Forwarding Tagged And Untagged Frames; Filtering Databases - ZyXEL Communications IES-1000 User Manual

IES-1000 User's Guide
TPID has a defined value of 8100 (hex). The first three bits of the TCI define user priority (giving eight priority
levels). The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. The
remaining twelve bits define the VLAN ID, giving a possible maximum number of 4,096 VLANs. Note that user
priority and VLAN ID are independent of each other. A frame with VID (VLAN Identifier) of null (0) is called a
priority frame, meaning that only the priority level is significant and the default VID of the ingress port is given as
the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095
(FFF) is reserved, so the maximum possible VLAN configurations are 4,094.
15.3

Forwarding Tagged and Untagged Frames

Each port on the switch is capable of receiving tagged or untagged frames. You can configure a network module to
receive only tagged or all frames on a port-by-port basis. If it is set to tagged-only on a port, then only tagged
frames are allowed to enter from that port and untagged frames are dropped; if set to all, then both tagged and
untagged frames are allowed to enter the switch. The network module does not alter the VID of a frame if it is
already tagged; however, when an untagged frame enters the switch, it is assigned the default port VID (PVID) of
the ingress (incoming) port. Thus a frame always has a VID inside the switch, regardless of whether it is tagged or
not on the wire. The default PVID is 1 for all ports, but this can be changed.
The egress (outgoing) port(s) of a frame is determined on the combination of the destination MAC address and the
VID of the frame. For a unicast frame, the egress port based by the destination address must be a member of the
VID, also; otherwise, the frame is blocked. For a broadcast (or multicast without IGMP snooping) frame, it is
duplicated only on ports (except the ingress port itself) that are members of the VID, thus confining the broadcast to
a specific domain.
Whether to tag an outgoing frame depends on the setting of the egress port on a per VLAN, per port basis (recall
that a port can be members of multiple VID). If the tagging on the egress port is enabled for the VID of a frame,
then the frame is transmitted as a tagged frame; otherwise, it is transmitted as an untagged frame.
15.4

Filtering Databases

A filtering database stores and organizes VLAN registration information useful for switching frames to and from a
LAN bridge. A filtering database consists of a static entries (Static VLAN or SVLAN table) and dynamic entries
(Dynamic VLAN or DVLAN table).
15.4.1 Static Entries (SVLAN Table)
Static entry registration information is added, modified and removed by management only.
15.4.2 Dynamic Entries (DVLAN Table)
Dynamic entries are learned by the bridge and cannot be created or updated by management. The bridge learns this
information by observing what port, source address and VLAN ID (or VID) is associated with a frame. Entries are
15-2
TPID User Priority
2 Bytes 3 Bits
CFI VLAN ID
1 Bit 12 bits
VLAN