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

· 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

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, static, OpenFlow, and Rule 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.

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

ARP creates Rule ARP entries by learning from the VXLAN, IPoE, and portal modules. A Rule ARP entry does not age out, and it cannot be updated. It can be overwritten by a static ARP entry. A Rule ARP entry can be used directly to forward packets.

For more information about IPoE, see BRAS Services Configuration Guide. For more information about VXLAN, see 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 dynamic ARP entry features

¡ 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

· (Optional.) Performing ARP entry synchronization

· (Optional.) Enabling recording user IP address conflicts

· (Optional.) Enabling recording conflicts between the endpoints and local device

· (Optional.) Enabling recording user port migrations

· (Optional.) Enabling an IP unnumbered interface to learn ARP entries for different subnets

· (Optional.) Enabling ARP safe-guard

· (Optional.) 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 static ARP entry.

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

Configuring a long static ARP entry

About this task

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 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 for a VLAN is deleted if the VLAN or VLAN interface is deleted.

Procedure

1. Enter system view.

system-view

2. Configure a static ARP entry.

arp static ip-address mac-address [ vlan-id interface-type interface-number | interface-type interface-number 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 ] [ description text ]

Configuring dynamic ARP entry features

Setting the dynamic ARP learning limit for a device

About this task

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.

Procedure

1. Enter system view.

system-view

2. Set the device dynamic ARP learning limit.

In standalone mode:

arp max-learning-number max-number slot slot-number [ cpu cpu-number ]

In IRF mode:

arp max-learning-number max-number chassis chassis-number slot slot-number [ cpu cpu-number ]

By default, a device can learn a maximum of 589824 dynamic ARP entries.

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

Setting the dynamic ARP learning limit for an interface

About this task

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

By default, an interface can learn a maximum of 589824 dynamic ARP entries.

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

Setting the aging timer for dynamic ARP entries

About this task

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.

You can set the aging timer for dynamic ARP entries in system view or in interface view. The aging timer set in interface view takes precedence over the aging timer set in system view.

Procedure

1. Enter system view.

system-view

2. Set the aging timer for dynamic ARP entries.

¡ Set the aging timer for dynamic ARP entries in system view.

arp timer aging aging-time

The default setting is 20 minutes.

¡ Execute the following commands in sequence to set the aging timer for dynamic ARP entries in interface view:

interface interface-type interface-number

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

By default, the aging timer set in system view applies.

Enabling dynamic ARP entry check

About this task

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.

Performing ARP entry synchronization

About this task

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

Enabling recording user IP address conflicts

About this task

This feature enables the device to detect and record user IP address conflicts. The device determines that a conflict occurs if an incoming ARP packet has the same sender IP address as an existing ARP entry but a different sender MAC address. The device generates a user IP address conflict record, logs the conflict, and sends the log to the information center. For information about the log destination and output rule configuration in the information center, see the information center in Network Management and Monitoring Configuration Guide.

After you enable user IP address conflict SNMP notifications for ARP, the device sends a notification to the SNMP module when a user IP address conflict occurs. The notification includes the sender IP address and sender MAC address in the conflicted ARP packet and the MAC address in the corresponding local ARP entry. For user IP address conflict event notifications to be sent correctly, you must also configure SNMP on the device. For more information about SNMP configuration, see Network Management and Monitoring Configuration Guide. For more information about SNMP notifications for ARP, see ARP attack protection commands in Security Command Reference.

Procedure

1. Enter system view.

system-view

2. (Optional.) Enable SNMP notifications for ARP.

snmp-agent trap enable arp [ user-ip-conflict ]

By default, SNMP notifications for ARP are disabled.

If you do not specify any keywords, this command enables all SNMP notifications for ARP. If you specify only the user-ip-conflict keyword, this command enables only user IP address conflict SNMP notifications for ARP.

3. Enable recording user IP address conflicts.

arp user-ip-conflict record enable

By default, recording user IP address conflicts is disabled.

Enabling recording conflicts between the endpoints and local device

About this task

This feature enables the device to detect and record conflicts between the endpoints and local device. The device determines that a conflict occurs if an incoming non-gratuitous ARP packet has the same sender IP address or sender MAC address as that of the device. The device generates an endpoint and local device conflict entry, logs the conflict, and sends the log to the information center. For information about the log destination and output rule configuration, see information center configuration in Network Management and Monitoring Configuration Guide.

After you enable endpoints and local device conflict SNMP notifications for ARP, the device sends a notification to the SNMP module when an endpoint and local device conflict occurs. The notification includes the sender IP address, sender MAC address, target IP address, and target MAC address in the conflicted packet. For the endpoints and local device conflict event notifications to be sent correctly, you must also configure SNMP on the device. For more information about SNMP configuration, see Network Management and Monitoring Configuration Guide. For more information about SNMP notifications for ARP, see ARP attack protection commands in Security Command Reference.

Restrictions and guidelines

An interface sends only one endpoint and local device conflict log to the information center per minute. The device supports a maximum of 128 interfaces sending conflict logs simultaneously. When the number of interfaces that send conflict logs reaches 128, the device stops another interface from sending conflict logs until an existing conflict is resolved.

Procedure

1. Enter system view.

system-view

2. (Optional.) Enable SNMP notifications for ARP.

snmp-agent trap enable arp [ local-conflict ]

By default, SNMP notifications for ARP are disabled.

If you do not specify any keywords, this command enables all SNMP notifications for ARP. If you specify only the local-conflict keyword, this command enables only endpoints and local device conflict SNMP notifications for ARP.

3. Enable recording conflicts between the endpoints and local device.

arp local-conflict record enable

By default, recording conflicts between the endpoints and local device is disabled.

Enabling recording user port migrations

About this task

This feature enables the device to detect and record user port migration events. A user port migrates if an incoming ARP packet has the same sender IP address and sender MAC address as an existing ARP entry but a different ingress port. The device generates a user port migration record, logs the migration event, sends the log to the information center, and updates the interface for the ARP entry. For information about the log destination and output rule configuration in the information center, see the information center in Network Management and Monitoring Configuration Guide.

After you enable user port migration notifications for ARP, the device sends a notification to the SNMP module when a user port migrates. The notification includes the IP address, MAC address, port before migration, and port after migration of the user. For user port migration event notifications to be sent correctly, you must also configure SNMP on the device. For more information about SNMP configuration, see Network Management and Monitoring Configuration Guide. For more information about SNMP notifications for ARP, see ARP attack protection commands in Security Command Reference.

Restrictions and guidelines

To avoid device performance degradation, disable recording user port migrations if too many user port migration logs are generated.

Procedure

1. Enter system view.

system-view

2. (Optional.) Enable SNMP notifications for ARP.

snmp-agent trap enable arp [ user-move ]

By default, SNMP notifications for ARP are disabled.

If you do not specify any keywords, this command enables all SNMP notifications for ARP. If you specify only the user-move keyword, this command enables only user port migration SNMP notifications for ARP.

3. Enable recording user port migrations.

arp user-move record enable

By default, recording user port migrations is disabled.

Enabling an IP unnumbered interface to learn ARP entries for different subnets

About this task

An IP unnumbered interface cannot learn the ARP entry of the peer device if the unnumbered interface and the peer device are on different subnets. To ensure communication between them, you can enable this feature on the IP unnumbered interface.

If an IP unnumbered interface is disabled from learning ARP entries for different subnets, existing ARP entries learned for different subnets are deleted after they age out.

Restrictions and guidelines

To avoid ARP learning anomalies, do not execute the arp ip-unnumbered learning enable command on a non-IP-unnumbered interface.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Configure the interface to borrow the IP address of the specified interface.

ip address unnumbered interface interface-type interface-number

By default, the interface does not borrow IP addresses from other interfaces.

4. Enable the IP unnumbered interface to learn ARP entries for different subnets.

arp ip-unnumbered learning enable

By default, an IP unnumbered interface cannot learn ARP entries for different subnets.

Enabling ARP safe-guard

About this task

The ARP safe-guard feature prevents traffic-intensive ARP packet attacks on a device. After you enable this feature, the device operates as follows:

· The device sends replies to all incoming ARP requests but do not generate corresponding ARP entries, which prevents gateway spoofing attacks.

· The device generates ARP entries for incoming ARP replies that the device requests.

· The device drops incoming ARP replies that are not requested by the device, which ensures that the device can learn correct ARP entries.

Restrictions and guidelines

This feature is available only for the following cards:

Card category	Cards
CEPC	CEPC-XP4LX, CEPC-XP24LX, CEPC-XP48RX, CEPC-CP4RX, CEPC-CP4RXA, CEPC-CP4RX-L, CEPC-CQ8L, CEPC-CQ8LA, CEPC-CQ8L1A, CEPC-CQ8L3A, CEPC-CQ16L1, CEPC-DQ2L1-G
CSPEX	CSPEX-1304X, CSPEX-1404X, CSPEX-1502X, CSPEX-1504X, CSPEX-1504XA, CSPEX-1602X, CSPEX-1602XA, CSPEX-1804X, CSPEX-1512X, CSPEX-1612X, CSPEX-1812X, CSPEX-1502XA, CSPEX-1802X, CSPEX-1802XA, CSPEX-1812X-E, CSPEX-2304X-G, CSPEX-2304X-LG, CSPEX-2612XA, CSPEX-2612X3A
SPE	RX-SPE200, RX-SPE200-E

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Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable the ARP safe-guard feature.

arp safe-guard enable

By default, the ARP safe-guard feature is disabled.

Enabling ARP logging

About this task

This feature enables a device to log ARP events when ARP cannot resolve IP addresses correctly. 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.

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 [ cpu cpu-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 [ cpu cpu-number ] ] \| vlan vlan-id \| interface interface-type interface-number ] [ count \| verbose ]
Display the ARP entry for an IP address.	In standalone mode: display arp ip-address [ slot slot-number [ cpu cpu-number ] ] [ verbose ] In IRF mode: display arp ip-address [ chassis chassis-number slot slot-number [ cpu cpu-number ] ] [ verbose ]
Display the maximum number of ARP entries that a device supports.	display arp entry-limit
Display brief ARP log information.	In standalone mode: display arp log [ interface interface-type interface-number \| ip ip-address ] [ slot slot-number [ cpu cpu-number ] ] In IRF mode: display arp log [ interface interface-type interface-number \| ip ip-address ] [ chassis chassis-number slot slot-number [ cpu cpu-number ] ]
Display ARP safe-guard statistics.	In standalone mode: display arp safe-guard statistics slot slot-number In IRF mode: display arp safe-guard statistics chassis chassis-number slot slot-number
Display the ARP entries for a VPN instance.	display arp vpn-instance vpn-instance-name [ count ]
Display the aging timer of dynamic ARP entries.	display arp timer aging
Display user IP address conflict records.	In standalone mode: display arp user-ip-conflict record [ slot slot-number [ cpu cpu-number ] ] In IRF mode: display arp user-ip-conflict record [ chassis chassis-number slot slot-number [ cpu cpu-number ] ]
Display user port migration records	In standalone mode: display arp user-move record [ slot slot-number [ cpu cpu-number ] ] In IRF mode: display arp user-move record [ chassis chassis-number slot slot-number [ cpu cpu-number ] ]
Clear ARP entries from the ARP table.	In standalone mode: reset arp { all \| dynamic \| interface interface-type interface-number \| multiport \| slot slot-number [ cpu cpu-number ] \| static } In IRF mode: reset arp { all \| chassis chassis-number slot slot-number [ cpu cpu-number ] \| dynamic \| interface interface-type interface-number \| multiport \| static }
Clear ARP safe-guard statistics.	In standalone mode: reset arp safe-guard statistics { all \| slot slot-number } In IRF mode: reset arp safe-guard statistics { all \| chassis chassis-number slot slot-number }

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 Ten-GigabitEthernet 3/1/1 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

# Create VLAN 10.

<DeviceB> system-view

[DeviceB] vlan 10

[DeviceB-vlan10] quit

# Add interface Ten-GigabitEthernet 3/1/1 to VLAN 10.

[DeviceB] interface ten-gigabitethernet 3/1/1

[DeviceB-Ten-GigabitEthernet3/1/1] port access vlan 10

[DeviceB-Ten-GigabitEthernet3/1/1] 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 Ten-GigabitEthernet 3/1/1 in VLAN 10.

[DeviceB] arp static 192.168.1.1 00e0-fc01-0000 10 ten-gigabitethernet 3/1/1

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 name Interface Aging Type

192.168.1.1 00e0-fc01-0000 10 XGE3/1/1 -- 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 Ten-GigabitEthernet 3/1/2.

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

# Configure an IP address for Ten-GigabitEthernet 3/1/2.

<DeviceB> system-view

[DeviceB] interface ten-gigabitethernet 3/1/2

[DeviceB-Ten-GigabitEthernet3/1/2] ip address 192.168.1.2 24

[DeviceB-Ten-GigabitEthernet3/1/2] 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 name Interface Aging Type

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

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.

· Update MAC entries of devices in the VLANs having ambiguous Dot1q or QinQ termination configured.

In VRRP configuration, if ambiguous Dot1q or QinQ termination is configured for multiple VLANs and VRRP groups, interfaces configured with VLAN termination must be disabled from transmitting broadcast/multicast packets. Also, a VRRP control VLAN must be configured so that VRRP advertisements can be transmitted within the control VLAN only. In such cases, you can enable periodic sending of gratuitous ARP packets containing the following addresses:

¡ The VRRP virtual IP address.

¡ The primary IP address or a manually configured secondary IP address of the sending interface on the subinterfaces.

When a VRRP failover occurs, devices in the VLANs can use the gratuitous ARP packets to update their corresponding MAC entries in a timely manner. For more information about ambiguous Dot1q or QinQ termination, see Layer 2—LAN Switching 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 this task

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 this task

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, local proxy ARP, and inter-VLAN proxy ARP.

Restrictions and guidelines: Proxy ARP

In a BRAS scenario, to enable proxy ARP features (including common proxy ARP, local proxy ARP, and inter-VLAN proxy ARP), you must execute the local-proxy-arp enable command. Executing this command enables all proxy ARP features.

Enabling common proxy ARP

About this task

As shown in Figure 5, Host A is at 192.168.10.100/16 and Host D is at 192.168.20.200/16. Host A and Host D is located in Subnet A and Subnet B, respectively.

Figure 5 Application scenario of common proxy ARP

Because Host A's IP address is on the same subnet as Host D's, Host A broadcasts an ARP request for Host D's MAC address. However, Host D cannot receive the ARP request because they reside in different broadcast domains.

To solve this problem, enable common ARP proxy on Interface A of the device. The communication process is as follows:

1. Upon receiving the ARP request from Host A, the device responds with its own MAC address.

2. Based on the reply, Host A uses the device's MAC address to encapsulate the packet destined for Host D.

3. When receiving the packet, the device searches for the ARP entry for Host D.

¡ If the ARP entry exists, it forwards the packet to Host D.

¡ If the ARP entry does not exist, the device broadcasts an ARP request to request the MAC address of Host D. After obtaining the MAC address of Host D, the device forwards the packet to Host D.

Similarly, you can enable common ARP proxy on Interface B of the device so that packets sent by the Host D can reach Host A.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable common proxy ARP.

proxy-arp enable

By default, common proxy ARP is disabled.

Enabling local proxy ARP

About this task

As shown in Figure 6, Host A, Host B, and Device B are in VLAN 2. Host A and Host B connect to Port B1 and Port B3, respectively. Port B1 and Port B3 are isolated ports in an isolation group.

Figure 6 Application scenario of local proxy ARP

Because Host A's IP address is on the same subnet as Host B's, Host A broadcasts an ARP request for Host B's MAC address. However, Host B cannot receive the ARP request because Host A and Host B are connected to different Device B's ports that are assigned to the same isolation group.

To solve this problem, enable local ARP proxy on Interface A of Device A. The communication process is as follows:

1. Upon receiving the ARP request from Host A, Device A responds with its own MAC address.

2. Based on the reply, Host A uses Device A's MAC address to encapsulate the packet destined for Host B.

3. When receiving the packet, Device A searches for the ARP entry of Host B.

¡ If the ARP entry exists, it forwards the packet to Host B.

¡ If the ARP entry does not exist, the device broadcasts an ARP request to request the MAC address of Host B. After obtaining the MAC address of Host B, the device forwards the packet to Host B.

Restrictions and guidelines

You can enable local proxy ARP to implement Layer 3 intercommunication in the following scenarios:

· Hosts connect to different ports that are assigned to the same VLAN and isolation group.

· Hosts connect to different ports that belong to different sub-VLANs associated with a super VLAN.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

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.

Enabling inter-VLAN proxy ARP

About this task

As shown in Figure 7, Host A belongs to VLAN 2 and Host B belongs to VLAN 3. Host A and Host B connect to Device B. The uplink port of Device B connects to Device A's Interface A.1, which is configured with VLAN termination.

Figure 7 Application scenario of inter-VLAN proxy ARP

To solve this problem, enable inter-VLAN proxy ARP on Interface A.1 of Device A. The communication process is as follows:

1. Upon receiving the ARP request from Host A, Device A searches for the ARP entry of Host B.

2. If the ARP entry does not exist, Device A broadcasts an ARP request for Host B's MAC address in all VLANs of which the packets are terminated on Interface A.1. After obtaining the MAC address of Host B, Device A determines whether Host A and Host B are in the same VLAN.

¡ If they are in the same VLAN, Device A does not respond to the ARP request from Host A.

¡ If they are in different VLANs, Device A responds to the ARP request from Host A with Host B's MAC address. When Host A sends packets to Host B, Device A forwards the packets to Host B.

Procedure

1. Enter system view.

system-view

2. Enter subinterface view.

interface interface-type interface-number.subnumber

3. Enable inter-VLAN proxy ARP.

proxy-arp enable

By default, inter-VLAN 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 ]

Common proxy ARP configuration examples

Example: Configuring common proxy ARP

Network configuration

As shown in Figure 8, Host A and Host D have the same prefix and mask, but they are located on different subnets. No default gateway is configured on Host A and Host D.

Configure common proxy ARP on the router to ensure communication between Host A and Host D.

Figure 8 Network diagram

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Procedure

# Configure the IP address of Ten-GigabitEthernet 3/1/2.

<Router> system-view

[Router] interface ten-gigabitethernet 3/1/2

[Router-Ten-GigabitEthernet3/1/2] ip address 192.168.10.99 255.255.255.0

# Enable common proxy ARP on Ten-GigabitEthernet 3/1/2.

[Router-Ten-GigabitEthernet3/1/2] proxy-arp enable

[Router-Ten-GigabitEthernet3/1/2] quit

# Configure the IP address of Ten-GigabitEthernet 3/1/1.

[Router] interface ten-gigabitethernet 3/1/1

[Router-Ten-GigabitEthernet3/1/1] ip address 192.168.20.99 255.255.255.0

# Enable common proxy ARP on Ten-GigabitEthernet 3/1/1.

[Router-Ten-GigabitEthernet3/1/1] proxy-arp enable

[Router-Ten-GigabitEthernet3/1/1] quit

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 [ cpu cpu-number ] ] [ count ] display arp snooping ip ip-address [ slot slot-number [ cpu cpu-number ] ] In IRF mode: display arp snooping [ vlan vlan-id ] [ chassis chassis-number slot slot-number [ cpu cpu-number ] ] [ count ] display arp snooping ip ip-address [ chassis chassis-number slot slot-number [ cpu cpu-number ] ]
Clear 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 [ cpu cpu-number ] ] [ count ]

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

In IRF mode:

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

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

Clear ARP snooping entries.

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

Configuring ARP suppression

About ARP suppression

The ARP suppression feature enables a device to directly answer ARP requests by using ARP suppression entries. The device generates ARP suppression entries based on dynamic ARP entries. This feature is typically configured on the PEs connected to base stations in an MPLS L2VPN that provides access to an L3VPN network.

You can also configure the ARP suppression push feature to push ARP suppression entries by broadcasting gratuitous ARP packets.

Figure 9 shows a typical application scenario. ARP suppression is enabled on the PE that connects to the base station. The PE generates ARP suppression entries for the base station, PE-agg 1, and PE-agg 2, and it directly replies subsequent ARP requests for these devices.

Figure 9 Typical application

Restrictions and guidelines: ARP suppression

ARP suppression does not take effect if QinQ is enabled on the device.

In standard system operating mode, this feature is available only for the following cards:

Card category	Cards
CEPC	CEPC-CQ8L, CEPC-CQ8LA, CEPC-CQ8L1A, CEPC-CQ8L3A, CEPC-CQ16L1, CEPC-DQ2L1-G
CSPEX	CSPEX-1502XA, CSPEX-1802X, CSPEX-1802XA, CSPEX-1812X-E, CSPEX-2304X-G, CSPEX-2304X-LG, CSPEX-2612XA, CSPEX-2612X3A
SPE	RX-SPE200-E

In SDN-WAN system operating mode, this feature is available only for the following cards:

Card category	Cards
CSPEX	CSPEX-1304X, CSPEX-1404X, CSPEX-1502X, CSPEX-1504X, CSPEX-1504XA, CSPEX-1602X, CSPEX-1602XA, CSPEX-1804X, CSPEX-1512X, CSPEX-1612X, CSPEX-1812X, CSPEX-1502XA, CSPEX-1802X, CSPEX-1802XA, CSPEX-1812X-E, CSPEX-2304X-G, CSPEX-2304X-LG, CSPEX-2612XA, CSPEX-2612X3A
SPE	RX-SPE200, RX-SPE200-E
CEPC	CEPC-XP4LX, CEPC-XP24LX, CEPC-XP48RX, CEPC-CP4RX, CEPC-CP4RXA, CEPC-CP4RX-L, CEPC-CQ8L, CEPC-CQ8LA, CEPC-CQ8L1A, CEPC-CQ8L3A, CEPC-CQ16L1, CEPC-DQ2L1-G

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Enabling ARP suppression

1. Enter system view.

system-view

2. (Optional.) Enable the ARP suppression push feature and set a push interval.

arp suppression push interval interval

By default, the ARP suppression push feature is disabled.

3. Create a cross-connect group and enter its view.

xconnect-group group-name

For more information about this command, see the MPLS L2VPN commands in MPLS Command Reference.

4. Create a cross-connect and enter its view.

connection connection-name

For more information about this command, see the MPLS L2VPN commands in MPLS Command Reference.

5. Enable ARP suppression.

arp suppression enable

By default, ARP suppression is disabled.

Display and maintenance commands for ARP suppression

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

Task	Command
Display ARP suppression entries.	In standalone mode: display arp suppression xconnect-group [ name group-name ] [ slot slot-number [ cpu cpu-number ] ] [ count ] In IRF mode: display arp suppression xconnect-group [ name group-name ] [ chassis chassis-number slot slot-number [ cpu cpu-number ] ] [ count ]
Clear ARP suppression entries.	reset arp suppression xconnect-group [ name group-name ]

Task

Command

Display ARP suppression entries.

In standalone mode:

display arp suppression xconnect-group [ name group-name ] [ slot slot-number [ cpu cpu-number ] ] [ count ]

In IRF mode:

display arp suppression xconnect-group [ name group-name ] [ chassis chassis-number slot slot-number [ cpu cpu-number ] ] [ count ]

Clear ARP suppression entries.

reset arp suppression xconnect-group [ name group-name ]

ARP suppression configuration examples

Example: Configuring ARP suppression

Network configuration

As shown in Figure 10, the base station, Router A, and Router B are in an MPLS L2VPN.

Enable ARP suppression on Router A to directly reply to ARP requests for Router B.

Figure 10 Network diagram

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Procedure

1. Configure IP addresses for the interfaces, and make sure the base station can reach the L3VE interface VE-L3VPN 1 of Router B. (Details not shown.)

2. Configure ARP suppression on Router A:

# Create a cross-connect group named vpna and create a cross-connect named svc in the group.

<RouterA> system-view

[RouterA] xconnect-group vpna

[RouterA-xcg-vpna] connection svc

# Enable ARP suppression for cross-connect svc in cross-connect group vpna.

[RouterA-xcg-vpna-svc] arp suppression enable

Verifying the configuration

1. On the base station, clear ARP entries, and ping the L3VE interface VE-L3VPN 1 of Router B. (Details not shown.)

2. Verify that Router A has ARP suppression entries for the base station and Router B.

[RouterA-xcg-vpna-svc] display arp suppression xconnect-group

IP address MAC address Xconnect-group Connection Aging

10.1.1.1 00e0-fc04-582c vpna svc 25

10.1.1.3 0023-89b7-0861 vpna svc 25

3. Enable ARP debugging on Router B to verify that Router B does not receive an ARP request from the base station under the following conditions (details not shown):

a. Clear ARP entries on the base station.

b. Ping the L3VE interface VE-L3VPN 1 of Router B from the base station.

Procedure

1. Configure IP addresses for the interfaces, and make sure the base station can reach the L3VE interface VE-L3VPN 1 of Switch B. (Details not shown.)

2. Configure ARP suppression on Switch A:

# Create a cross-connect group named vpna and create a cross-connect named svc in the group.

<SwitchA> system-view

[SwitchA] xconnect-group vpna

[SwitchA-xcg-vpna] connection svc

# Enable ARP suppression for cross-connect svc in cross-connect group vpna.

[SwitchA-xcg-vpna-svc] arp suppression enable

Verifying the configuration

1. On the base station, clear ARP entries, and ping the L3VE interface VE-L3VPN 1 of Switch B. (Details not shown.)

2. Verify that Switch A has ARP suppression entries for the base station and Switch B.

[SwitchA-xcg-vpna-svc] display arp suppression xconnect-group

IP address MAC address Xconnect-group Connection Aging

10.1.1.1 00e0-fc04-582c vpna svc 25

10.1.1.3 0023-89b7-0861 vpna svc 25

3. Enable ARP debugging on Switch B to verify that Switch B does not receive an ARP request from the base station under the following conditions (details not shown):

a. Clear ARP entries on the base station.

b. Ping the L3VE interface VE-L3VPN 1 of Switch B from the base station.

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.

Application in Layer 3 access networks

As shown in Figure 11, ARP direct route advertisement is enabled on Interface A and Interface B. This feature generates a host route to Server A and a host route to Server B for the routing protocols to advertise. So each device forwards only the traffic destined to the server within the network, which saves bandwidth.

Figure 11 Application in a Layer 3 access network

If you enable ARP direct route advertisement on the public network or in a VPN instance, all interfaces on the public network or in the VPN instance are enabled with ARP direct route advertisement.

Application in L2VPN access to L3VPN networks

As shown in Figure 12, the ARP direct route feature is configured on PE-aggs to advertise host routes to the connected PEs in the L3VPN.

The PE in the L3VPN has ECMP routes destined to a base station in the L2VPN. Traffic from the PE in the L3VPN to the base station can be load shared by PE-agg 1 and PE-agg 2. If PE-agg 1 fails, the PE uses the host route through PE-agg 2 to forward traffic.

For information about L2VPN access to L3VPN, see MPLS Configuration Guide.

Figure 12 Application in L2VPN access to L3VPN networks

Enabling ARP direct route advertisement

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable ARP direct route advertisement.

¡ Enable ARP direct route advertisement in system view.

arp route-direct advertise

¡ Execute the following commands in sequence to enable ARP direct route advertisement on an interface:

interface interface-type interface-number

arp route-direct advertise

¡ Execute the following commands in sequence to enable ARP direct route advertisement in a VPN instance:

ip vpn-instance vpn-instance-name

arp route-direct advertise

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

06-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 this task

Procedure

About this task

Procedure

Setting the dynamic ARP learning limit for an interface

About this task

Procedure

Setting the aging timer for dynamic ARP entries

About this task

Procedure

About this task

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Procedure

ARP configuration examples

Network configuration

Procedure

Verifying the configuration

Example: Configuring a short static ARP entry

Network configuration

Procedure

Verifying the configuration

Gratuitous ARP tasks at a glance

About this task

Procedure

Restrictions and guidelines

Procedure

About this task

Procedure

Configuring proxy ARP

Enabling common proxy ARP

About this task

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Procedure

Network configuration

Procedure

Verifying the configuration

Configuring ARP suppression

Display and maintenance commands for ARP suppression

Network configuration

Procedure

Verifying the configuration

Procedure

Verifying the configuration

Configuring ARP direct route advertisement

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