Planet GS-4210-8P2S User Manual download pdf (Page 118)

User’s Manual of GS-4210 Series

The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether Type/Length or Logical

Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be

recalculated.

Adding an IEEE802.1Q Tag

Dest. Addr. Src. Addr. Length/E. type Data Old CRC

Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC

Priority CFI VLAN ID

New Tagged Packet

Original Ethernet

■ Port VLAN ID

Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network

device to another with the VLAN information intact. This allows 802.1Q VLAN to span network devices (and indeed, the entire

network – if all network devices are 802.1Q compliant).

Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no VLAN are

defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets are assigned the

PVID of the port on which they were received. Forwarding decisions are based upon this PVID, in so far as VLAN are concerned.

Tagged packets are forwarded according to the VID contained within the tag. Tagged packets are also assigned a PVID, but the

PVID is not used to make packet forwarding decisions, the VID is.

Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will compare the VID

of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VID are different the switch will drop

the packet. Because of the existence of the PVID for untagged packets and the VID for tagged packets, tag-aware and

tag-unaware network devices can coexist on the same network.

A switch port can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to store them.

Because some devices on a network may be tag-unaware, a decision must be made at each port on a tag-aware device before

packets are transmitted – should the packet to be transmitted have a tag or not? If the transmitting port is connected to a

tag-unaware device, the packet should be untagged. If the transmitting port is connected to a tag-aware device, the packet

should be tagged.

■ Default VLANs

The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the Switch to

the "default". As new VLAN are configured in Port-based mode, their respective member ports are removed from the "default."

118

1 2 ... 113 114 115 116 117 118 119 120 121 122 123 ... 352 353

No comments

Planet GS-4210-8P2S User Manual Page 118