CTS FOS-3128 Series User Manual

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Introduction to 802.1Q frame format:
Preamble
SFD
DA
SA
Type/LEN
PAYLOAD
FCS
Original frame
Preamble
SFD
DA
SA
TAG
TCI/P/C/VID
Type/LEN
PAYLOAD
FCS
802.1q
frame
PRE Preamble
62 bits
Used to synchronize traffic
SFD Start Frame Delimiter
2 bits
Marks the beginning of the header
DA Destination Address
6 bytes
The MAC address of the destination
SA Source Address
6 bytes
The MAC address of the source
TCI Tag Control Info
2 bytes set to 8100 for 802.1p and Q tags
P Priority
3 bits
Indicates 802.1p priority level 0-7
C Canonical Indicator
1 bit
Indicates if the MAC addresses are in
Canonical format - Ethernet set to "0"
VID VLAN Identifier
12 bits
Indicates the VLAN (0-4095)
T/L Type/Length Field
2 bytes
Ethernet II "type" or 802.3 "length"
Payload < or = 1500 bytes User data
FCS Frame Check Sequence
4 bytes
Cyclical Redundancy Check
Important VLAN Concepts for 802.1Q VLAN Configuration:
There are two key concepts to understand.
- Access-VLAN specifies the VLAN ID to the switch port that will assign the VLAN ID to
untagged traffic from that port. A port can only be assigned to one Access-VLAN at a time.
When the port is configured as Access Mode, the port is called an Access Port, the link
to/from this port is called an Access Link. The VLAN ID assigned is called PVID.
- Trunk-VLAN specifies the set of VLAN IDs that a given port is allowed to receive and send
tagged packets. A port can be assigned to multiple Trunk-VLANs at a time. When the port is
configured as Trunk Mode, the port is called a Trunk Port, the link to/from this port is called a
Trunk Link. The VLAN ID assigned is called VID.
A port can be configured as below 802.1q VLAN modes :
- Access Mode :
Access Links (the link to/from access ports) are the most common type of links on any VLAN
switch. All network hosts (such as PCs) connect to the switch's Access Links in order to
gain access to the local network. We configure only one Access-VLAN per port, that is, the
network hosts will be allowed to access.
It is important to note at this point that any network host connected to an Access Port is
totally unaware of the VLAN assigned to the port. The network host simply assumes it is part
of a single broadcast domain, just as it happens with any normal switch. During data transfers,
any VLAN information or data from other VLANs is removed so the recipient has no
information about them.
- Trunk Mode :
Trunk Links (the link to/from trunk ports) is configured to carry packets for multiple VLANs.
These types of ports are usually found in connections between switches. These links require
the ability to carry packets from multiple VLANs because VLANs span over multiple switches.

CTS FOS-3128 Series Specifications

General

Brand	CTS
Model	FOS-3128 Series
Category	Switch
Language	English