3Com 4007, Implementation Manual, Page: 298 / 684

298 C
HAPTER
14: V
IRTUAL
LAN
S
(VLAN
S
)
Egress Rules These rules determine whether the outgoing frame is forwarded, filtered
(dropped), or flooded. They also determine the frame’s tag status. The
same standard bridging rules apply to both open and closed VLANs, but
they result in different behavior depending on the allOpen mode (one
address table for the module) versus allClosed mode (one address table
for each VLAN). For example, on a Multilayer Switching Module, if a
frame is associated with a VLAN that uses VID 1 and has a destination
address associated with a VLAN that uses VID 2, the frame is flooded over
the VID 1 VLAN in allClosed mode but forwarded untagged in allOpen
mode.
Standard Bridging Rules for Outgoing Frames
The frame is handled according to these bridging rules:
■
If the frame’s destination address matches an address that was
previously learned on the receive port, it is
filtered (dropped).
■
If the frame’s destination address matches an address that was learned
on a port other than the receive port, it is
forwarded to that port.
■
If a frame with an unknown, multicast, or broadcast destination
address is received, then it is
flooded (that is, forwarded to all ports on
the VLAN that is associated with the frame, except the port on which
it was received). See “Examples of Flooding and Forwarding
Decisions” later in this chapter.
■
If the frame’s destination address matches a MAC address of one of
the bridge’s ports, or it matches an appropriate multicast address such
as STP (if STP is enabled on the module), it is further processed and
not forwarded immediately. This type of frame is either a
management/configuration frame (such as a RIP update, SNMP get/set
PDU, or an Administration Console Telnet packet), or it is a routed
packet. If it is a routed packet, the Multilayer Switching Module
performs the routing functions that is described in the appropriate
routing chapter (for example, IP, IPX, or AppleTalk).
Tag Status Rules
After the VLAN and the transmit ports are determined for the frame, the
Tag Status rules determine whether the frame is transmitted with an
IEEE 802.1Q tag. For Multilayer Switching Modules, priority tagged
frames for QoS use the same frame format as IEEE 802.1Q tagging but
with a VID of 0. Priority tagged frames received by the Multilayer
Switching Module are transmitted as either untagged frames (that is, no
priority tagging) or IEEE 802.1Q tagged frames.