580 Configuring VLANsConfiguring the PVID for an interface is useful when untagged and taggedpackets will be sent and received on that port and a device connected to theinterface does not support VLAN tagging.GVRPThe GARP VLAN Registration Protocol (GVRP) helps to dynamicallymanage VLAN memberships on trunk ports. When GARP is enabled,switches can dynamically register (and de-register) VLAN membershipinformation with other switches attached to the same segment.Information about the active VLANs is propagated across all networkingswitches in the bridged LAN that support GVRP. You can configure ports toforbid dynamic VLAN assignment through GVRP.The operation of GVRP relies upon the services provided by the GenericAttribute Registration Protocol (GARP). GVRP can create up to 1024 VLANs.For information about GARP timers, see "What Are GARP and GMRP?" onpage 718.Double-VLAN TaggingFor trunk ports, which are ports that connect one switch to another switch,the PowerConnect M6220, M6348, M8024, and M8024-k switches supportdouble-VLAN tagging. This feature allows service providers to create VirtualMetropolitan Area Networks (VMANs). With double-VLAN tagging, serviceproviders can pass VLAN traffic from one customer domain to anotherthrough a metro core in a simple and cost-effective manner. By using anadditional tag on the traffic, the switch can differentiate between customersin the MAN while preserving an individual customer’s VLAN identificationwhen the traffic enters the customer’s 802.1Q domain.With the introduction of this second tag, customers are no longer required todivide the 4-byte VLAN ID space to send traffic on a Ethernet-based MAN.In short, every frame that is transmitted from an interface has a double-VLANtag attached, while every packet that is received from an interface has a tagremoved (if one or more tags are present).In Figure 21-2, two customers share the same metro core. The serviceprovider assigns each customer a unique ID so that the provider candistinguish between the two customers and apply different rules to each.When the configurable EtherType is assigned to something different than the
