Table of Contents

Chapter 1 MAC Address Table Management 1-1

1.1 Introduction to MAC Address Table Management 1-1

1.1.1 Introduction to MAC Address Table. 1-1

1.1.2 Classification and Characteristics of MAC Address Entries. 1-2

1.2 Configuring the MAC Address Table. 1-3

1.2.1 Configuring MAC Address Table Entries. 1-3

1.2.2 Configuring MAC Address Aging Time for the System.. 1-3

1.2.3 Configuring the Maximum Number of MAC Addresses that an Ethernet Port Can Learn. 1-4

1.3 Displaying the MAC Address Table Configuration. 1-5

1.4 MAC Address Table Management Configuration Example. 1-6

 

Chapter 1  MAC Address Table Management

1.1  Introduction to MAC Address Table Management

1.1.1  Introduction to MAC Address Table

An Ethernet switch needs to maintain a MAC address table to speed up packet forwarding. A table entry includes the MAC address of a device connected to the Ethernet switch, the interface number, and VLAN ID of the Ethernet switch connected to the device.

A MAC address table includes both static and dynamic address entries.

l           Static entries are manually configured by users and will never be aged out.

l           Dynamic entries can be manually configured by users or dynamically learned by the Ethernet switch. Dynamic entries can be aged out (if a dynamic entry has its aging time configured as aging, it will be aged out; if its aging time is configured as no-aging, the entry will not be aged out).

Figure 1-1 illustrates how an Ethernet switch learns a MAC addresses: after receiving a data frame from a port (assumed as port A), the Ethernet switch analyzes its source MAC address (assumed as MAC-SOURCE) and considers that the packets destined for MAC-SOURCE can be forwarded through port A. If the table contains the MAC-SOURCE, the Ethernet switch will update the corresponding entry, otherwise, it will add the new MAC address and the corresponding port A as a new entry to the table.

Figure 1-1 MAC address learning process

 

 

The Ethernet switch forwards packets whose destination MAC addresses can be found in the MAC address table and broadcasts those whose destination MAC addresses are not in the table. After the Ethernet switch broadcasts the packet:

l           If the packet reaches the destination network device, the device sends a response packet back which contains the MAC address of this device. The Ethernet switch learns and adds this new MAC address to the MAC address table of the device. The consequent packets destined for the same MAC address can be forwarded directly thereafter.

l           If the packet fails to reach the network device that the MAC address corresponds to, the switch discards the packet.

1.1.2  Classification and Characteristics of MAC Address Entries

Depending on their characteristics and how they are configured, MAC address entries fall into the following three categories:

l           Static MAC address entries: Also known as "permanent addresses", this type of entries are added or deleted manually and will never be aged out. Static address entries can be added manually to reduce the broadcast traffic on a network with minor device changes.

l           Dynamic MAC address entries: These are the MAC address entries that will be aged out after the aging timer configured expires. Dynamic MAC address entries can be generated by the switch through MAC address learning or configured manually.

l           Blackhole MAC address entries: These are special MAC addresses manually configured by users. When the switch receives a packet whose source address or destination address is a blackhole MAC address, the switch discards the packet.

Table 1-1 shows the classification and characteristics of MAC address entries.

Table 1-1 Classification and characteristics of MAC address entries

 

1.2  Configuring the MAC Address Table

1.2.1  Configuring MAC Address Table Entries

Administrators can manually add, modify, or delete the entries in a MAC address table according to actual needs.

Table 1-2 Configure MAC Address Table Entries

 

1.2.2  Configuring MAC Address Aging Time for the System

Setting the aging time too long will result in a large number of outdated table entries being kept in the MAC address table, and thereby exhausting the MAC address table resources and making it impossible for the Ethernet switch to update the MAC address table according to the network change. On the other hand, if the aging time is set too short, valid MAC address table entries may be deleted by the Ethernet switch. This causes the switch to identify a large number of packets as unicasts with unknown destination address and broadcast them within the VLAN, resulting in degradation of the switch performance. Therefore, it is important that subscribers set an appropriate aging time according to the actual network environment in order to implement MAC address aging effectively.

Table 1-3 Configure MAC address aging time for the system

 

 

1.2.3  Configuring the Maximum Number of MAC Addresses that an Ethernet Port Can Learn

If the switch maintains an oversize MAC address table, it may take longer for the switch to find the entry corresponding to the destination MAC address for packet forwarding, thus degrading the forwarding performance of the switch. By setting the maximum number of MAC addresses that an Ethernet port can learn, you can limit the number of entries in the Ethernet port MAC address table maintained by the Ethernet switch. If the maximum number of MAC address is set to count, then after the number of learned MAC addresses has reached to count, the interface will no longer learn any more MAC addresses.

Table 1-4 Configure the maximum number of MAC addresses that an Ethernet port can learn

 

1.3  Displaying the MAC Address Table Configuration

After the above-mentioned configuration, you can use the display command in any view to display MAC address table operating information, so as to verify configuration result.

Table 1-5 Display the MAC address table configuration

 

1.4  MAC Address Table Management Configuration Example

I. Network requirements

The user logs on to the device through the Console port. Enable the MAC address table management function. Configure the aging time for dynamic table entries as 500 seconds. Add a static address table entry “00e0-fc35-dc71” to GigabitEthernet1/0/7 in VLAN 1.

II. Configuration procedure

# Enter system view.

<Sysname> system-view

# Add a static MAC address.

[Sysname] mac-address static 00e0-fc35-dc71 interface GigabitEthernet 1/0/7 vlan 1

# Configure the aging time for dynamic MAC address table entries as 500 seconds.

[Sysname] mac-address timer aging 500

# Display the MAC address information on GigabitEthernet1/0/7.

[Sysname] display mac-address interface GigabitEthernet 1/0/7

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

00e0-fc35-dc71   1       Config static   GigabitEthernet1/0/7 NOAGED

 

  ---  1 mac address(es) found  ---