概念定义：
所谓地址族，就是把ipv4、ipv6和vpnv4分成不同的“块”，对每个“块”单独进行配置，如下所示：
Rack1R3(config-router)#address-family ? ipv4 Address family ipv6 Address family vpnv4 Address family
由于ipv6现在应用较少，所以暂时不予讨论，如果大家配置过MPLS VPN，那么对ipv4和vpnv4这两个地址族应该有些了解。
其实如果不是有特殊应用（MPLS、Ipv6），也不会用到这些地址族的概念，直接默认建立全局BGP邻居及属性就可以了。所以可以理解地址组是BGP的一个扩展。 下面我们就这两个地址族进行分析。
Ipv4和vpnv4地址族的分析：
特殊说明：“no bgp default ipv4-unicast” 命令是在全局BGP下禁用单播的传播，在MPLS中，正确的配置就是打上此命令，然后如果需要全局BGP连接或在BGP vpnv4连接，那么就在各个地址族中激活此邻居就可以了。
但有些全局命令是可以被继承到各个地址族中的。
以下分别分析几种常见的情况
1、如果没有配置“no bgp default ipv4-unicast”，直接在全局配置BGP邻居关系，这样ipv4地址族会默认被激活的，但是不会显示在“show run”中，如下所示：
Rack1R1#config ter
Enter configuration commands, one per line. End with CNTL/Z. Rack1R1(config)#router bgp 1
Rack1R1(config-router)#bgp router-id 150.1.1.1
Rack1R1(config-router)#nei 150.1.2.2 remote-as 1 Rack1R1(config-router)#nei 150.1.2.2 update-source lo0 Rack1R1(config-router)#address-family vpnv4 Rack1R1(config-router-af)#nei 150.1.2.2 activate
Rack1R1(config-router-af)#nei 150.1.2.2 send-community ex Rack1R1(config-router-af)#exit Rack1R1(config-router)#end Rack1R1#
Rack1R1#sh run | b r b router bgp 1 no synchronization bgp router-id 150.1.1.1 neighbor 150.1.2.2 remote-as 1
neighbor 150.1.2.2 update-source Loopback0 no auto-summary
! >>>>> 没有ipv4地址族 address-family vpnv4 neighbor 150.1.2.2 activate
neighbor 150.1.2.2 send-community extended exit-address-family !
2、此时如果打上“no bgp default ipv4-unicast” ，address-family ipv4地址族就会自动显示出来，此时如果再建立BGP全局邻居，这条命令就会生效：
Rack1R1#config ter
Enter configuration commands, one per line. End with CNTL/Z. Rack1R1(config)#router bgp 1
Rack1R1(config-router)#no bgp default ipv4-unicast Rack1R1(config-router)#end
Rack1R1#
Rack1R1#sh run | b r b router bgp 1
bgp router-id 150.1.1.1 no bgp default ipv4-unicast neighbor 150.1.2.2 remote-as 1
neighbor 150.1.2.2 update-source Loopback0 !
address-family ipv4 neighbor 150.1.2.2 activate
no auto-summary >>>>> 这些命令从全局BGP配置中继承到了ipv4的地址族中 no synchronization exit-address-family !
address-family vpnv4 neighbor 150.1.2.2 activate
neighbor 150.1.2.2 send-community extended exit-address-family
再加一个全局BGP邻居会怎么样呢？
Rack1R1#config ter
Enter configuration commands, one per line. End with CNTL/Z. Rack1R1(config)#router bgp 1
Rack1R1(config-router)#nei 150.1.3.3 remot 1
Rack1R1(config-router)#nei 150.1.3.3 update-source lo0 Rack1R1(config-router)#end Rack1R1#
Rack1R1#sh run | b r b router bgp 1
bgp router-id 150.1.1.1 no bgp default ipv4-unicast neighbor 150.1.2.2 remote-as 1
neighbor 150.1.2.2 update-source Loopback0 neighbor 150.1.3.3 remote-as 1
neighbor 150.1.3.3 update-source Loopback0 !
address-family ipv4 >>>>> 在ipv4地址族中没有150.1.3.3 的邻居 neighbor 150.1.2.2 activate no auto-summary no synchronization exit-address-family !
address-family vpnv4 neighbor 150.1.2.2 activate
neighbor 150.1.2.2 send-community extended exit-address-family !
由于“no bgp default ipv4-unicast”命令的关系，虽然建立了全局BGP的邻居，但它是不会出现在任何地址族中的。如果想在ipv4或vpnv4中起效果，就得在地址族中用“nei 150.1.3.3 active”把邻居激活：
Rack1R1(config-router)#address-family ipv4 Rack1R1(config-router-af)#nei 150.1.3.3 activate Rack1R1(config-router-af)#exit Rack1R1(config-router)#end Rack1R1#
Rack1R1#sh run | b r b router bgp 1
bgp router-id 150.1.1.1 no bgp default ipv4-unicast neighbor 150.1.2.2 remote-as 1
neighbor 150.1.2.2 update-source Loopback0 neighbor 150.1.3.3 remote-as 1
neighbor 150.1.3.3 update-source Loopback0 !
address-family ipv4 neighbor 150.1.2.2 activate neighbor 150.1.3.3 activate no auto-summary no synchronization exit-address-family !
address-family vpnv4 neighbor 150.1.2.2 activate
neighbor 150.1.2.2 send-community extended exit-address-family !
3、如果在没打“no bgp default ipv4-unicast”时，配置“address-family ipv4” 址族，进去后直接end退出），会怎么样呢？
Rack1R2#sh run | b r b router bgp 1 no synchronization bgp router-id 150.1.2.2 neighbor 150.1.1.1 remote-as 1
neighbor 150.1.1.1 update-source Loopback0 no auto-summary
（不用配置地