Dell PowerConnect 5548 User Guide - Page 461

Frame Flow Figure 16-1 describes the flow of VLAN frames from the Ingress port to the Egress port: Figure 14-1. Frame Flow Through a VLAN Received Fra me Ingress VLAN C la ssifica tio n Ingress Filtering Pr og re ss Fo rwa rding De cisio n Egress Egress Filt e ring T ransmitted Frame Filte ring D at aba se When a frame is received, it must be assigned a VLAN. VLAN assignment is accomplished by the following steps: 1 If the frame contains a VLAN tag, that tag is used, otherwise the frame is classified by the port's default VLAN (PVID), if it is defined. 2 After classification, the frame may pass (if enabled) through ingress filtering, where the frame is dropped if the frame's VLAN ID is not one of the VLANs to which the ingress port belongs. 3 A forwarding decision is made, as a function of the VLAN ID and the destination MAC address. 4 The egress rules define whether the frame is to be sent as tagged or untagged. Special-case VLANs VLAN#1 and VLAN#4095 are special-case VLANs: • VLAN1 - Defined as the default VLAN, and may only be used as a Ports Default VLAN ID (PVID). This means that if the VLAN, whose VID is the current port's PVID, is deleted from the port (or from the system), that port's PVID is set to 1. VLAN#1 cannot be deleted from the system. • VLAN #4095 - Defined (according to standard and industry practice) as the "discard" VLAN. A frame classified to this VLAN is silently dropped. Dell PowerConnect 55xx Systems User Guide 461