06-Layer 3—IP Services Configuration Guide

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01-ARP configuration
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Contents

Configuring ARP·· 1

About ARP· 1

ARP message format 1

ARP operating mechanism·· 1

ARP entry types· 2

ARP tasks at a glance· 3

Configuring a static ARP entry· 4

Configuring a short static ARP entry· 4

Configuring a long static ARP entry· 4

Configuring a multiport ARP entry· 4

Configuring features for dynamic ARP entries· 5

Setting the dynamic ARP learning limit for a device· 5

Setting the dynamic ARP learning limit for an interface· 6

Setting the aging timer for dynamic ARP entries· 6

Enabling dynamic ARP entry check· 6

Enabling an IP unnumbered interface to learn ARP entries for different subnets· 7

Enabling ARP logging· 7

Display and maintenance commands for ARP· 8

ARP configuration examples· 9

Example: Configuring a long static ARP entry· 9

Example: Configuring a short static ARP entry· 10

Configuring gratuitous ARP·· 11

About gratuitous ARP· 11

IP conflict detection· 11

Gratuitous ARP packet learning· 11

Periodic sending of gratuitous ARP packets· 11

Gratuitous ARP tasks at a glance· 12

Enabling IP conflict notification· 12

Enabling gratuitous ARP packet learning· 13

Enabling periodic sending of gratuitous ARP packets· 13

Enabling sending gratuitous ARP packets for ARP requests with sender IP address on a different subnet 13

Configuring gratuitous ARP packet retransmission for the device MAC address change· 14

Configuring proxy ARP·· 15

About proxy ARP· 15

Enabling common proxy ARP· 15

Enabling local proxy ARP· 15

Display and maintenance commands for proxy ARP· 16

Common proxy ARP configuration example· 16

Example: Configuring common proxy ARP· 16

Configuring ARP snooping· 18

About ARP snooping· 18

Creation of ARP snooping entries· 18

Aging of ARP snooping entries· 18

Protection for ARP snooping· 18

Enabling ARP snooping· 18

Display and maintenance commands for ARP snooping· 18

Configuring ARP fast-reply· 20

About ARP fast-reply· 20

Enabling ARP fast-reply· 20

ARP fast-reply configuration example· 21

Example: Configuring ARP fast-reply· 21

Configuring ARP PnP·· 22

About ARP PnP· 22

Prerequisites· 22

Enabling ARP PnP· 22

Display and maintenance commands for ARP PnP· 22

ARP PnP configuration example· 23

Example: Configuring ARP PnP· 23

Configuring ARP suppression· 25

About ARP suppression· 25

Enabling ARP suppression· 25

Display and maintenance commands for ARP suppression· 26

ARP suppression configuration example· 26

Example: Configuring ARP suppression· 26

Configuring ARP direct route advertisement 28

About ARP direct route advertisement 28

Mechanism of ARP direct route advertisement 28

Application in Layer 3 networks· 28

Application in L2VPN access to L3VPN networks· 28

Enabling ARP direct route advertisement 29

 


Configuring ARP

About ARP

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 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 IGMP snooping configuraiton in 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:

·     Portal. For more information about portal, see Security Configuration Guide.

·     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

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

·     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 ] [ 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 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 ] [ vpn-instance vpn-instance-name ] [ description text ]

Configuring a multiport ARP entry

About this task

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 ] [ description text ]

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 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. 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

In IRF mode:

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

By default, the device can learn a maximum of 65536 dynamic ARP entries.

To disable the 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 65536 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.

Procedure

1.     Enter system view.

system-view

2.     Set the aging timer for dynamic ARP entries.

arp timer aging aging-time

The default setting is 20 minutes.

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.

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.

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 logging

About this task

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

IMPORTANT

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.

In standalone mode:

display arp [ [ all | dynamic | multiport | static ] | vlan vlan-id | interface interface-type interface-number ] [ count | verbose ]

In IRF mode:

display arp [ [ all | dynamic | multiport | static ] [ slot slot-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 [ verbose ]

In IRF mode:

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

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 | static }

In IRF mode:

reset arp { all | dynamic | interface interface-type interface-number | multiport | slot slot-number | static }

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 GigabitEthernet 1/0 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

Procedure

# Create VLAN 10.

<DeviceB> system-view

[DeviceB] vlan 10

[DeviceB-vlan10] quit

# Add interface GigabitEthernet 1/0 to VLAN 10.

[DeviceB] interface gigabitethernet 1/0

[DeviceB-GigabitEthernet1/0] port access vlan 10

[DeviceB-GigabitEthernet1/0] 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 GigabitEthernet 1/0 in VLAN 10.

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

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/Link ID        Aging Type

192.168.1.1      00e0-fc01-0000 10        GE1/0                    --    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 GigabitEthernet 2/0.

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

Procedure

# Configure an IP address for GigabitEthernet 2/0.

<DeviceB> system-view

[DeviceB] interface gigabitethernet 2/0

[DeviceB-GigabitEthernet2/0] ip address 192.168.1.2 24

[DeviceB-GigabitEthernet2/0] 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/Link ID        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 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 ]

Common proxy ARP configuration example

Example: Configuring common proxy ARP

Network configuration

As shown in Figure 5, 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 5 Network diagram

Procedure

# Configure the IP address of GigabitEthernet 2/0.

<Router> system-view

[Router] interface gigabitethernet 2/0

[Router-GigabitEthernet2/0] ip address 192.168.10.99 255.255.255.0

# Enable common proxy ARP on GigabitEthernet 2/0.

[Router-GigabitEthernet2/0] proxy-arp enable

[Router-GigabitEthernet2/0] quit

# Configure the IP address of GigabitEthernet 1/0.

[Router] interface gigabitethernet 1/0

[Router-GigabitEthernet1/0] ip address 192.168.20.99 255.255.255.0

# Enable common proxy ARP on GigabitEthernet 1/0.

[Router-GigabitEthernet1/0] proxy-arp enable

[Router-GigabitEthernet1/0] 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 and MFF can use the ARP snooping entries. For more information about MFF, see Security Configuration Guide.

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 ] [ count ]

display arp snooping ip ip-address

In IRF mode:

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

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

Delete ARP snooping entries.

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


Configuring ARP fast-reply

About ARP fast-reply

ARP fast-reply enables a device to directly answer ARP requests according to IPSG bindings or ARP snooping entries. ARP fast-reply functions in a VLAN.

The device processes a received ARP request as follows:

1.     Checks the target IP address of the ARP request.

¡     If the target IP address is the IP address of the VLAN interface, the device delivers the request to the ARP module.

¡     If the target IP address is not the IP address of the VLAN interface, the process goes to step 2.

2.     Searches IPSG bindings by using the target IP address.

¡     If a match is found and the interface in the matching entry is a wireless interface, the device returns a reply according to the matching entry.

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

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

For information about IPSG, see IP source guard configuration in Security Configuration Guide.

3.     Searches the ARP snooping table if ARP snooping is enabled on the device. If ARP snooping is disabled on the device, the process goes to step 4.

¡     If a match is found and the interface in the matching entry is a wireless interface, the device returns a reply according to the ARP snooping entry.

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

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

¡     If no match is found, the process goes to step 4.

4.     Forwards the ARP request to other interfaces except the receiving interface in the VLAN, or delivers the ARP request  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 6, 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 6 Network diagram

Procedure

# 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 PnP

About ARP PnP

The ARP plug and play (PnP) feature is typically configured on a gateway. This feature allows end users to access the gateway without changing their IP addresses on subnets different from the subnet where the gateway resides.

After ARP PnP is enabled on an interface, it provides the following functions:

·     ARP PnP enables the interface to always reply to users' ARP requests with the interface's MAC address.

·     Upon receiving a packet from the user, ARP PnP replaces the source IP address of the packet with an agent IP address. The agent IP address is on the same subnet as the interface IP address.

·     Upon receiving the return packet destined to the agent IP address, ARP PnP replaces the agent IP with the user's original IP address.

Prerequisites

Before you configure the ARP PnP feature on an interface, perform the following tasks:

·     Assign the interface a primary IP address. ARP PnP generates agent IP addresses based on the primary IP address and mask length of the interface.

·     Use the reset arp command to delete all ARP entries on the interface.

·     Configure NAT on the device. The configuration includes configuring an address group, adding an IP address range to the address group, and configuring outbound dynamic NAT on the interface that connects to the external network. For more information about NAT, see "Configuring NAT."

Enabling ARP PnP

1.     Enter system view.

system-view

2.     Enter interface view of the interface that connects to the internal network.

interface interface-type interface-number

The following interface types are supported:

¡     Layer 3 Ethernet interfaces.

¡     Layer 3 Ethernet subinterfaces.

3.     Enable the ARP PnP feature.

arp pnp

By default, the ARP PnP feature is disabled.

Display and maintenance commands for ARP PnP

Execute display commands in any view.

 

Task

Command

Display ARP PnP mappings.

display arp pnp [ interface interface-type interface-number ]

ARP PnP configuration example

Example: Configuring ARP PnP

Network configuration

As shown in Figure 7, configure the ARP PnP feature to allow the host at 1.2.3.4 to access the external server through GigabitEthernet 1/0.

Figure 7 Network diagram

Procedure

1.     Configure NAT:

# Specify IP addresses for GigabitEthernet 1/0 and GigabitEthernet 2/0.

<Router> system-view

[Router] interface gigabitethernet 1/0

[Router-GigabitEthernet1/0] ip address 192.168.0.2 24

[Router-GigabitEthernet1/0] quit

[Router] interface gigabitethernet 2/0

[Router-GigabitEthernet2/0] ip address 202.38.1.100 24

[Router-GigabitEthernet2/0] quit

# Configure ACL 2000 to identify packets from subnet 192.168.0.0/24.

[Router] acl basic 2000

[Router-acl-ipv4-basic-2000] rule permit source 192.168.0.0 0.0.0.255

[Router-acl-ipv4-basic-2000] quit

# Create address group 1, and add address 202.38.1.100 to the group.

[Router] nat address-group 1

[Router-nat-address-group-1] address 202.38.1.100 202.38.1.100

[Router-nat-address-group-1] quit

# Enable outbound PAT on interface GigabitEthernet 2/0 to translate the source address of outgoing packets matching ACL 2000 into the address in address group 1.

[Router] interface gigabitethernet 2/0

[Router-GigabitEthernet2/0] nat outbound 2000 address-group 1

2.     Enable the ARP PnP feature on GigabitEthernet 1/0.

[Router] interface gigabitethernet 1/0

[Router-GigabitEthernet1/0] arp pnp

[Router-GigabitEthernet1/0] quit

Verifying the configuration

# Verify that the router creates an ARP PnP mapping for the host IP address 1.2.3.4 on GigabitEthernet 1/0.

[Router] display arp pnp interface gigabitethernet 1/0

Total number of entries : 1

Agent IP address   User IP address   MAC address      Interface   Aging

192.168.0.3        1.2.3.4           00e0-fc00-0001   GE1/0       10


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 8 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 8 Typical application

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 ] [ count ]

In IRF mode:

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

Clear ARP suppression entries.

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

ARP suppression configuration example

Example: Configuring ARP suppression

Network configuration

As shown in Figure 9, 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 9 Network diagram

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.


Configuring ARP direct route advertisement

About ARP direct route advertisement

Mechanism of ARP direct route advertisement

The ARP direct route advertisement feature generates host routes based on ARP entries for packet forwarding and route advertisement.

Application in Layer 3 networks

As shown in Figure 10, ARP direct route advertisement is enabled on Interface A and Interface B. This feature advertises a host route to Server A and a host route to Device B, respectively. The routing protocol advertises these host routes rather than the network routes, reducing unexpected traffic caused by network route advertisements.

Figure 10 Application in a Layer 3 network

Application in L2VPN access to L3VPN networks

As shown in Figure 11, 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 11 Application in an 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 the ARP direct route advertisement feature.

arp route-direct advertise

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