3Com 4007, Implementation Manual, Page: 295 / 684

Rules of VLAN Operation 295
Rules of VLAN
Operation
After you select a VLAN mode for your modules and create VLAN
interfaces with VLAN characteristics such as IEEE 802.1Q or no tagging,
port membership, protocol type, and Layer 3 (network) address
information, the system determines the details of VLAN operation by
observing two main types of rules:
■
Ingress rules — Assign an incoming frame to a specific VLAN.
■
Egress rules — Use standard bridging rules to determine whether the
frame is forwarded, flooded, or filtered. These rules also determine
the tag status of the transmitted frame.
These rules are classified in the IEEE 802.1Q standard. In addition, the
system relies on some module-specific rules, discussed next.
Ingress Rules These rules determine the VLAN to which an incoming frame belongs.
The frame is assigned to the VLAN that has the most specific match. The
system uses this protocol match hierarchy to find the most specific match.
The ingress rules use the following hierarchy to determine the most
specific match:
1 IEEE 802.1Q tag VID value.
2 For Multilayer Switching Modules, a specific protocol match (for example,
IP, IPX, or AppleTalk).
3 The default VLAN (an untagged, unspecified protocol type VLAN with all
ports and a VID of 1), or any VLAN that has the unspecified protocol type.
4 The null VLAN , a special VLAN that the system uses if the frame cannot
be assigned to any VLAN. This VLAN has no ports and has no address
table (in allClosed mode).
The Release 3.0 ingress rules are classified according to the tag status of
the frame and the VLAN mode (allOpen for open VLANs or allClosed for
closed VLANs). For the ingress rules, the system considers a priority
tagged frame an untagged frame.