User`s guide

Chapter 2: VLAN Operation
2-6 DIGITAL MultiSwitch 700 Port Based VLAN User’s Guide
In Open mode, the forwarding decision is based only on the destination
MAC address of the frame. As an example, assume that a frame is
received by the switch depicted in Figure 2-1. This frame is a unicast
untagged frame received from Interface 3. The frame is classified for
VLAN B. The switch makes its forwarding decision by comparing the
destination MAC address to its filtering database. The switch recognizes
the destination MAC address of the frame as being located out Interface 1.
Having made the forwarding decision, the switch now examines the
Egress List of Interface 1 to determine if it may transmit the frame. If
Interface 1 was only a member of VLAN A, the frame would be
discarded. Because the Egress List for Interface 1 does not contain an
eligibility for VLAN B, the frame is discarded.
Secure Mode
If the switch is operating in Secure mode, it makes its forwarding decision
based on different criteria. While Open mode bases the forwarding
decision on only one item in the Filtering Database (the MAC address),
Secure mode makes the decision based on the combination of MAC
address and VLAN match.
For example, the switch shown in Figure 2-1 receives an untagged frame
from Interface 1, and the frame is a unicast intended for a MAC address
that the switch Source Address Table recognizes as being located out
Interface 3. Immediately upon reception, this untagged frame is classified
into a VLAN, VLAN A.
The forwarding decision for the frame is made by examining the Filtering
Database for a match of the destination MAC address and membership in
VLAN A. The switch examines the Filtering Database and finds no
matches for this combination of MAC address and VLAN.
Since no matches were found containing both the destination MAC
address and the desired VLAN, the frame will be forwarded to all ports in
the Forwarding List for VLAN A.