Contents

Configuring ARP· 2

Overview· 2

ARP message format 2

ARP operating mechanism·· 2

ARP table· 3

Configuring a static ARP entry· 4

Setting the maximum number of dynamic ARP entries for a device· 5

Setting the maximum number of dynamic ARP entries for an interface· 5

Setting the aging timer for dynamic ARP entries· 5

Enabling dynamic ARP entry check· 6

Enabling ARP logging· 6

Displaying and maintaining ARP· 7

Configuration examples· 7

Static ARP configuration example· 7

Configuring gratuitous ARP· 8

Overview· 8

Gratuitous ARP packet learning· 9

Periodic sending of gratuitous ARP packets· 9

Configuration procedure· 9

Enabling IP conflict notification· 10

Configuring gratuitous ARP packet retransmission for the device MAC address change· 10

Configuring proxy ARP· 12

Enabling common proxy ARP· 12

Enabling local proxy ARP· 12

Displaying proxy ARP· 12

Common proxy ARP configuration example· 13

Network requirements· 13

Configuration procedure· 13

Verifying the configuration· 14

Configuring ARP snooping· 15

Configuration procedure· 15

Displaying and maintaining ARP snooping· 15

Configuring ARP fast-reply· 16

Overview· 16

Operation· 16

Configuration procedure· 16

ARP fast-reply configuration example· 16

Network requirements· 16

Configuration procedure· 17

 

Configuring ARP

Overview

ARP resolves IP addresses into MAC addresses on Ethernet networks.

ARP message format

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 table

An ARP table stores dynamic, static, and OpenFlow 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 contains the IP address, MAC address, VLAN, and output interface. It is directly used for forwarding packets.

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

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 tasks:

¡  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.

Configure a short static ARP entry when the device communicates with a host by using a fixed IP-to-MAC mapping. Configure a long static ARP entry when the device communicates with a host by using a fixed IP-to-MAC mapping through an interface in a VLAN.

OpenFlow ARP entry

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

Configuring a static ARP entry

A static ARP entry is effective when the device functions correctly. If a VLAN or VLAN interface is deleted, long static ARP entries in the VLAN are deleted, and resolved short static ARP entries in the VLAN become unresolved.

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

A long static ARP entry is ineffective in either of the following situations:

·           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.

Follow these guidelines when you configure a long static ARP entry:

·           The vlan-id argument must be the ID of an existing VLAN where the ARP entry resides. The specified Ethernet interface must belong to that VLAN.

·           The VLAN interface must be created. Its IP address and the specified IP address must be on the same subnet.

To configure a static ARP entry:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Configure a static ARP entry.	·          Configure a long static ARP entry: arp static ip-address mac-address vlan-id interface-type interface-number [ vpn-instance vpn-instance-name ] ·          Configure a short static ARP entry: arp static ip-address mac-address [ vpn-instance vpn-instance-name ]	By default, no static ARP entry is configured.

 

Setting the maximum number of dynamic ARP entries for a device

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

If you set a value lower than the number of existing dynamic ARP entries, the device does not remove the existing entries unless they are aged out.

To set the maximum number of dynamic ARP entries for a device:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Set the maximum number of dynamic ARP entries for the device.	arp max-learning-number number	By default, a device can learn a maximum of 1024 dynamic ARP entries. If the value for the number argument is set to 0, the device is disabled from learning dynamic ARP entries.

 

Setting the maximum number of dynamic ARP entries 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 maximum number is reached, the interface stops learning ARP entries.

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.

To set the maximum number of dynamic ARP entries for an interface:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enter interface view.	interface interface-type interface-number	N/A
3.      Set the maximum number of dynamic ARP entries for the interface.	arp max-learning-num number	By default, an interface can learn a maximum of 1024 dynamic ARP entries. If the value of the number argument is set to 0, the interface is disabled from learning dynamic ARP entries.

 

Setting 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.

To set the aging timer for dynamic ARP entries:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Set the aging timer for dynamic ARP entries.	arp timer aging aging-time	By default, the aging time for dynamic ARP entries is 20 minutes.

 

Enabling dynamic ARP entry check

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

When this function is disabled, ARP entries containing multicast MAC addresses are supported. The device can learn dynamic ARP entries from ARP packets containing multicast sender MAC addresses. You can manually add static ARP entries containing multicast MAC addresses.

To enable dynamic ARP entry check:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enable dynamic ARP entry check.	arp check enable	By default, dynamic ARP entry check is enabled.

 

Enabling ARP logging

This function 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 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 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.

To enable the ARP logging function:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enable the ARP logging function.	arp check log enable	By default, ARP logging is disabled.

 

Displaying and maintaining ARP

	IMPORTANT: 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.	display arp [ [ all | dynamic | static ] [ slot slot-number ] | vlan vlan-id | interface interface-type interface-number ] [ count | verbose ]
Display the ARP entry for an IP address.	display arp ip-address [ slot slot-number ] [ verbose ]
Display the maximum number of ARP entries that a device supports.	display arp entry-limit
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
Clear ARP entries from the ARP table.	reset arp { all | dynamic | interface interface-type interface-number | slot slot-number | static }

 

Configuration examples

Static ARP configuration example

Network requirements

As shown in Figure 3, hosts are connected to the switch. The switch is connected to the router through interface Ten-GigabitEthernet 1/0/1 in VLAN 10.

To ensure secure communications between the router and switch, configure a static ARP entry for the router on the switch.

Figure 3 Network diagram

 

Configuration procedure

# Create VLAN 10.

<Switch> system-view

[Switch] vlan 10

[Switch-vlan10] quit

# Add interface Ten-GigabitEthernet 1/0/1 to VLAN 10.

[Switch] interface Ten-GigabitEthernet 1/0/1

[Switch-Ten-GigabitEthernet1/0/1] port access vlan 10

[Switch-Ten-GigabitEthernet1/0/1] quit

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

[Switch] interface vlan-interface 10

[Switch-vlan-interface10] ip address 192.168.1.2 8

[Switch-vlan-interface10] quit

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

[Switch] arp static 192.168.1.1 00e0-fc01-0000 10 Ten-GigabitEthernet 1/0/1

# Display information about static ARP entries.

[Switch] display arp static

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

IP address       MAC address     VLAN     Interface              Aging Type

192.168.1.1      00e0-fc01-0000  10       XGE1/0/1               N/A   S

Configuring gratuitous ARP

Overview

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.

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

Enabling periodic sending of gratuitous ARP packets helps downstream devices update ARP entries or MAC entries in a timely manner. This feature implements 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 then 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. The receiving hosts can update ARP entries in time.

·           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.

Configuration procedure

The following conditions apply to the gratuitous ARP configuration:

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

·           Periodic sending of gratuitous ARP packets takes effect only when the link of the enabled interface goes up and an IP address has been assigned to the interface.

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

·           The frequency of sending gratuitous ARP packets might be much lower than the sending interval set by the user in any of the following circumstances:

¡  This function 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.

To configure gratuitous ARP:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enable learning of gratuitous ARP packets.	gratuitous-arp-learning enable	By default, learning of gratuitous ARP packets is enabled.
3.      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.
4.      Enter interface view.	interface interface-type interface-number	N/A
5.      Enable periodic sending of gratuitous ARP packets and set the sending interval.	arp send-gratuitous-arp [ interval milliseconds ]	By default, periodic sending of gratuitous ARP packets is disabled.

 

Enabling IP conflict notification

By default, if the sender IP address of an ARP packet is being used by the receiving device, the receiving device sends a gratuitous ARP request. It also displays an error message after it receives an ARP reply about the conflict.

You can use this command to enable the device to display error messages before sending a gratuitous ARP reply or request for conflict confirmation.

To enable IP conflict notification:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enable IP conflict notification.	arp ip-conflict log prompt	By default, IP conflict notification is disabled.

 

Configuring 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.

To configure gratuitous ARP packet retransmission for the device MAC address change:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
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

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

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enter VLAN interface view.	interface interface-type interface-number	N/A
3.      Enable common proxy ARP.	proxy-arp enable	By default, common proxy ARP is disabled.

 

Enabling local proxy ARP

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enter VLAN interface view.	interface interface-type interface-number	N/A
3.      Enable local proxy ARP.	local-proxy-arp enable [ ip-range startIP to endIP ]	By default, local proxy ARP is disabled.

 

Displaying 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 example

Network requirements

As shown in Figure 4, 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 4 Network diagram

 

Configuration procedure

# 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

ARP snooping is used in Layer 2 switching networks. It creates ARP snooping entries by using information in ARP packets. Manual-mode MFF (MAC–Forced Forwarding) can use the ARP snooping entries. For more information about MFF, see Security Configuration Guide.

If you enable ARP snooping for a VLAN, ARP packets received by any interface 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.

The aging time and valid period of an ARP snooping entry are 25 minutes and 15 minutes. 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 entry expires, the entry is removed.

An ARP packet that has the same sender IP address as a valid ARP snooping entry, but with a different sender MAC address, is considered an attack. The ARP snooping entry becomes invalid, and is removed after 25 minutes.

Configuration procedure

To enable ARP snooping for a VLAN:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enter VLAN view.	vlan vlan-id	N/A
3.      Enable ARP snooping	arp snooping enable	By default, ARP snooping is disabled.

 

Displaying and maintaining ARP snooping

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

 

Task	Command
Display ARP snooping entries.	display arp snooping [ vlan vlan-id ] [ slot slot-number ] [ count ] display arp snooping ip ip-address [ slot slot-number ]
Remove ARP snooping entries.	reset arp snooping [ ip ip-address | vlan vlan-id ]

 

Configuring ARP fast-reply

Overview

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 more information about DHCP snooping, see "Configuring DHCP snooping."

Operation

Upon receiving an ARP request with the target IP address being the IP address of the VLAN interface, the device processes it as a normal ARP packet. If not, it takes the following steps to process the packet:

1.      Search the DHCP snooping table for a match.

2.      If the interface in the matching entry is the Ethernet interface that received the ARP request, the device returns no reply. Otherwise, it returns a reply.

3.      If no match 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 returns no reply. Otherwise, it returns a reply.

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.

Configuration procedure

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

To configure ARP fast-reply:

 

Step	Command	Remarks
1.      Enter system view.	system-view	N/A
2.      Enter VLAN view.	vlan vlan-id	N/A
3.      Enable ARP fast-reply.	arp fast-reply enable	By default, ARP fast-reply is disabled.

ARP fast-reply configuration example

Network requirements

As shown in Figure 5, 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 5 Network diagram

 

Configuration procedure

# Enable ARP snooping for VLAN 2 on the switch.

<Switch> system-view

[Switch] vlan 2

[Switch-vlan2] arp snooping enable

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

[Switch-vlan2] arp fast-reply enable

[Switch-vlan2] quit