability Configuration Guide.

Gratuitous ARP tasks at a glance

All gratuitous ARP tasks are optional. If all of the following features are disabled, gratuitous ARP still provides the IP conflict detection function.

· Enabling IP conflict notification

· Enabling gratuitous ARP packet learning

· Enabling periodic sending of gratuitous ARP packets

· Enabling sending gratuitous ARP packets for ARP requests with sender IP address on a different subnet

· Configuring gratuitous ARP packet retransmission for the device MAC address change

Enabling IP conflict notification

About IP conflict notification

Upon detecting an IP conflict, the device will sends a gratuitous ARP request. By default, the device displays an error message only after it receives an ARP reply. You can enable this feature to allow the device to display an error message immediately upon detecting an IP conflict.

Procedure

1. Enter system view.

system-view

2. Enable IP conflict notification.

arp ip-conflict log prompt

By default, IP conflict notification is disabled.

Enabling gratuitous ARP packet learning

1. Enter system view.

system-view

2. Enable gratuitous ARP packet learning.

gratuitous-arp-learning enable

By default, gratuitous ARP packet learning is enabled.

Enabling periodic sending of gratuitous ARP packets

Restrictions and guidelines

· You can enable periodic sending of gratuitous ARP packets on a maximum of 1024 interfaces.

· Periodic sending of gratuitous ARP packets takes effect on an interface only when the following conditions are met:

¡ The data link layer state of the interface is up.

¡ The interface has an IP address.

· If you change the sending interval for gratuitous ARP packets, the configuration takes effect at the next sending interval.

· The sending interval for gratuitous ARP packets might be much longer than the specified sending interval in any of the following circumstances:

¡ This feature is enabled on multiple interfaces.

¡ Each interface is configured with multiple secondary IP addresses.

¡ A small sending interval is configured when the previous two conditions exist.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable periodic sending of gratuitous ARP packets.

arp send-gratuitous-arp [ interval interval ]

By default, periodic sending of gratuitous ARP packets is disabled.

Enabling sending gratuitous ARP packets for ARP requests with sender IP address on a different subnet

1. Enter system view.

system-view

2. Enable the device to send gratuitous ARP packets upon receiving ARP requests whose sender IP address belongs to a different subnet.

gratuitous-arp-sending enable

By default, a device does not send gratuitous ARP packets upon receiving ARP requests whose sender IP address belongs to a different subnet.

Configuring gratuitous ARP packet retransmission for the device MAC address change

About gratuitous ARP packet retransmission for the device MAC address change

The device sends a gratuitous ARP packet to inform other devices of its MAC address change. However, the other devices might fail to receive the packet because the device sends the gratuitous ARP packet once only by default. Configure the gratuitous ARP packet retransmission feature to ensure that the other devices can receive the packet.

Procedure

1. Enter system view.

system-view

2. Set the times and the interval for retransmitting a gratuitous ARP packet for the device MAC address change.

gratuitous-arp mac-change retransmit times interval seconds

By default, the device sends a gratuitous packet to inform its MAC address change once only.

Configuring proxy ARP

About proxy ARP

Proxy ARP enables a device on one network to answer ARP requests for an IP address on another network. With proxy ARP, hosts on different broadcast domains can communicate with each other as they would on the same broadcast domain.

Proxy ARP includes common proxy ARP and local proxy ARP.

· Common proxy ARP—Allows communication between hosts that connect to different Layer 3 interfaces and reside in different broadcast domains.

· Local proxy ARP—Allows communication between hosts that connect to the same Layer 3 interface and reside in different broadcast domains.

Enabling common proxy ARP

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

The following interface types are supported:

¡ VLAN interface.

¡ Layer 3 Ethernet interface.

¡ Layer 3 Ethernet subinterface.

¡ Layer 3 aggregate interface.

¡ Layer 3 aggregate subinterface.

3. Enable common proxy ARP.

proxy-arp enable

By default, common proxy ARP is disabled.

Enabling local proxy ARP

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

The following interface types are supported:

¡ VLAN interface.

¡ Layer 3 Ethernet interface.

¡ Layer 3 Ethernet subinterface.

¡ Layer 3 aggregate interface.

¡ Layer 3 aggregate subinterface.

3. Enable local proxy ARP.

local-proxy-arp enable [ ip-range start-ip-address to end-ip-address ]

By default, local proxy ARP is disabled.

Display and maintenance commands for proxy ARP

Execute display commands in any view.

Task	Command
Display common proxy ARP status.	display proxy-arp [ interface interface-type interface-number ]
Display local proxy ARP status.	display local-proxy-arp [ interface interface-type interface-number ]

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Common proxy ARP configuration example

Example: Configuring common proxy ARP

Network configuration

As shown in Figure 6, Host A and Host D have the same IP prefix and mask, but they are located on different subnets separated by the switch. Host A belongs to VLAN 1, and Host D belongs to VLAN 2. No default gateway is configured on Host A and Host D.

Configure common proxy ARP on the switch to enable communication between the two hosts.

Figure 6 Network diagram

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Procedure

IMPORTANT:

By default, interfaces on the device are disabled (in ADM or Administratively Down state). To have an interface operate, you must use the undo shutdown command to enable that interface.

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# Create VLAN 2.

<Switch> system-view

[Switch] vlan 2

[Switch-vlan2] quit

# Configure the IP address of VLAN-interface 1.

[Switch] interface vlan-interface 1

[Switch-Vlan-interface1] ip address 192.168.10.99 255.255.255.0

# Enable common proxy ARP on VLAN-interface 1.

[Switch-Vlan-interface1] proxy-arp enable

[Switch-Vlan-interface1] quit

# Configure the IP address of VLAN-interface 2.

[Switch] interface vlan-interface 2

[Switch-Vlan-interface2] ip address 192.168.20.99 255.255.255.0

# Enable common proxy ARP on VLAN-interface 2.

[Switch-Vlan-interface2] proxy-arp enable.

Verifying the configuration

# Verify that Host A and Host D can ping each other.

Configuring ARP snooping

About ARP snooping

ARP snooping is used in Layer 2 switching networks. It creates ARP snooping entries by using information in ARP packets. ARP fast-reply can use the ARP snooping entries.

Creation of ARP snooping entries

If you enable ARP snooping for a VLAN, ARP packets received in the VLAN are redirected to the CPU. The CPU uses the sender IP and MAC addresses of the ARP packets, and receiving VLAN and port to create ARP snooping entries.

Aging of ARP snooping entries

The aging timer and valid period of an ARP snooping entry are 25 minutes and 15 minutes. If an ARP snooping entry is not updated in 12 minutes, the device sends an ARP request. The ARP request uses the IP address of the entry as the target IP address. If an ARP snooping entry is not updated in 15 minutes, it becomes invalid and cannot be used. After that, if an ARP packet matching the entry is received, the entry becomes valid, and its aging timer restarts.

If the aging timer of an ARP snooping entry expires, the entry is removed.

Protection for ARP snooping

An attack occurs if an ARP packet has the same sender IP address as a valid ARP snooping entry but a different sender MAC address. The ARP snooping entry becomes invalid, and it is removed in 1 minute.

Enabling ARP snooping

1. Enter system view.

system-view

2. Enter VLAN view.

vlan vlan-id

3. Enable ARP snooping

arp snooping enable

By default, ARP snooping is disabled.

Display and maintenance commands for ARP snooping

Execute display commands in any view and reset commands in user view.

Task	Command
Display ARP snooping entries.	In standalone mode: display arp snooping [ vlan vlan-id ] [ slot slot-number ] [ count ] display arp snooping ip ip-address [ slot slot-number ] In IRF mode: display arp snooping [ vlan vlan-id ] [ chassis chassis-number slot slot-number ] [ count ] display arp snooping ip ip-address [ chassis chassis-number slot slot-number ]
Delete ARP snooping entries.	reset arp snooping [ ip ip-address \| vlan vlan-id ]

Task

Command

Display ARP snooping entries.

In standalone mode:

display arp snooping [ vlan vlan-id ] [ slot slot-number ] [ count ]

display arp snooping ip ip-address [ slot slot-number ]

In IRF mode:

display arp snooping [ vlan vlan-id ] [ chassis chassis-number slot slot-number ] [ count ]

display arp snooping ip ip-address [ chassis chassis-number slot slot-number ]

Delete ARP snooping entries.

reset arp snooping [ ip ip-address | vlan vlan-id ]

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Configuring ARP fast-reply

About ARP fast-reply

ARP fast-reply enables a device to directly answer ARP requests according to DHCP snooping entries or ARP snooping entries. ARP fast-reply functions in a VLAN. For information about DHCP snooping, see "Configuring DHCP snooping."

If the target IP address of a received ARP request is the IP address of the VLAN interface, the device delivers the request to the ARP module. If not, the device takes the following steps to process the packet:

1. Search the DHCP snooping table for a match by using the target IP address.

2. If a match is found, whether the device returns a reply depends on the type of interface in the matching entry.

¡ If the interface is the Ethernet interface that received the ARP request, the device does not return any reply.

¡ If the interface is an Ethernet interface other than the receiving interface, the device returns a reply according to the matching entry.

3. If no matching DHCP snooping entry is found and ARP snooping is enabled, the device searches the ARP snooping table.

¡ If the interface in the matching entry is the Ethernet interface that received the ARP request, the device does not return any reply.

¡ If the interface is an Ethernet interface other than the receiving interface, the device returns a reply according to the ARP snooping entry.

4. If no match is found in both tables, the ARP request is forwarded to other interfaces except the receiving interface in the VLAN, or delivered to other modules.

Enabling ARP fast-reply

Restrictions and guidelines

To improve the availability of ARP fast-reply, enable ARP snooping at the same time.

Procedure

1. Enter system view.

system-view

2. Enter VLAN view.

vlan vlan-id

3. Enable ARP fast-reply.

arp fast-reply enable

By default, ARP fast-reply is disabled.

ARP fast-reply configuration example

Example: Configuring ARP fast-reply

Network configuration

As shown in Figure 7, all clients are in VLAN 2, and access the network through the switch. They have obtained IP addresses through DHCP.

Enable ARP snooping and ARP fast-reply for VLAN 2. The switch directly returns an ARP reply without broadcasting received ARP requests in the VLAN.

Figure 7 Network diagram

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Procedure

IMPORTANT:

By default, interfaces on the device are disabled (in ADM or Administratively Down state). To have an interface operate, you must use the undo shutdown command to enable that interface.

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# Enable ARP snooping for VLAN 2 on the switch.

<Switch> system-view

[Switch] vlan 2

[Switch-vlan2] arp snooping enable

# Enable ARP fast-reply for VLAN 2 on the switch.

[Switch-vlan2] arp fast-reply enable

[Switch-vlan2] quit

Configuring ARP direct route advertisement

About ARP direct route advertisement

Mechanism of ARP direct route advertisement

The ARP direct route advertisement feature advertises host routes instead of advertising the network route.

Cooperation with BGP

As shown in Figure 8, this feature can cooperate with BGP in an EVPN network. The Layer 3 Ethernet interfaces that directly connect a leaf node and a spine node borrow IP addresses with the subnet mask 255.255.255.255, for example, from loopback interfaces. For communication, the leaf node and the spine node learn the ARP entry of each other through LLDP.

Because the IP addresses of the interfaces that directly connect the spine node and the leaf node belong to different subnets, one node does not have the direct route to the other node. If BGP is used to transmit underlay network routes, BGP cannot perform correct route recursion. To resolve this problem, enable ARP direct route advertisement on the Layer 3 Ethernet interfaces that directly connect the spine node and the leaf node. One node generates a direct route to the other node and BGP can perform correct route recursion.

For information about EVPN, see EVPN Configuration Guide. For information about LLDP, see Layer 2—LAN Switching Configuration Guide. For information about BGP, see Layer 3—IP Routing Configuration Guide.

Figure 8 ARP direct route with BGP

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Enabling ARP direct route advertisement

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable the ARP direct route advertisement feature.

arp route-direct advertise

By default, the ARP direct route advertisement feature is disabled.

04-Layer 3-IP Services Configuration Guide

Configuring ARP

About ARP

ARP message format

ARP entry types

Dynamic ARP entry

Static ARP entry

OpenFlow ARP entry

Rule ARP entry

ARP tasks at a glance

Restrictions and guidelines

Procedure

Configuring a long static ARP entry

About long static ARP entries

Procedure

About multiport ARP entries

Restrictions and guidelines

Procedure

About the dynamic ARP learning limit for a device

Procedure

Setting the dynamic ARP learning limit for an interface

About setting the dynamic ARP learning limit for an interface

Procedure

Setting the aging timer for dynamic ARP entries

About the aging timer for dynamic ARP entries

Procedure

About dynamic ARP entry check

Procedure

About ARP entry synchronization

Restrictions and guidelines

Procedure

About the customer-side and network-side port

Restrictions and guidelines

Procedure

About interface consistency check for ARP entries and MAC address entries

Software version and feature compatibility

Procedure

About ARP logging

Procedure

Network configuration

Procedure

Verifying the configuration

Example: Configuring a short static ARP entry

Network configuration

Procedure

Verifying the configuration

Network configuration

Procedure

Verifying the configuration

Gratuitous ARP tasks at a glance

About IP conflict notification

Procedure

Restrictions and guidelines

Procedure

About gratuitous ARP packet retransmission for the device MAC address change

Procedure

Configuring proxy ARP

Enabling common proxy ARP

Network configuration

Procedure

Verifying the configuration

Restrictions and guidelines

Procedure

Network configuration

Procedure

Configuring ARP direct route advertisement

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