8.3.17.0 - Force10 Networks

Port-Based VLANs
Port-based VLANs are a broadcast domain defined by different ports or interfaces. In FTOS, a port-based
VLAN can contain interfaces from different stack units within the chassis. FTOS supports 4094 port-based
VLANs.
Port-based VLANs offer increased security for traffic, conserve bandwidth, and allow switch
segmentation. Interfaces in different VLANs do not communicate with each other, adding some security to
the traffic on those interfaces. Different VLANs can communicate between each other by means of IP
routing. Because traffic is only broadcast or flooded to the interfaces within a VLAN, the VLAN conserves
bandwidth. Finally, you can have multiple VLANs configured on one switch, thus segmenting the device.
Interfaces within a port-based VLAN must be in Layer 2 mode and can be tagged or untagged in the
VLAN ID.
VLANs and Port Tagging
To add an interface to a VLAN, it must be in Layer 2 mode. After you place an interface in Layer 2 mode,
it is automatically placed in the default VLAN. FTOS supports IEEE 802.1Q tagging at the interface level
to filter traffic. When you enable tagging, a tag header is added to the frame after the destination and
source MAC addresses. That information is preserved as the frame moves through the network.
Figure 10-7 shows the structure of a frame with a tag header. The VLAN ID is inserted in the tag header.
Figure 10-7. Tagged Frame Format
Ethernet
Preamble Destination
Address
Source
Address
6 octets 6 octets
Tag
Header
Protocol
Type
The tag header contains some key information used by FTOS:
• The VLAN protocol identifier identifies the frame as tagged according to the IEEE 802.1Q
specifications (2 bytes).
• Tag control information (TCI) includes the VLAN ID (2 bytes total). The VLAN ID can have 4,096
values, but two are reserved.
Note: The insertion of the tag header into the Ethernet frame increases the size of the frame to more than
the 1518 bytes specified in the IEEE 802.3 standard. Some devices that are not compliant with IEEE 802.3
may not support the larger frame size.
Information contained in the tag header allows the system to prioritize traffic and to forward information to
ports associated with a specific VLAN ID. Tagged interfaces can belong to multiple VLANs, while
untagged interfaces can belong only to one VLAN.
Data
Frame
Check
Sequence
4 octets 2 octets 45 - 1500 octets 4 octets
FN00001B
Interfaces | 125