H3C S3100-52P Operation Manual-Release 1602(V1.01)

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

When configuring MAC address table management, go to these sections for information you are interested in:

l           Overview

l           Configuring MAC Address Table Management

l           Displaying MAC Address Table Information

l           Configuration Example

 

&  Note:

l      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 part related to multicast protocol.

l      The function of destination MAC address triggered update was introduced. For more information, refer to Enabling Destination MAC Address Triggered Update.

l      The function of assigning MAC addresses to Ethernet ports was introduced. For more information, refer to Assigning MAC Addresses for Ethernet Ports.

 

1.1  Overview

1.1.1  Introduction to MAC Address Table

An Ethernet switch is mainly used to forward packets at the data link layer, that is, transmit the packets to the corresponding ports according to the destination MAC address of the packets. To forward packets quickly, a switch maintains a MAC address table, which is a Layer 2 address table recording the MAC address-to-forwarding port association. 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 egress port numbers on the local switch

When forwarding a packet, an Ethernet switch adopts one of the two forwarding methods based upon the MAC address table entries.

l           Unicast forwarding: If the destination MAC address carried in the packet is included in a MAC address table entry, the switch forwards the packet through the forwarding egress port in the entry.

l           Broadcast forwarding: If the destination MAC address carried in the packet is not included in the MAC address table, the switch broadcasts the packet to all ports except the one receiving the packet.

1.1.2  Introduction to MAC Address Learning

MAC address table entries can be updated and maintained through the following two ways:

l           Manual configuration

l           MAC address learning

Generally, the majority of MAC address entries are created and maintained through MAC address learning. The following describes the MAC address learning process of a switch:

1)         As shown in Figure 1-1, User A and User B are both in VLAN 1. When User A communicates with User B, the packet from User A needs to be transmitted to Ethernet 1/0/1. At this time, the switch records the source MAC address of the packet, that is, the address MAC-A of User A to the MAC address table of the switch, forming an entry shown in Figure 1-2.

Figure 1-1 MAC address learning diagram (1)

 

Figure 1-2 MAC address table entry of the switch (1)

2)         After learning the MAC address of User A, the switch starts to forward the packet. Because there is no MAC address and port information of User B in the existing MAC address table, the switch forwards the packet to all ports except Ethernet 1/0/1 to ensure that User B can receive the packet.

Figure 1-3 MAC address learning diagram (2)

3)         Because the switch broadcasts the packet, both User B and User C can receive the packet. However, User C is not the destination device of the packet, and therefore does not process the packet. Normally, User B will respond to User A, as shown in Figure 1-4. When the response packet from User B is sent to Ethernet 1/0/4, the switch records the association between the MAC address of User B and the corresponding port to the MAC address table of the switch.

 

Figure 1-4 MAC address learning diagram (3)

4)         At this time, the MAC address table of the switch includes two forwarding entries shown in Figure 1-5. When forwarding the response packet, the switch unicasts the packet instead of broadcasting it to User A through Ethernet 1/0/1, because MAC-A is already in the MAC address table.

Figure 1-5 MAC address table entries of the switch (2)

5)         After this interaction, the switch directly unicasts the packets destined for User A and User B based on the corresponding MAC address table entries.

 

&  Note:

l      Under some special circumstances, for example, User B is unreachable or User B receives the packet but does not respond to it, the switch cannot learn the MAC address of User B. Hence, the switch still broadcasts the packets destined for User B.

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

 

1.1.3  Managing MAC Address Table

I. Aging of MAC address table

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 starts an aging timer for an entry when dynamically creating the entry. The switch removes the MAC address entry if no more packets with the MAC address recorded in the entry are received within the aging time.

 

&  Note:

l      The MAC address aging timer only takes effect on dynamic MAC address entries.

l      With the destination MAC address triggered update function enabled, when a switch finds a packet with a destination address matching one MAC address entry within the aging time, it updates the entry and restarts the aging timer.

 

II. 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

1.2.1  MAC Address Table Management Configuration Task List

Complete the following tasks to configure MAC address table management:

Task

Remarks

Configuring a MAC Address Entry

Required

Setting the MAC Address Aging Timer

Optional

Setting the Maximum Number of MAC Addresses a Port Can Learn

Optional

Enabling Destination MAC Address Triggered Update

Optional

Assigning MAC Addresses for Ethernet Ports

Optional

 

1.2.2  Configuring a MAC Address Entry

You can add, modify, or remove a MAC address entry, remove all MAC address entries 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

Follow these steps to add a MAC address entry in system view:

To do…

Use the command…

Remarks

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

Follow these steps to add a MAC address entry in Ethernet port view:

To do…

Use the command…

Remarks

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.

 

1.2.3  Setting the MAC Address Aging Timer

Setting an appropriate MAC address aging timer is important for the switch to run efficiently.

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

Follow these steps to set aging time of MAC address entries:

To do…

Use the command…

Remarks

Enter system view

system-view

Set the MAC address aging timer

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

Required

The default is 300 seconds.

 

Normally, you are recommended to use the default aging timer, namely, 300 seconds. The no-aging keyword specifies that MAC address entries never age out.

 

&  Note:

MAC address aging configuration applies to all ports, but only takes effect on dynamic MAC addresses, which are either learnt or configured.

 

1.2.4  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. By searching the MAC address table, the switch directly forwards the packets destined for these MAC addresses through the hardware, improving the forwarding efficiency. A MAC address table too big in size may prolong the time for searching MAC address entries, thus decreasing the forwarding performance of the switch.

By setting the maximum number of MAC addresses that can be learnt from individual ports, the administrator 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.

Follow these steps to set the maximum number of MAC addresses a port can learn:

To do…

Use the command…

Remarks

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.

 

&  Note:

If you have configured the maximum number of MAC addresses that a port can learn, you cannot enable the MAC address authentication or port security functions on the port, and vice versa.

 

1.2.5  Enabling Destination MAC Address Triggered Update

By default, a switch updates its MAC address entries based on the source MAC addresses of packets. However, this may cause the switch to perform unnecessary broadcasts in some applications.

The destination MAC address triggered update function solves the above problem by allowing the switch to update its MAC address entries according to destination MAC addresses in addition to source MAC addresses. This function improves the availability of the MAC address table.

Follow these steps to enable destination MAC address triggered update:

To do…

Use the command…

Remarks

Enter system view

system-view

Enable destination MAC address triggered update

mac-address aging destination-hit enable

Required

Disabled by default

 

1.2.6  Assigning MAC Addresses for Ethernet Ports

By default, no Ethernet port of an S3100-52P switch is configured with a MAC address. Therefore, when the switch sends Layer 2 protocol packets, for example, BPDUs of STP, it uses the MAC address predefined in the protocol as the source address to send the BPDUs. As switches in your network may use the same source MAC address for Layer 2 protocol data units (PDUs), different ports on a switch may learn the same MAC address, thus affecting the maintenance of the MAC address table.

To avoid the problem, you are allowed to assign MAC addresses to the Ethernet ports on an S3100-52P switch. The idea is to assign a MAC address (called the start port MAC address) for the start Ethernet port, that is, Ethernet 1/0/1, and each of the following ports uses the MAC address of the preceding port plus 1 as its MAC address.

For example, if you configure 000f-e200-0001 as the start port MAC address, then port Ethernet 1/0/2 will take MAC address 000f-e200-0002, and so on.

Follow these steps to configure the start port MAC address:

To do…

Use the command…

Remarks

Enter system view

system-view

Configure the start port MAC address

port-mac start-mac-address

Required

No start port MAC address is configured by default.

 

&  Note:

The start port MAC address must be a valid unicast address.

 

With the port MAC address configuration, the switch uses the MAC address of a port as the source MAC address when sending the following Layer 2 PDUs out of the port:

l           LACP

l           STP

l           NDP/NTDP

l           GVRP

l           DLDP

Port MAC address configuration does not affect service packet forwarding.

1.3  Displaying MAC Address Table Information

To do…

Use the command…

Remarks

Display information about the MAC address table

display mac-address [ display-option ]

Available in any view

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

display mac-address aging-time

Display the configured start port MAC address

display port-mac

 

1.4  Configuration Example

1.4.1  Adding a Static MAC Address Entry Manually

I. Network requirements

The server connects to the switch through Ethernet 1/0/2. To prevent the switch from broadcasting packets destined for the server, it is required to add the MAC address of the server to the MAC address table of the switch, which then forwards packets destined for the server through Ethernet 1/0/2.

l           The MAC address of the server is 000f-e20f-dc71.

l           Port Ethernet 1/0/2 belongs to VLAN 1.

II. Configuration procedure

# Enter system view.

<Sysname> system-view

[Sysname]

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

[Sysname] mac-address static 000f-e20f-dc71 interface Ethernet 1/0/2 vlan 1

# Display information about the current MAC address table.

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

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

000f-e20f-dc71    1        Config static   Ethernet1/0/2    NOAGED

000f-e20f-a7d6    1        Learned         Ethernet1/0/2    AGING

000f-e20f-b1fb    1        Learned         Ethernet1/0/2    AGING

000f-e20f-f116    1        Learned         Ethernet1/0/2    AGING

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

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