Certified Router Support Professional | VTP Pruning

VTP Pruning
 

VTP Pruning

VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used to manage VLANs in a switched network. VTP Pruning is an optimization feature of VTP that reduces unnecessary broadcast, multicast, and unicast traffic across VLANs on trunk links. This tutorial covers the fundamentals, advantages, configuration, and troubleshooting of VTP Pruning.

1. What is VTP Pruning?

In a multi-switch network, all VLAN traffic is typically forwarded over trunk links, even if the connected switch doesn’t have any ports assigned to that VLAN. VTP Pruning eliminates this inefficiency by stopping traffic for unnecessary VLANs from being sent across trunk links.

  • Without Pruning: Broadcast traffic for all VLANs is sent over all trunk links, even to switches where the VLAN isn’t needed.
  • With Pruning: Traffic for a VLAN is only sent on trunk links to switches where the VLAN is active (i.e., has ports assigned to it).

2. Benefits of VTP Pruning

  1. Reduces Bandwidth Usage: Prevents unnecessary VLAN traffic from traversing trunk links, freeing up bandwidth for other traffic.
  2. Improves Network Efficiency: Ensures VLAN traffic is forwarded only where it’s required.
  3. Minimizes Broadcast Traffic: Limits broadcast domains to relevant switches, reducing overall broadcast traffic in the network.

3. VTP Modes and Pruning

VTP operates in three modes, and pruning can only be enabled in specific modes:

  1. Server Mode:

    • Switches in server mode manage VLAN configurations.
    • VTP Pruning can only be enabled on a switch in server mode.

  2. Client Mode:

    • Switches in client mode receive VLAN updates from VTP servers.
    • Pruning configuration cannot be performed directly but is applied if the server enables it.

  3. Transparent Mode:

    • Switches do not participate in VTP but forward VTP messages.
    • Pruning is not supported.

4. VTP Pruning Configuration

Follow these steps to enable VTP Pruning on a Cisco switch:

Step 1: Check the Current VTP Status

Use the following command to view the VTP status:

Switch# show vtp status

This displays the VTP domain name, mode, and whether pruning is enabled.

Step 2: Enable VTP Pruning

Pruning can only be enabled on a switch in VTP server mode:

Switch(config)# vtp pruning

This command enables pruning for the entire VTP domain.

Step 3: Verify Pruning

To confirm pruning is enabled, use the following command:

Switch# show vtp status

Look for the line: "VTP Pruning Mode: Enabled".

Step 4: Configure Trunk Ports

Ensure trunk ports are properly configured to carry VLAN traffic:

Switch(config)# interface FastEthernet0/1
Switch(config-if)# switchport mode trunk

5. VLANs Excluded from Pruning

By default, VLANs 1 and VLANs 1002-1005 are not pruned, as these are reserved VLANs in Cisco switches. To modify which VLANs are pruned, you must configure the trunk port to allow or disallow specific VLANs.

Example: Allowing Specific VLANs on a Trunk Port

Switch(config)# interface FastEthernet0/1
Switch(config-if)# switchport trunk allowed vlan 10,20,30

6. Testing and Verifying VTP Pruning

  • Command to Check Pruning Effectiveness:
    Use the following command to verify which VLANs are pruned:

    Switch# show interface trunk

    This command shows the VLANs allowed and pruned on each trunk port.

  • Scenario to Test Pruning:
    Disconnect all devices in a VLAN on a downstream switch. VTP Pruning will stop forwarding traffic for that VLAN over the trunk link.

7. Troubleshooting VTP Pruning

  1. Pruning Not Working:

    • Ensure the switch is in VTP server mode.
    • Verify the trunk ports are configured and operational.

  2. VLAN Traffic Not Forwarded:

    • Check that the VLAN is allowed on the trunk port.
    • Ensure the downstream switch has active ports in the VLAN.

  3. Inconsistent VTP Domain Names:

    • All switches in the VTP domain must have the same domain name. Use:
      Switch(config)# vtp domain <domain-name>

  4. VTP Version Mismatch:

    • Ensure all switches are using the same VTP version (1, 2, or 3). Use:
      Switch(config)# vtp version <1/2/3>

8. VTP Pruning Best Practices

  1. Enable on Large Networks: Use pruning in networks with multiple VLANs and trunk links to optimize bandwidth.
  2. Avoid VLAN 1 for User Traffic: VLAN 1 is excluded from pruning and should not be used for regular traffic.
  3. Document VLAN Assignments: Keep a record of VLANs and their associated switches for easier troubleshooting.
  4. Test in a Lab Environment: Before deploying VTP Pruning in a production network, test its behavior in a lab setup.


VTP Pruning is a powerful feature for optimizing VLAN traffic in a Cisco network. By ensuring that only necessary traffic is forwarded on trunk links, it improves bandwidth efficiency and reduces unnecessary load on network devices. For the Certified Router Support Professional Exam, understanding how to configure, verify, and troubleshoot VTP Pruning is a critical skill.

 

Enabling VTP Pruning

To enable VTP pruning, do the following:

 
Step
Action

1 Access the Virtual LAN Menu.

Select [V] from the Main Menu.

2 Access the VTP Pruning Mode screen.

Select [F] VTP Pruning Mode.

3 Enable VTP pruning.

Enter Enable at the selection prompt. The VLAN Configuration Menu reappears.


 

Verifying VTP Pruning

To verify that you have enabled VTP pruning, select [F] VTP Pruning Mode, and view the VTP pruning state.

 

 For Support