Contents

Configuring the MAC address table· 2

Overview· 2

How a MAC address entry is created· 2

Types of MAC address entries· 2

MAC address table configuration task list 3

Configuring MAC address entries· 3

Configuration guidelines· 3

Adding or modifying a static or dynamic MAC address entry globally· 4

Adding or modifying a static or dynamic MAC address entry on an interface· 4

Adding or modifying a blackhole MAC address entry· 4

Setting the aging timer for dynamic MAC address entries· 5

Setting the MAC learning limit on an interface· 5

Configuring the unknown frame forwarding rule after the MAC learning limit is reached· 6

Assigning MAC learning priority to interfaces· 6

Displaying and maintaining the MAC address table· 7

MAC address table configuration example· 7

Network requirements· 7

Configuration procedure· 8

Verifying the configuration· 8

 

Configuring the MAC address table

Overview

An Ethernet device uses a MAC address table to forward frames. A MAC address entry includes a destination MAC address, an outgoing interface, and a VLAN ID. When the device receives a frame, it uses the destination MAC address of the frame to look for a match in the MAC address table.

·     The device forwards the frame out of the outgoing interface in the matching entry if a match is found.

·     The device floods the frame in the VLAN of the frame if no match is found.

How a MAC address entry is created

The entries in the MAC address table include entries automatically learned by the device and entries manually added.

MAC address learning

The device can automatically populate its MAC address table by learning the source MAC addresses of incoming frames on each interface.

The device performs the following operations to learn the source MAC address of incoming packets:

1.     Checks the source MAC address (for example, MAC-SOURCE) of the frame.

2.     Looks up the source MAC address in the MAC address table.

¡     The device updates the entry if an entry is found.

¡     The device adds an entry for MAC-SOURCE and the incoming port if no entry is found.

When the device receives a frame destined for MAC-SOURCE after learning this source MAC address, the device performs the following operations:

1.     Finds the MAC-SOURCE entry in the MAC address table.

2.     Forwards the frame out of the port in the entry.

The device performs the learning process for each incoming frame with an unknown source MAC address until the table is fully populated.

Manually configuring MAC address entries

Dynamic MAC address learning does not distinguish between illegitimate and legitimate frames, which can invite security hazards. When Host A is connected to port A, a MAC address entry will be learned for the MAC address of Host A (for example, MAC A). When an illegal user sends frames with MAC A as the source MAC address to port B, the device performs the following operations:

1.     Learns a new MAC address entry with port B as the outgoing interface and overwrites the old entry for MAC A.

2.     Forwards frames destined for MAC A out of port B to the illegal user.

As a result, the illegal user obtains the data of Host A. To improve the security for Host A, manually configure a static entry to bind Host A to port A. Then, the frames destined for Host A are always sent out of port A. Other hosts using the forged MAC address of Host A cannot obtain the frames destined for Host A.

Types of MAC address entries

A MAC address table can contain the following types of entries:

·     Static entries—A static entry is manually added to forward frames with a specific destination MAC address out of the associated interface, and it never ages out. A static entry has higher priority than a dynamically learned one.

·     Dynamic entries—A dynamic entry can be manually configured or dynamically learned to forward frames with a specific destination MAC address out of the associated interface. A dynamic entry might age out. A manually configured dynamic entry has the same priority as a dynamically learned one.

·     Blackhole entries—A blackhole entry is manually configured and never ages out. A blackhole entry is configured for filtering out frames with a specific source or destination MAC address. For example, to block all frames destined for or sourced from a user, you can configure the MAC address of the user as a blackhole MAC address entry. A blackhole entry has higher priority than a dynamically learned one.

A static or blackhole MAC address entry can overwrite a dynamic MAC address entry, but not vice versa. A static entry and a blackhole entry cannot overwrite each another.

MAC address table configuration task list

The configuration tasks discussed in the following sections can be performed in any order.

This document covers only the configuration of unicast MAC address entries, including static, dynamic, and blackhole MAC address entries. For information about configuring static multicast MAC address entries, see IP Multicast Configuration Guide. For information about MAC address table configuration in VPLS, see MPLS Configuration Guide.

To configure the MAC address table, perform the following tasks:

 

Tasks at a glance
(Optional.) Configuring MAC address entries ·     Adding or modifying a static or dynamic MAC address entry globally ·     Adding or modifying a static or dynamic MAC address entry on an interface ·     Adding or modifying a blackhole MAC address entry
(Optional.) Setting the aging timer for dynamic MAC address entries
(Optional.) Setting the MAC learning limit on an interface
(Optional.) Configuring the unknown frame forwarding rule after the MAC learning limit is reached
(Optional.) Assigning MAC learning priority to interfaces

 

Configuring MAC address entries

Configuration guidelines

·     You cannot add a dynamic MAC address entry if a learned entry already exists with a different outgoing interface for the MAC address.

·     The manually configured static and blackhole MAC address entries cannot survive a reboot if you do not save the configuration. The manually configured dynamic MAC address entries are lost upon reboot whether or not you save the configuration.

A frame whose source MAC address matches different types of MAC address entries is processed differently.

 

Type	Description
Static MAC address entry	Forwards the frame according to the destination MAC address regardless of whether the frame's ingress interface is the same as that in the entry.
Blackhole MAC address entry	Drops the frame.
Dynamic MAC address entry	·     Learns the MAC address of the frames received on a different interface from that in the entry and overwrites the original entry. ·     Forwards the frame received on the same interface as that in the entry and updates the aging timer for the entry.

 

Adding or modifying a static or dynamic MAC address entry globally

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Add or modify a static or dynamic MAC address entry.	mac-address { dynamic | static } mac-address interface interface-type interface-number vlan vlan-id	By default, no MAC address entry is configured globally. Make sure you have created the VLAN and assigned the interface to the VLAN.

 

Adding or modifying a static or dynamic MAC address entry on an interface

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter interface view.	·     Enter Layer 2 Ethernet interface view: interface interface-type interface-number ·     Enter Layer 2 aggregate interface view: interface bridge-aggregation interface-number	N/A
3.     Add or modify a static or dynamic MAC address entry.	mac-address { dynamic | static } mac-address vlan vlan-id	By default, no MAC address entry is configured on the interface. Make sure you have created the VLAN and assigned the interface to the VLAN.

 

Adding or modifying a blackhole MAC address entry

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Add or modify a blackhole MAC address entry.	mac-address blackhole mac-address vlan vlan-id	By default, no blackhole MAC address entry is configured. Make sure you have created the VLAN.

 

Setting the aging timer for dynamic MAC address entries

For security and efficient use of table space, the MAC address table uses an aging timer for each dynamic MAC address entry. If a dynamic MAC address entry is not updated before the aging timer expires, the device deletes the entry. This aging mechanism ensures that the MAC address table can promptly update to accommodate latest network topology changes.

A stable network requires a longer aging interval, and an unstable network requires a shorter aging interval.

An aging interval that is too long might cause the MAC address table to retain outdated entries. As a result, the MAC address table resources might be exhausted, and the MAC address table might fail to update its entries to accommodate the latest network changes.

An interval that is too short might result in removal of valid entries, which would cause unnecessary floods and possibly affect the device performance.

To reduce floods on a stable network, set a long aging timer or disable the timer to prevent dynamic entries from unnecessarily aging out. Reducing floods improves the network performance. Reducing flooding also improves the security because it reduces the chances for a data frame to reach unintended destinations.

To set the aging timer for dynamic MAC address entries:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Set the aging timer for dynamic MAC address entries.	mac-address timer { aging seconds | no-aging }	The default setting is 300 seconds. The no-aging keyword disables the aging timer.

 

Setting the MAC learning limit on an interface

This feature limits the MAC address table size. A large MAC address table will degrade forwarding performance.

To set the MAC learning limit on an interface:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter interface view.	·     Enter Layer 2 Ethernet interface view: interface interface-type interface-number ·     Enter Layer 2 aggregate interface view: interface bridge-aggregation interface-number	N/A
3.     Set the MAC learning limit on the interface.	mac-address max-mac-count count	By default, the MAC learning limit on the interface is restricted only by hardware capacity.

 

Configuring the unknown frame forwarding rule after the MAC learning limit is reached

You can enable or disable forwarding of unknown frames after the MAC learning limit on an interface is reached.

 

	NOTE: In this document, unknown frames refer to frames whose source MAC addresses are not in the MAC address table.

 

To configure the device to forward unknown frames received on the interface after the MAC learning limit on the interface is reached:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter interface view.	·     Enter Layer 2 Ethernet interface view. interface interface-type interface-number ·     Enter Layer 2 aggregate interface view. interface bridge-aggregation interface-number	N/A
3.     Configure the device to forward unknown frames received on the interface after the MAC learning limit on the interface is reached.	mac-address max-mac-count enable-forwarding	By default, the device can forward unknown frames received on an interface after the MAC learning limit on the interface is reached.

 

Assigning MAC learning priority to interfaces

The MAC learning priority mechanism assigns either low priority or high priority to an interface. An interface with high priority can learn MAC addresses as usual. However, an interface with low priority is not allowed to learn MAC addresses already learned on a high-priority interface.

The MAC learning priority mechanism can help defend your network against MAC address spoofing attacks. In a network that performs MAC-based forwarding, an upper layer device MAC address might be learned by a downlink interface because of a loop or attack to the downlink interface. To avoid this problem, perform the following tasks:

·     Assign high MAC learning priority to an uplink interface.

·     Assign low MAC learning priority to a downlink interface.

To assign MAC learning priority to an interface:

 

Step	Command	Remarks
1.     Enter system view.	system-view	N/A
2.     Enter interface view.	·     Enter Layer 2 Ethernet interface view: interface interface-type interface-number ·     Enter Layer 2 aggregate interface view: interface bridge-aggregation interface-number	N/A
3.     Assign MAC learning priority to the interface.	mac-address mac-learning priority { high | low }	By default, low MAC learning priority is used.

 

Displaying and maintaining the MAC address table

Execute display commands in any view.

 

Task	Command
Display MAC address table information.	display mac-address [ mac-address [ vlan vlan-id ] | [ [ dynamic | static ] [ interface interface-type interface-number ] | blackhole ] [ vlan vlan-id ] [ count ] ]
Display the aging timer for dynamic MAC address entries.	display mac-address aging-time

 

MAC address table configuration example

Network requirements

As shown in Figure 1:

·     Host A at MAC address 000f-e235-dc71 is connected to interface GigabitEthernet 1/1/1 of Device and belongs to VLAN 1.

·     Host B at MAC address 000f-e235-abcd, which behaved suspiciously on the network, also belongs to VLAN 1.

Configure the MAC address table as follows:

·     To prevent MAC address spoofing, add a static entry for Host A in the MAC address table of Device.

·     To drop all frames destined for Host B, add a blackhole MAC address entry for Host B.

·     Set the aging timer to 500 seconds for dynamic MAC address entries.

Figure 1 Network diagram

 

Configuration procedure

# Add a static MAC address entry for MAC address 000f-e235-dc71 on GigabitEthernet 1/1/1 that belongs to VLAN 1.

<Device> system-view

[Device] mac-address static 000f-e235-dc71 interface gigabitethernet 1/1/1 vlan 1

# Add a blackhole MAC address entry for MAC address 000f-e235-abcd that belongs to VLAN 1.

[Device] mac-address blackhole 000f-e235-abcd vlan 1

# Set the aging timer to 500 seconds for dynamic MAC address entries.

[Device] mac-address timer aging 500

Verifying the configuration

# Display the static MAC address entries for interface GigabitEthernet 1/1/1.

[Device] display mac-address static interface gigabitethernet 1/1/1

MAC Address      VLAN ID    State            Port/NickName            Aging

000f-e235-dc71   1          Static           GE1/1/1                   N

# Display the blackhole MAC address entries.

[Device] display mac-address blackhole

MAC Address      VLAN ID    State            Port/NickName            Aging

000f-e235-abcd   1          Blackhole        N/A                      N

# Display the aging time of dynamic MAC address entries.

[Device] display mac-address aging-time

MAC address aging time: 500s.