A. Why do we need the trunk link 
computers on the same piece we switch connected to communicate with each of our two switches and hosts have their own broadcast domain, we realize now how the communication between the VLAN switch it? We need Trunk link
principle two .trunk link 
vlan10 vlan10 across switches and other communications, we need to connect a line, 
but if we have one hundred VLAN, let's switch only, 8,16,24 , 48 ports is not enough, this time we need to trunk link 
Firstly, our trunk link is a common link, from how data VLAN10 come to VLAN10 in it this time you need to distinguish between different VLAN identifier 
first Cisco's proprietary standard
and the second is our common identity 802.1q 
we generally understand what this frame format is what
DA: destination address, SA: source address, type, data
tAG: sign marking a message with this frame of 802.1q
Priority : user priority 3, this field is not used 802.1Q
the CFI: commonly used in Ethernet and token Ring, this value is typically set bit 0
the VLAN ID
three by what we GNS3 Trunk link configuration.
1. we Enhancing line and written on the IP address, VLAN written 
2. let's configured capacity 
3. Close route by VLAN10 
4. create a VLAN20, see VLAN 
5. enter the port, access the link, fill in VLAN10 
6. SW2 enter them, creating Build the VLAN, an access link and 
7. We enter the relay link, 1/2-port, SWl and SW2 are set up
8. we try to PC1, PC4 which can not pass 
9. We use capture software test at 
10, which will have two general companies ready for use when the relay link 
11. We again to create one 
12. we were then SW1 and SW2 which are set up 
13. we have to set up open 
after opening 14. we need two links tied together 
15. If we try to disconnect F1 / 2 port can not Unicom
first, we often use the PING command 
16 to SW1 and SW2 among the F1 / 2 turn off, shut down shut down 
17. we often PING test it just can not pass up the 
IV. the principle three switches
switcher no three layer switch full-featured 
switch has a three-tier exchange of the three switches is part of router functions, the most important objective is to accelerate the large-scale three switches within the LAN data exchange, but also has the function of routing services for this purpose, can do once routing, multiple forwarding. For the regular high-speed packet forwarding process implemented in hardware, but as routing updates, routing table maintenance, route calculation, route determination and other functions, realized by software. Machine following the routing function also functions as a switch! []
Based on the exchange process of 
our three switches will go to transfer data from this table
V. Let's look at this experiment three switches
1. Let's add two routers and three host
2. SW1 to configure a VLAN, port, TRUNK 
3. Layer we give good conversion tag address and the VLAN 
4. we give SW2 with a good TRUNK, with the IP address to the port 
5. We address to each host with a good card, and 
6. we have to test three hosts can communicate