Specifications

Installation and Operation Manual Appendix C Operating Environment
FCD-E1LC Ver. 1.0 Ethernet Transmission Technology C-15
Bridging
Communication between Nodes on Same LAN
Unlike IP addresses, a MAC address is unique and identifies a single physical port.
Therefore, two Ethernet nodes attached to the same LAN exchange frame
directly, by specifying the desired MAC destination address, together with the
source MAC address. The node that identifies its MAC address in the destination
field can send a response by copying the source address of the frame to the
destination address field.
Communication between Nodes on Different LANs
To enable nodes on different LANs to communicate, it is necessary to transfer
frames between the two LANs. The device used for this purpose is called
MAC
bridge
, or just bridge . Two types of bridges are used:
• Local bridges, which have Ethernet ports attached to the two LANs. The
bridge control mechanism learns the nodes attached to each LAN by reading
the source MAC addresses of the frames generated by the nodes. When the
destination address of a frame is not on the LAN from which it was received,
the bridge transfers it to the other LAN.
• Remote bridges, which are used in pairs. A basic remote bridge has one LAN
port and one WAN port. The WAN port communicates through a link with the
WAN port of the remote bridge connected to the desired remote LAN. In this
case, the traffic addressed to destinations not located on the local LAN is
transferred through the WAN link to the remote bridge.
Using Virtual Bridged LANs
VLAN can be used to provide separation between traffic from different sources
sharing the same physical transmission facilities, and provide information on the
relative priority the user assigns to each frame. The characteristics and use of
virtual LANs (VLANs) and of the MAC bridges capable of handling tagged frames
are defined in IEEE Standard 802.1Q.
VLANs are made possible by a slight modification to the Ethernet frame structure
shown in
Figure A-4.
The structure of an Ethernet frame with VLAN support is shown in Figure A-5
(for simplicity, the figure does not include the preamble and SFD fields).