- Objectives
- Key Terms
- Introduction (3.0.1.1)
- VLAN Segmentation (3.1)
- VLANs in a Multiswitched Environment (3.1.2)
- VLAN Implementations (3.2)
- VLAN Trunks (3.2.2)
- Dynamic Trunking Protocol (3.2.3)
- Troubleshoot VLANs and Trunks (3.2.4)
- VLAN Security and Design (3.3)
- Design Best Practices for VLANs (3.3.2)
- Summary (3.4)
- Practice
- Class Activities
- Labs
- Packet Tracer Activities
- Check Your Understanding Questions
This chapter is from the book
This chapter is from the book
This chapter is from the book
VLAN Trunks (3.2.2)
Trunks are commonly used between switches and other network devices such as a router, another switch, or a server. A network technician must be very familiar with configuring a trunk and ensuring it works properly.
Configuring IEEE 802.1Q Trunk Links (3.2.2.1)
A VLAN trunk is an OSI Layer 2 link between two switches that carries traffic for all VLANs (unless the allowed VLAN list is restricted manually or dynamically). To enable trunk links, configure the ports on either end of the physical link with parallel sets of commands.
To configure a switch port on one end of a trunk link, use the switchport mode trunk command. With this command, the interface changes to permanent trunking mode. The port enters into a Dynamic Trunking Protocol (DTP) negotiation to convert the link into a trunk link even if the interface connecting to it does not agree to the change. DTP is described in the next topic. In this course, the switchport mode trunk command is the only method implemented for trunk configuration.
The Cisco IOS command syntax to specify a native VLAN (other than VLAN 1) is shown in Table 3-6. In the example, VLAN 99 is configured as the native VLAN using the switchport trunk native vlan 99 command.
Table 3-6 Switch Port Trunk Commands
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Enter global configuration mode. |
S1# configure terminal |
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Enter interface configuration mode for a particular port number. |
S1(config)# interface interface_id |
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Optionally, put the trunk in the appropriate trunking mode if the switch supports more than one mode. |
S1(config-if)# switchport trunk encapsulation [dot1q | isl] |
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Force the link to be a trunk link. |
S1(config-if)# switchport mode trunk |
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Specify a native VLAN for untagged 802.1Q frames. |
S1(config-if)# switchport trunk native vlan vlan_id |
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Specify the list of VLANs to be allowed on the trunk link. |
S1(config-if)# switchport trunk allowed vlan vlan-list |
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Return to the privileged EXEC mode. |
S1(config-if)# end |
Use the Cisco IOS switchport trunk allowed vlan vlan-list command to specify the list of VLANs to be allowed on the trunk link.
In Figure 3-19, VLANs 10, 20, and 30 support the Faculty, Student, and Guest computers (PC1, PC2, and PC3). The F0/1 port on switch S1 is configured as a trunk port and forwards traffic for VLANs 10, 20, and 30. VLAN 99 is configured as the native VLAN.
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Figure 3-19 Sample VLAN Design
Look at the configuration of port F0/1 on switch S1 as a trunk port. The native VLAN is changed to VLAN 99 and the allowed VLAN list is restricted to 10, 20, and 30. If the native VLAN is not allowed on the trunk link, the trunk will not allow any data traffic for the native VLAN.
S1(config)# interface fastethernet0/1 S1(config-if)# switchport mode trunk S1(config-if)# switchport trunk native vlan 99 S1(config-if)# switchport trunk allowed vlan 10,20,30 S1(config-if)# end
Resetting the Trunk to Default State (3.2.2.2)
Table 3-7 shows the commands to remove the allowed VLANs and reset the native VLAN of the trunk. When reset to the default state, the trunk allows all VLANs and uses VLAN 1 as the native VLAN.
Table 3-7 Resetting Configures Values on Trunk Lines
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Enter global configuration mode. |
S1# configure terminal |
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Enter interface configuration mode for a particular port number. |
S1(config)# interface interface_id |
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Set trunk to allow all VLANs. |
S1(config-if)# no switchport trunk allowed vlan |
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Reset the native VLAN to the default. |
S1(config-if)# no switchport trunk allowed vlan |
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Configure the port in access mode. |
S1(config-if)# switchport mode access |
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Optionally, remove the trunk mode if it was entered. |
S1(config-if)# no switchport trunk encapsulation [dot1q | isl] |
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Return to the privileged EXEC mode. |
S1(config-if)# end |
The command to reset the switch port to an access port and, in effect, delete the trunk configuration is also shown.
The following output shows the commands used to reset all trunking characteristics of a trunking interface to the default settings. The show interfaces f0/1 switchport command reveals that the trunk has been reconfigured to a default state.
S1(config)# interface f0/1S1(config-if)# no switchport trunk allowed vlanS1(config-if)# no switchport trunk native vlanS1(config-if)# end S1# show interfaces f0/1 switchport Name: Fa0/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: OnAccess Mode VLAN: 1 (default)Trunking Native Mode VLAN: 1 (default) Administrative Native VLAN tagging: enabled <output omitted> Administrative private-vlan trunk mappings: none Operational private-vlan: noneTrunking VLANs Enabled: AllPruning VLANs Enabled: 2-1001 <output omitted>
The following sample output shows the commands used to remove the trunk feature from the F0/1 switch port on switch S1. The show interfaces f0/1 switchport command reveals that the F0/1 interface is now in static access mode.
S1(config)# interface f0/1S1(config-if)# switchport mode accessS1(config-if)# end S1# show interfaces f0/1 switchport Name: Fa0/1 Switchport: EnabledAdministrative Mode: static accessOperational Mode: static access Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: nativeNegotiation of Trunking: OffAccess Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Administrative Native VLAN tagging: enabled <output omitted>
Verifying Trunk Configuration (3.2.2.3)
The following output displays the configuration of switch port F0/1 on switch S1. The configuration is verified with the show interfaces interface-ID switchport command.
S1(config)# interface f0/1 S1(config-if)# switchport mode trunk S1(config-if)# switchport trunk native vlan 99 S1(config-if)# end S1# show interfaces f0/1 switchport Name: Fa0/1 Switchport: EnabledAdministrative Mode: trunkOperational Mode: trunkAdministrative Trunking Encapsulation: dot1qOperational Trunking Encapsulation: dot1qNegotiation of Trunking: On Access Mode VLAN: 1 (default)Trunking Native Mode VLAN: 99 (VLAN0099)Administrative Native VLAN tagging: enabledVoice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk Native VLAN tagging: enabled Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk associations: none Administrative private-vlan trunk mappings: none Operational private-vlan: noneTrunking VLANs Enabled: AllPruning VLANs Enabled: 2-1001 <output omitted>
The top highlighted area shows that port F0/1 has its administrative mode set to trunk. The port is in trunking mode. The next highlighted area verifies that the native VLAN is VLAN 99. Further down in the output, the bottom highlighted area shows that all VLANs are enabled on the trunk.
Activity 3.2.2.3: Configuring and Verifying a Trunk
Go to the online curriculum and select the second graphic to use the Syntax Checker to configure a trunk supporting all VLANs on interface F0/1, with native VLAN 99. Verify the trunk configuration with the show interfaces f0/1 switchport command.
Packet Tracer Activity 3.2.2.4: Configuring Trunks
VLAN trunks are required to pass VLAN information between switches. A port on a switch is either an access port or a trunk port. Access ports carry traffic from a specific VLAN assigned to the port. A trunk port by default is a member of all VLANs; therefore, it carries traffic for all VLANs. This activity focuses on creating trunk ports and assigning them to a native VLAN other than the default.
Lab 3.2.2.5: Configuring VLANs and Trunking
In this lab, you will complete the following objectives:
- Part 1: Build the Network and Configure Basic Device Settings
- Part 2: Create VLANs and Assign Switch Ports
- Part 3: Maintain VLAN Port Assignments and the VLAN Database
- Part 4: Configure an 802.1Q Trunk Between the Switches
- Part 5: Delete the VLAN Database