CTS FOS-3148 Series, Руководство пользователя

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VLAN membership information to Ethernet frames, the IEEE 802.1Q can help network
administrators break large switched networks into smaller segments so that broadcast and
multicast traffic will not occupy too much available bandwidth as well as provide a higher level
security between segments of internal networks.
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
VLAN ID 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.