VLANs and Trunking

Chapter Description

Learn how to configure, assign ports, and trunk VLANs on Cisco hardware.

6-5: GVRP

  • Generic Attribute Registration Protocol (GARP) VLAN Registration Protocol (GVRP) is an application defined in the IEEE 802.1Q standard that allows for the control of VLANs.

  • GVRP runs only on 802.1Q trunk links.

  • GVRP prunes trunk links so that only active VLANs will be sent across trunk connections.

  • GVRP expects to hear join messages from the switches before it will add a VLAN to the trunk.

  • GVRP updates and hold timers can be altered.

  • GVRP ports run in various modes to control how they will prune VLANs.

  • GVRP can be configured to dynamically add and manage VLANS to the VLAN database for trunking purposes.

Configuring GVRP

GVRP is supported only on COS switches. GVRP will run only on 802.1Q trunk ports and is used primarily to prune traffic from VLANs that does not need to be passed between trunking switches. Use the following steps to configure GVRP.

  1. Enable GVRP globally:


    set gvrp enable

    By default GVRP is not enabled for the switch. You must first enable GVRP on the switch before you can configure the 802.1Q ports for GVRP operation.

  2. Configure the port for 802.1Q operation:


    set trunk mod/port [auto | desirable | on ] dot1q

    GVRP will run only on ports that are configured for 802.1Q trunking. See section "6-3: Trunking" for more information on trunking.

  3. Configure the port GVRP:


    set port gvrp mod/port enable

    This command enables GVRP on the individual 802.1Q trunk port. GVRP must be configured on both sides of the trunk to work correctly.

  4. (Optional) Configure the port registration mode:


    set gvrp registration [normal | fixed | forbidden] mod/port

    By default GVRP ports are in normal registration mode. These ports use GVRP join messages from neighboring switches to prune the VLANs running across the 802.1Q trunk link. If the device on the other side is not capable of sending GVRP messages, or if you do not want to allow the switch to prune any of the VLANs, use the fixed mode. Fixed mode ports will forward for all VLANs that exist in the switch database. Ports in forbidden mode forward only for VLAN 1.

Configuring GVRP for Dynamic VLAN Creation

Like VTP, GVRP can dynamically create VLANs on switches for trunking purposes. By enabling GVRP dynamic VLAN creation, a switch will add VLANs to its database when it receives GVRP join messages about VLANs it does not have.

  1. (Optional) Enable dynamic VLAN creation:


    set gvrp dynamic-vlan-creation enable

    Dynamic VLAN creation is configured on a switch-by-switch basis. GVRP does not synchronize between switches, but only adds VLANs on devices that have dynamic creation enabled in order to pass traffic between trunks. To enable dynamic VLAN creation, all the trunk ports on the switch have to be 802.1Q and they all must be GVRP-enabled ports. If the switch has any non-802.1Q trunk ports or if the 802.1Q ports that exist are not configured for GVRP, this feature will not be enabled. VLANs will be added only for join messages received across a normal registration port. You must also have configured VTP in transparent or off mode, because VTP and dynamic VLAN creation cannot both be enabled at the same time.


    The trunk ports 15/1 and 16/1 on a 5000 or 6000 series switch do not count as ISL trunks when enabling dynamic-vlan-creation and will not prevent the function from operating.

Verifying GVRP Operation

After you have configured GVRP, use the following command to verify operation:


show gvrp configuration

Feature Example

In this example, the switch Access_1 is connected to Distribution_1 via an 802.1Q trunk shown in Figure 6-4. Distribution_1 is also connected to Core_1 via an 802.1Q trunk. GVRP is enabled on both the distribution and core switches and on each GVRP port on those switches. Dynamic VLAN creation has also been enabled on the switches, and the port from Distribution_1 to Access_1 has been set to GVRP fixed mode because the Access_1 device will not send join messages and the distribution switch would prune all VLANs if it were in the normal default mode.

Figure 6-4Figure 6-4 Network Diagram for GVRP Configuration on Access_1, Distribution_1, and Core_1

An example of the Catalyst OS configuration for Core_1 follows:

Core_1 (enable)>set vtp mode transparent
Core_1 (enable)>set trunk 1/1 on dot1Q
Core_1 (enable)>set gvrp enable
Core_1 (enable)>set port gvrp 1/1 enable
Core_1 (enable)>set gvrp dynamic-vlan-creation enable

An example of the Catalyst OS configuration for Distribution_1 follows:

Distribution_1 (enable)>set vtp mode transparent
Distribution_1 (enable)>set trunk 1/1 on dot1q
Distribution_1 (enable)>set trunk 2/1 on dot1q
Distribution_1 (enable)>set gvrp enable
Distribution_1 (enable)>set gvrp enable 1/1
Distribution_1 (enable)>set gvrp enable 2/1 
Distribution_1 (enable)>set gvrp registration fixed 2/1
Distribution_1 (enable)>set gvrp dynamic-vlan-creation enable

An example of the Layer 2 IOS configuration for Access_1 follows:

Access_1 #config t
Access_1 (config)#interface gigabitethernet 0/1
Access_1 (config-if)#switchport mode trunk
Access_1 (config-if)#switchport trunk encapsulation dot1Q
Access_1 (config-if)#end
Access_1#copy running-config startup-config
6. Private VLANs | Next Section Previous Section

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