Specifications

Switching 8-17
Software Release 2.7.3
C613-03098-00 REV A
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 2 or Layer 3. In Layer 3 switching, the switch
determines the destination VLAN from its routing tables. The VID of the
destination VLAN is 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 is associated with it on egress.
VLAN membership using VLAN tags
Ports can belong to many VLANs as tagged ports. Because VLAN tags
determine to which VLAN a packet belongs, 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
transmitted only 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 8-2 on page 8-18 shows a network configured with VLAN tagging.
Table 8-4 on page 8-18 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.