1.1 Introduction to MAC Address Table Management
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.
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Static entries are manually configured by users
and will never be aged out.
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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
During MAC address learning, static MAC addresses that are manually
configured by users will not be overwritten by dynamic MAC addresses. However,
the latter can be overwritten by the former.
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:
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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.
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If the packet fails to reach the network device
that the MAC address corresponds to, the switch discards the packet.
Depending on their characteristics and how
they are configured, MAC address entries fall into the following three
categories:
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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.
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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.
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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
|
MAC address entry
|
Configuration mode
|
To be aged out or not
|
Reserved or not after the device is
reset (with the configuration saved)
|
|
Static MAC
address entry
|
Configured
manually by users
|
No
|
Yes
|
|
Dynamic
MAC address entry
|
Generated
by the device through MAC address learning or configured manually by users
|
Yes
|
No
|
|
Blackhole MAC address entry
|
Configured manually by users
|
No
|
Yes
|
1.2 Configuring the MAC Address Table
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
|
Operation
|
Command
|
Remarks
|
|
Enter system view
|
system-view
|
—
|
|
Add/modify a MAC address entry
|
Add/modify a MAC address entry in system
view
|
mac-address
{ static | dynamic | blackhole } mac-address
interface interface-type interface-number vlan
vlan-id
|
Either is required.
By default, the MAC address table
maintained by the switch contains only the MAC addresses learned.
|
|
Add/modify a MAC address entry in Ethernet
port view
|
interface interface-type interface-number
|
|
mac-address
{ static | dynamic| blackhole } mac-address
vlan vlan-id
|
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
|
To do…
|
Use the command…
|
Remarks
|
|
Enter system view
|
system-view
|
—
|
|
Configure the dynamic MAC address aging
time
|
mac-address
timer { aging seconds | no-aging }
|
Optional
300 seconds by default
|
This command takes
effect on all ports. However, the address aging only functions on dynamic MAC
address entries (those learned by the switch or configured by the user).
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
|
To do…
|
Use the command…
|
Remarks
|
|
Enter
system view
|
system-view
|
—
|
|
Enter
Ethernet port view or port group view
|
Enter the
Ethernet port view of a specified port
|
interface
interface-type interface-number
|
At least
one required
The
consequent configurations apply to the current interface only after entering
its interface view; the consequent configurations apply to all ports in a
port group after entering the port group view
|
|
Enter the
port group view of a specified port group
|
port-group
{ aggregation agg-id | manual
port-group-name }
|
|
Configure
the maximum number of MAC addresses that can be learned by an Ethernet port.
|
mac-address
max-mac-count count
|
Required
By
default, no limit is configured on 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
|
To do...
|
Use the command…
|
Remarks
|
|
Display
the information in the address table
|
display
mac-address [ mac-address [ vlan vlan-id ] | [ static | dynamic|
blackhole] [ interface interface-type interface-number ]
[ vlan vlan-id ] [ count ] ]
|
Available in any view
|
|
Display
the aging time of dynamic address table entries
|
display
mac-address aging-time
|
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 ---
