H3C S3100-52P Ethernet Switch Operation Manual-Release 1500(V1.02)

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19-ARP Operation
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Chapter 1  ARP Configuration

1.1  Introduction to ARP

Address resolution protocol (ARP) is used to map IP addresses to the corresponding MAC addresses so that packets can be delivered to their destinations correctly.

1.1.1  Necessity of the Address Resolution

After a packet is forwarded to the destination network, MAC address is necessary for the packet to reach the very device. So the destination IP address carried in a packet need to be translated into the corresponding MAC address.

1.1.2  ARP Packet Structure

ARP packets are classified as ARP request packets and ARP reply packets. Figure 1-1 illustrates the structure of these two types of ARP packets.

l           As for an ARP request packet, all the fields except the hardware address of the receiver field are set. The hardware address of the receiver is what the sender request for.

l           As for an ARP reply packets, all the fields are set.

Figure 1-1 Structure of an ARP request/reply packet

Table 1-1 describes the fields of an ARP packet.

Table 1-1 Description on the fields of an ARP packet

Field

Description

Hardware Type

Identifies the type of the hardware interface. Refer to Table 1-2 for the information about the field values.

Protocol type

Identifies the type of the protocol used by the sending device. Normally, the field takes the value of 1 in TCP/IP networks, which stands for EtherType.

Length of the hardware address

Hardware address length (in bytes)

Length of protocol address

Protocol address length (in bytes)

Operator

Indicates the type of a data packets, which can be:

l      1: ARP request packets

l      2: ARP reply packets

l      3: RARP request packets

l      4: RARP reply packets

Hardware address of the sender

Hardware address of the sender

IP address of the sender

IP address of the sender

Hardware address of the receiver

l      For an ARP request packet, this field is null.

l      For an ARP reply packet, this field carries the hardware address of the receiver.

IP address of the receiver

IP address of the receiver

 

Table 1-2 Description on the values of the hardware type field

Value

Description

1

Ethernet

2

Experimental Ethernet

3

X.25

4

Proteon ProNET (Token Ring)

5

Chaos

6

IEEE802.X

7

ARC network

 

1.1.3  ARP Table

In an Ethernet, the MAC addresses of two hosts must be available for the two hosts to communicate with each other. Each host in an Ethernet maintains an ARP mapping table, where the latest used IP address-to-MAC address mapping entries are stored. Note that this manual only introduces the basic implementation of the mapping table. Different products of different manufactures may provide more information about the mapping table. S3100-52P Ethernet switches provide the display arp command to display the information about ARP mapping entries. Figure 1-2 shows the structure of an ARP mapping table.

Figure 1-2 An ARP mapping table

Table 1-3 describes the APR mapping table fields.

Table 1-3 Description on the fields of an ARP table

Field

Description

IF index

Index of the physical interface/port on the device owning the physical address and IP address contained in the entry

Physical address

Physical address of the device, that is, the MAC address

IP address

IP address of the device

Type

Entry type, which can be:

l      1: An entry falling out of the following three cases

l      2: Invalid entry

l      3: Dynamic entry

l      4: Static entry

 

1.1.4  ARP Implementation Procedure

The ARP mapping table of a host is empty when the host is just started up. And when a dynamic ARP mapping entry is not in use for a specified period of time, it is removed from the ARP mapping table so as to save the memory space and shorten the interval for the switch to look up entries in the ARP mapping table. For details, refer to Figure 1-3.

l           Suppose there are two hosts on the same network segment: Host A and Host B. The IP address of Host A is IP_A and that of Host B is IP_B. To send a packet to Host B, Host A checks its own ARP mapping table first to see if the ARP entry corresponding to IP_B exists. If yes, Host A encapsulates the IP packet into a frame with the MAC address of Host B inserted to it and sends it to Host B.

l           If the corresponding MAC address is not found in the ARP mapping table, Host A adds the packet in the transmission queue, creates an ARP request packet and broadcasts it throughout the Ethernet. As mentioned earlier, the ARP request packet contains the IP address of Host B, the IP address of Host A, and the MAC address of Host A. Since the ARP request packet is broadcasted, all hosts on the network segment can receive it. However, only the requested host (namely, Host B) processes the request.

l           Host B saves the IP address and the MAC address carried in the request packet (that is, the IP address and the MAC address of the sender, Host A) to its ARP mapping table and then sends back an ARP reply packet to the sender (Host A), with its MAC address carried in the packet. Note that the ARP reply packet is a unicast packet instead of a broadcasted packet.

l           Upon receiving the ARP reply packet, Host A extracts the IP address and the corresponding MAC address of Host B from the packet, adds them to its ARP mapping table, and then transmits all the packets in the queue with their destination being Host B.

Figure 1-3 ARP work flow

Normally, a device automatically triggers the ARP calculation in the IP addressing process.

1.1.5  Introduction to Gratuitous ARP

The following are the characteristics of gratuitous ARP packets:

l           Both source and destination IP addresses carried in a gratuitous ARP packet are the local addresses, and the source MAC address carried in it is the local MAC addresses.

l           If a device finds that the IP addresses carried in a received gratuitous packet conflict with those of its own, it returns an ARP response to the sending device to notify of the IP address conflict.

By sending gratuitous ARP packets, a network device can:

l           Determine whether or not IP address conflicts exist between it and other network devices.

l           Trigger other network devices to update its hardware address stored in their caches.

The gratuitous ARP packet learning function:

When the gratuitous ARP packet learning function is enabled on a switch and the switch receives a gratuitous ARP packet, the switch updates the existing ARP entry (contained in the cache of the switch) that matches the received gratuitous ARP packet using the hardware address of the sender carried in the gratuitous ARP packet. A switch operates like this whenever it receives a gratuitous ARP packet.

1.2  ARP Configuration

ARP entries in an S3100-52P Ethernet switch can either be static entries or dynamic entries, as described in Table 1-4.

Table 1-4 ARP entries

ARP entry

Generation Method

Maintenance Mode

Static ARP entry

Manually configured

Manual maintenance

Dynamic ARP entry

Dynamically generated

ARP entries of this type age with time. The aging period is set by the ARP aging timer.

 

1.2.1  Adding a Static ARP Mapping Entry Manually

Table 1-5 Add a static ARP mapping entry manually

Operation

Command

Description

Enter system view

system-view

Add a static ARP mapping entry manually

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

Required

By default, the ARP mapping table is empty, and the address mapping entries are created dynamically by ARP.

 

  Caution:

l      Static ARP mapping entries are valid as long as the Ethernet switch operates. But some operations, which make the ARP entries invalid, result in ARP entries being removed, such as changing/removing a VLAN interface, removing a VLAN, or removing a port from a VLAN.

l      As for the arp static command, the value of the vlan-id argument must be the ID of an existing VLAN, and the port identified by the interface-type and interface-number arguments must belong to the VLAN.

l      Currently, static ARP mapping entries cannot be configured on aggregation ports.

 

1.2.2  Configuring the ARP Aging Timer for Dynamic ARP Entries

The ARP aging timer applies to all dynamic ARP mapping entries.

Table 1-6 Configure the ARP aging timer for dynamic ARP entries

Operation

Command

Description

Enter system view

system-view

Configure the ARP aging timer

arp timer aging aging-time

Optional

By default, the ARP aging timer is set to 20 minutes.

 

1.2.3  Enabling the ARP Entry Checking Function

When multiple hosts share one multicast MAC address, you can specify whether or not to create ARP entries for the multicast MAC address by performing the operations listed in Table 1-7.

Table 1-7 Enable the ARP entry checking function

Operation

Command

Description

Enter system view

system-view

Enable the ARP entry checking function (that is, disable the switch from creating ARP entries for multicast MAC addresses)

arp check enable

Optional

By default, the ARP entry checking function is enabled.

 

1.3  Gratuitous ARP Packet Configuration

1.3.1  Configuring Sending of Gratuitous ARP Packets

Sending of gratuitous ARP packets is enabled as long as an S3100-52P switch operates. And no command is needed for this function.

1.3.2  Configuring the Gratuitous ARP packet Learning Function

Table 1-8 lists the operations to configure the gratuitous ARP packet learning function.

Table 1-8 Configure the gratuitous ARP packet learning function

Operation

Command

Description

Enter system view

system-view

Enable the gratuitous ARP packet learning function

gratuitous-arp-learning enable

Required

By default, the gratuitous ARP packet learning function is enabled.

 

1.4  Displaying and Debugging ARP

After the above configuration, you can execute the display command in any view to display the running of the ARP configuration, and to verify the effect of the configuration.

You can execute the reset command in user view to clear ARP mapping entries.

Table 1-9 Display and debug ARP

Operation

Command

Description

Display specific ARP mapping table entries

display arp [ static | dynamic | ip-address ]

These commands can be executed in any view.

Display the ARP mapping entries related to a specified string in a specified way

display arp [ dynamic | static | ip-address ] | { begin | include | exclude } text

Display the number of the ARP mapping entries of a specified type

display arp count [ [ dynamic | static ] [ | { begin | include | exclude } text ] | ip-address ]

Display the setting of the ARP aging timer

display arp timer aging

Clear specific ARP mapping entries

reset arp [ dynamic | static | interface interface-type interface-number ]

Execute this command in user view.