1. Defining VLANs
Implementing Trunks

2. Maintaining Specific VLAN Identification
Specifically developed for multi-VLAN interswitch communications
Places a unique identifier in each frame(每个帧有唯一标识)
Functions at Layer 2

3. VLAN Trunking

4. Comparing ISL and 802.1Q ISL 802.1Q Proprietary(私有) Nonproprietary（公有）
Encapsulated（封装式）
Tagged（标记式）
Protocol independent
Protocol dependent
Encapsulates the old frame in a new frame
Adds a field to the frame header
Note: The 1900 only supports ISL trunking.
ISL is Cisco proprietary Q is an IEEE standard.
Other trunk types:
LANE (VLANSs over ATM)
(FDDI trunk)

5. Trunking with ISL Is a Cisco proprietary protocol Supports PVST
Uses an encapsulation process
Does not modify the original frame

6. ISL Encapsulation

7. Trunking with 802.1Q An IEEE standard
Adds a 4-byte tag to the original frame
Additional tag includes a priority field
Does not tag frames that belong to the native VLAN
Supports Cisco IP telephony

8. The 802.1Q Tagging Process

9. 802.1Q Native VLAN
Native VLAN frames are carried over the trunk link untagged.
（本征vlan不会标记就能穿越干道）

10. VLAN Ranges VLAN Range Use 0, 4095 Reserved for system use only 1
Cisco default
2–1001
For Ethernet VLANs
1002–1005
Cisco defaults for FDDI and Token Ring
1006–4094
Ethernet VLANs only, unusable on specific legacy platforms
透明模式的交换机可使用扩展的vlan号（扩展vlan不在vlan数据库中，不能共享通告。如果把透明改回服务器或客户，得把扩展vlan删除）

11. Trunking Configuration Commands
Trunks can be configured statically or via DTP.
DTP provides the ability to negotiate the trunking method.
（DTP思科动态中继协议用于自动协商中继模式,并选择适当的封装协议）
Configuring a Trunk
switchport trunk
switchport mode
switchport nonegotiate

12. Trunking Configuration Commands
switchport mode trunk （永久成为trunk）
dynamic desirable（默认模式，端口主动尝试 将链路变为trunk）
dynamic auto （被动模式仅当远端设备请求式才成为trunk）
switchport nonegotiate （关闭dtp自动协商）
switchport trunk encapsulation {isl | dot1q |negotiate（默认） }
switchport trunk allowed {all | vlan x}（允许vlan通行）

13. Switchport Mode Interactions
Dynamic Auto
Dynamic Desirable
Trunk
Access
Not recommended
Note: Table assumes DTP is enabled at both ends.
show dtp interface – to determine current setting

14. How to Configure Trunking
Enter interface configuration mode.
Shut down interface.
Select the encapsulation (802.1Q or ISL).
Configure the interface as a Layer 2 trunk.
Specify the trunking native VLAN (for 802.1Q).
Configure the allowable VLANs for this trunk.
Use the no shutdown command on the interface to activate the trunking process.
Verify the trunk configuration.

15. 802.1Q Trunk Configuration
Switch(config)#interface fastethernet 5/8 Switch(config-if)#shutdown Switch(config-if)#switchport trunk encapsulation dot1q Switch(config-if)#switchport trunk allowed vlan 1,5,11, Switch(config-if)#switchport mode trunk Switch(config-if)#switchport trunk native vlan 99
Switch(config-if)#switchport nonegotiate Switch(config-if)#no shutdown

16. Verifying the 802.1Q Configuration
Switch#show running-config interface {fastethernet | gigabitethernet} slot/port
Switch#show interfaces [fastethernet | gigabitethernet] slot/port [ switchport | trunk ]
Switch#show interfaces fastEthernet 5/8 switchport
Name: fa5/8
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 99 (trunk_only)
Trunking VLANs Enabled: 1,5,11,
Pruning VLANs Enabled:
. . .

17. Verifying a 802.1Q Dynamic Trunk Link
Switch#show running-config interface fastethernet 5/8
Building configuration...
Current configuration: !
interface FastEthernet5/8
switchport mode dynamic desirable
switchport trunk encapsulation dot1q
Switch#show interfaces fastethernet 5/8 trunk
Port Mode Encapsulation Status Native vlan
Fa5/8 desirable q trunking
Port Vlans allowed on trunk
Fa5/ ,5,11,
Port Vlans allowed and active in management domain
Fa5/ ,5,
Port Vlans in spanning tree forwarding state and not pruned

18. ISL Trunk Configuration
Switch(config)#interface fastethernet 2/1 Switch(config-if)#shutdown Switch(config-if)#switchport trunk encapsulation isl Switch(config-if)#switchport trunk allowed vlan 1-5, Switch(config-if)#switchport mode trunk Switch(config-if)#switchport nonegotiate
Switch(config-if)#no shutdown

19. Verifying ISL Trunking
Switch#show running-config interface {fastethernet | gigabitethernet} slot/port
Switch#show interfaces [fastethernet | gigabitethernet] slot/port [ switchport | trunk ]
Switch#show interfaces fastethernet 2/1 trunk
Port Mode Encapsulation Status Native VLAN
Fa2/1 trunk isl trunking
Port VLANs allowed on trunk
Fa2/ ,
Port VLANs allowed and active in management domain
Fa2/ ,
Port VLANs in spanning tree forwarding state and not pruned

20. Summary Trunk links carry traffic from multiple VLANs.
ISL is Cisco proprietary and encapsulates the Layer 2 frames.
802.1Q is an IEEE standard for trunking, which implements a 4-byte tag.
The 802.1Q native VLANs forward frames without the tag.
VLAN numbers have specific ranges and purposes.
Various commands are used to configure and verify ISL and 802.1Q trunk links.
Allow only required VLANs over the trunk.