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

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11-MAC Address Table Operation
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Chapter 1  MAC Address Table Management

 

&  Note:

This chapter describes the management of static, dynamic, and blackhole MAC address entries. For information about the management of multicast MAC address entries, refer to the section related to multicast protocol in H3C S3100-52P Ethernet Switch  Operation Manual.

 

1.1  Overview

1.1.1  Introduction to MAC Address Learning

An Ethernet switch maintains a MAC address table to forward packets quickly. A MAC address table is a port-based Layer 2 address table. It is the base for Ethernet switch to forward Layer 2 packet. Each entry in a MAC address table contains the following fields:

l           Destination MAC address

l           ID of the VLAN which a port belongs to

l           Forwarding port number

Upon receiving a packet, a switch queries its MAC address table for the forwarding port number according to the destination MAC address carried in the packet and then forwards the packet through the port.

The dynamic address entries (not configured manually) in the MAC address table are learned by the Ethernet switch. When an Ethernet switch learns a MAC address, the following occurs:

When a switch receives a packet from one of its ports (referred to as Port 1), the switch extracts the source MAC address (referred to as MAC-SOURCE) of the packet and considers that the packets destined for MAC-SOURCE can be forwarded through Port 1.

l           If the MAC address table already contains MAC-SOURCE, the switch updates the corresponding MAC address entry.

l           If MAC-SOURCE does not exist in the MAC address table, the switch adds MAC-SOURCE and Port 1 as a new MAC address entry to the MAC address table.

Figure 1-1 A switch uses a MAC address table to forward packets

After learning the source address of the packet, the switch searches the MAC address table for the destination MAC address of the received packet:

l           If it finds a match, it directly forwards the packet.

l           If it finds no match, it forwards the packet to all ports, except the receiving port, within the VLAN to which the receiving port belongs. Normally, this is referred to as broadcasting the packet.

After the packet is broadcast:

l           If the network device returns a packet to the switch, this indicates the packet has been sent to the destination device. The MAC address of the device is carried in the packet. The switch adds the new MAC address to the MAC address table through address learning. After that, the switch can directly forward other packets destined for the same network device by using the newly added MAC address entry.

l           If the destination device does not respond to the packet, this indicates that the destination device is unreachable or that the destination device receives the packet but gives no response. In this case, the switch still cannot learn the MAC address of the destination device. Therefore, the switch will still broadcast any other packet with this destination MAC address.

To fully utilize a MAC address table, which has a limited capacity, the switch uses an aging mechanism for updating the table. That is, the switch removes the MAC address entries related to a network device if no packet is received from the device within the aging time. Aging time only applies to dynamic MAC address entries.

You can manually configure (add or modify) a static or dynamic MAC address entry based on the actual network environment.

 

&  Note:

The switch learns only unicast addresses by using the MAC address learning mechanism but directly drops any packet with a broadcast source MAC address.

 

1.1.2  Entries in a MAC Address Table

Entries in a MAC address table fall into the following categories according to their characteristics and configuration methods:

l           Static MAC address entry: Also known as permanent MAC address entry. This type of MAC address entries are added/removed manually and can not age out by themselves. Using static MAC address entries can reduce broadcast packets remarkably and are suitable for networks where network devices seldom change.

l           Dynamic MAC address entry: This type of MAC address entries age out after the configured aging time. They are generated by the MAC address learning mechanism or configured manually.

l           Blackhole MAC address entry: This type of MAC address entries are configured manually. A switch discards the packets destined for or originated from the MAC addresses contained in blackhole MAC address entries.

Table 1-1 lists the different types of MAC address entries and their characteristics.

Table 1-1 Characteristics of different types of MAC address entries

MAC address entry

Configuration method

Aging time

Reserved or not at reboot (if the configuration is saved)

Static MAC address entry

Manually configured

Unavailable

Yes

Dynamic MAC address entry

Manually configured or generated by MAC address learning mechanism

Available

No

Blackhole MAC address entry

Manually configured

Unavailable

Yes

 

1.2  Configuring MAC Address Table Management

The configuration to manage a MAC address table includes:

l           Configuring a MAC address entry

l           Configuring the aging time of MAC address entries

l           Configuring the maximum number of MAC addresses a port can learn

1.2.1  Configuring a MAC Address Entry

You can add, modify, or remove one MAC address entry, remove all MAC address entries (unicast MAC addresses only) concerning a specific port, or remove specific type of MAC address entries (dynamic or static MAC address entries).

You can add a MAC address entry in either system view or Ethernet port view.

I. Adding a MAC address entry in system view

Table 1-2 Add a MAC address entry in system view

Operation

Command

Description

Enter system view

system-view

Add a MAC address entry

mac-address { static | dynamic | blackhole } mac-address interface interface-type interface-number vlan vlan-id

Required

 

  Caution:

l      When you add a MAC address entry, the port specified by the interface argument must belong to the VLAN specified by the vlan argument in the command. Otherwise, the entry will not be added.

l      If the VLAN specified by the vlan argument is a dynamic VLAN, after a static MAC address is added, it will become a static VLAN.

 

II. Adding a MAC address entry in Ethernet port view

Table 1-3 Add a MAC address entry in Ethernet port view

Operation

Command

Description

Enter system view

system-view

Enter Ethernet port view

interface interface-type interface-number

Add a MAC address entry

mac-address { static | dynamic | blackhole } mac-address vlan vlan-id

Required

 

  Caution:

l      When you add a MAC address entry, the current port must belong to the VLAN specified by the vlan argument in the command. Otherwise, the entry will not be added.

l      If the VLAN specified by the vlan argument is a dynamic VLAN, after a static MAC address is added, it will become a static VLAN.

 

1.2.2  Setting the Aging Time of MAC Address Entries

Setting aging time properly helps implement effective MAC address aging. The aging time that is too long or too short results in a large amount of broadcast packets wandering across the network and decreases the performance of the switch.

l           If the aging time is too long, excessive invalid MAC address entries maintained by the switch may fill up the MAC address table. This prevents the MAC address table from being updated with network changes in time.

l           If the aging time is too short, the switch may remove valid MAC address entries. This decreases the forwarding performance of the switch.

Table 1-4 Set aging time of MAC address entries

Operation

Command

Description

Enter system view

system-view

Set the aging time of MAC address entries

mac-address timer { aging age | no-aging }

Required

The default aging time is 300 seconds.

 

This command is used in system view and applies to all ports. Aging applies to only dynamic MAC addresses that are learnt or configured to age.

Normally, you are recommended to use the default aging time, namely, 300 seconds. The no-aging keyword specifies that MAC address entries do not age out.

1.2.3  Setting the Maximum Number of MAC Addresses a Port Can Learn

The MAC address learning mechanism enables an Ethernet switch to acquire the MAC addresses of the network devices on the segment connected to the ports of the switch. The switch directly forwards the packets destined for these MAC addresses. A MAC address table too big in size may decrease the forwarding performance of the switch.

By setting the maximum number of MAC addresses that can be learnt from individual ports, you can control the number of the MAC address entries the MAC address table can dynamically maintain. When the number of the MAC address entries learnt from a port reaches the set value, the port stops learning MAC addresses.

Table 1-5 Set the maximum number of MAC addresses a port can learn

Operation

Command

Description

Enter system view

system-view

Enter Ethernet port view

interface interface-type interface-number

Set the maximum number of MAC addresses the port can learn

mac-address max-mac-count count

Required

By default, the number of the MAC addresses a port can learn is not limited.

 

1.3  Displaying and Maintaining MAC Address Table Configuration

To verify your configuration, you can display information about the MAC address table by executing the display command in any view.

Table 1-6 Display and maintain MAC address table configuration

Operation

Command

Description

Display information about the MAC address table

display mac-address [ display-option ]

The display command can be executed in any view.

Display the aging time of the dynamic MAC address entries in the MAC address table

display mac-address aging-time

 

1.4  Configuration Example

I. Network requirements

l           Log in to the switch through the Console port and enable address table configuration.

l           Set the aging time of dynamic MAC address entries to 500 seconds.

l           Add a static MAC address entry 00e0-fc35-dc71 for Ethernet1/0/2 port (assuming that the port belongs to VLAN 1)

II. Network diagram

Figure 1-2 Network diagram for MAC address table configuration

III. Configuration procedure

# Enter system view.

<H3C> system-view

[H3C]

# Add a MAC address, with the VLAN, ports, and states specified.

[H3C] mac-address static 00e0-fc35-dc71 interface Ethernet 1/0/2 vlan 1

# Set the aging time of dynamic MAC addresses to 500 seconds.

[H3C] mac-address timer aging 500

# Display the information about the MAC address entries in system view.

[H3C] display mac-address interface Ethernet 1/0/2

MAC ADDR         VLAN ID STATE   PORT INDEX      AGING TIME(s)

00-e0-fc-35-dc-71    1       Static  Ethernet1/0/2   NOAGED

00-e0-fc-17-a7-d6    1       Learned Ethernet1/0/2   AGING

00-e0-fc-5e-b1-fb    1       Learned Ethernet1/0/2   AGING

00-e0-fc-55-f1-16    1       Learned Ethernet1/0/2   AGING

---  4 mac address(es) found on port Ethernet1/0/2 ---