MAC（Medium/Media Access Control）地址，采用十六进制数表示，共六个字节（48位）。其中，前三个字节是由IEEE的注册管理机构RA负责给不同厂家分配的代码(高位24位），也称为“编制上唯一的标识符”（Organizationally Unique Identifier），后三个字节(低位24位)由各厂家自行指派给生产的适配器接口，称为扩展标识符（唯一性).

交换机或hub转发数据时会利用MAC和ARP来通讯，当主机要发送目的MAC不知道时用全1的地址广播给同一网段的所有主机，主机收到后会进行单播应答；然后主机互相添加对方MAC和IP关联表，这样就可以单播通讯了。cisco设备更改MAC 地址表的分两类

  Dynamic address: a source MAC address that the switch learns and then ages when it is not 

    in use.  动态地址：交换机学习的源mac；当不适用时会老化。
Static address: a manually entered unicast address that does not age and that is not lost when

   the  switch resets 静态地址：手工输入的单播地址，若没有手动改动就不会老化.

更改cisco设备mac

Step 1 configure terminal                 //Enter global configuration mode.
Step 2 switch(config) #  mac-address-table static mac_address vlan vlan-id {
drop | interface {
type slot/port} | port-channel number} [auto-learn]  //手工指定mac

Setp 3 no mac-address-table learning {vlan
vlan-id [,vlan-id | -vlan-id] | interface
interface slot/port}      //关闭接口学习mac 功能
//Disable MAC address learning on an interface or on a specified VLAN or
VLANs.
You can specify a single VLAN ID or a range of VLAN IDs separated by
a hyphen or comma. Valid VLAN IDs 1 to 4094. It cannot be an internal
VLAN.
Step 4 end Return to privileged EXEC mode.
Step 5 show mac address-table learning [vlan
vlan-id | interface interface slot/port]
Verify the configuration.
Step 5 copy running-config startup-config  //(Optional) Save your entries in the configuration file.