trunk怎么了？
实验拓扑：
实验需求：R2交换机上不添加VLAN2，会怎样呢？验证其C2发往C1数据的路径过程
配置如下：
R1： R3:
vlan 2 vlan 2
exit exit
int f0/0 int f0/0
switch mode access switch mode access
switch access vlan 2 swtich access vlan 2
exit exit
int f0/15 int f0/15
switch mode trunk switch mode trunk
switch trunk encapsulation dot1q switch trunk encapsulation dot1q
switch trunk allowed vlan except 3-1001 switch trunk allowed vlan except 3-1001
R2:
interface range f0/14 - 15
switch mode trunk
switch trunk encapsulation dot1q
switch trunk allowd vlan except 3-1001
C1的IP地址为：172.16.1.1/24 C2的IP地址为：172.16.1.2/24
C2 ping C1 不通
通过抓包得知：
C2将产生ARP广播包，经过R3的f0/0，查CAM表知道，需要将该包泛洪到所有VLAN2接口和
Switch2#show mac-address-table
Destination Address Address Type VLAN Destination Port
------------------- ------------ ---- --------------------
cc02.17e8.0000 Self 1 Vlan1
cc02.17e8.0000 Self 2 Vlan2
0024.8cbf.42ee Dynamic 2 FastEthernet0/0
0c72.2ced.826c Dynamic 2 FastEthernet0/0
继续发往F0/15 TRUNK口，打上VLAN2标签，发往R2的F0/15口
查看R2的CAM表
Destination Address Address Type VLAN Destination Port
------------------- ------------ ---- --------------------
cc01.17e8.0000 Self 1 Vlan
通过查表得知，没有VLAN2的信息，丢弃帧。
通过在R2添加VLAN2后
R2#show mac-address-table
Destination Address Address Type VLAN Destination Port
------------------- ------------ ---- --------------------
cc01.17e8.0000 Self 1 Vlan1
c008.14c8.0000 Dynamic 2 FastEthernet0/15
c007.14c8.0000 Dynamic 2 FastEthernet0/14
看到了VLAN2的转发信息，通了
备注：
一个交换机在刚配置了VLAN，而所有接口都没有接入设备的情况下
R1(config)#do show ip inter brie
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up down
FastEthernet0/1 unassigned YES unset up down
FastEthernet0/2 unassigned YES unset up down
FastEthernet0/3 unassigned YES unset up down
FastEthernet0/4 unassigned YES unset up down
FastEthernet0/5 unassigned YES unset up down
FastEthernet0/6 unassigned YES unset up down
FastEthernet0/7 unassigned YES unset up down
FastEthernet0/8 unassigned YES unset up down
FastEthernet0/9 unassigned YES unset up down
FastEthernet0/10 unassigned YES unset up down
FastEthernet0/11 unassigned YES unset up down
FastEthernet0/12 unassigned YES unset up down
FastEthernet0/13 unassigned YES unset up down
FastEthernet0/14 unassigned YES unset up down
FastEthernet0/15 unassigned YES unset up down
Vlan1 unassigned YES unset up down
看，VLAN 1是down的，而新添加的VLAN 2还没有出现在这张表里
那么怎样才能让VLAN 2出现在这个表里面呢？
很简单：interface vlan2
no shut
就出现，但是protocol也是down的，知道为什么了吗？
那就是还没有设备接入相应VLAN的端口，使其up；如果VLAN的相应端口up了，vlan也就up了
Vlan1 unassigned YES NVRAM up up
Vlan2 unassigned YES NVRAM up up