Layer 2 Switching 75
Software Release 2.6.1
C613-02039-00 REV A
Every frame admitted by the switch has a VID associated with it. If a frame
arrives on a tagged port, the associated VID is determined from the VLAN tag
the frame had when it arrived. If a frame arrives on an untagged port, it is
associated with the VID of the VLAN for which the incoming port is untagged.
When the switch forwards a frame over a tagged port, it adds a VLAN tag to
the frame. When the switch forwards the frame over an untagged port, it
transmits the frame as a VLAN-untagged frame, not including the VID in the
frame.
The VLAN tag that the switch adds to a frame on egress depends on whether
the frame is switched in Layer 3 or Layer 2. In Layer 3 switching, the switch
determines the destination VLAN from its routing tables. The VID of the
destination VLAN will be added to the frame on egress. In Layer 2 switching,
the frame’s source and destination VLANs are the same. The VID that was
associated with the frame on ingress will be associated with it on egress.
VLAN Membership using VLAN Tags
Ports can belong to many VLANs as tagged ports. Therefore, when the VLAN
tag is used to determine which VLAN a packet belongs to, it is easy to:
■Share network resources, such as servers and printers, across several
VLANs
■Configure VLANs that span several switches
For tagged ports, the switch uses the VID of incoming frames, and the frame’s
destination field to switch traffic through a VLAN aware network. Frames are
only transmitted on ports belonging to the required VLAN. Other vendors’
VLAN aware devices on the network can be configured to accept traffic from
one or more VLANs. A VLAN-aware server can be configured to accept traffic
from many different VLANs, and then return data to each VLAN without
mixing or leaking data into the wrong VLANs.
Figure 14 on page 76 shows a network configured with VLAN tagging.
Table12 on page 76 shows the VLAN membership. The server on port 2 on
Switch A belongs to both the admin and marketing VLANs. The two switches
are connected through uplink port 26 on Switch A and uplink port 25 on
Switch B, which belong to both the marketing VLAN and the training VLAN, so
devices on both VLANs can use this link.