ZyXEL Communications 2002 Series Betriebsanweisung PDF herunterladen (Seite 46)

VES-1000 Series Ethernet Switch

4-8 General, Switch and IP Setup

In MTU applications, VLAN is vital in providing isolation and security among the subscribers. When properly

configured, VLAN prevents one subscriber from accessing the network resources of another on the same LAN,

thus a user will not see the printers and hard disks of another user in the same building.

VLANs also increase network performance by limiting broadcasts to a smaller and more manageable logical

broadcast domain. In traditional switched environments, all broadcast packets go to each and every individual

port. With VLAN, broadcasts are confined to the members of the VLAN. Note that VLANs are unidirectional -

they only govern outgoing traffic.

Port-based VLANS

Port-based VLANs are VLANs where the packet forwarding decision is based on the destination MAC address

and its associated port.

As previously mentioned, VLANs are unidirectional. Therefore, if you wish to allow two subscriber ports to talk

to each other, for example, between conference rooms in a hotel, you must define the egress (an egress port is an

outgoing port, that is, a port through which a data packet leaves) for both ports.

Port-based VLANs are specific only to the switch on which they were created. The factory default port-based

VLAN settings for VES-1000 Series switches are summarized below.

• Port 0 (the CPU management port) forms a VLAN with uplink ports and can use all Ethernet ports as the

uplink.

• The VDSL ports cannot talk to each other.

• Each VDSL port forms a VLAN with the Ethernet ports and vice-versa.

Tagged VLANs (IEEE 802.1Q)

Tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of a frame

across bridges – they are not confined to the switch on which they were created. The VLANs can be created

statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a specific VLAN and

provides the information that switches need to process the frame across the network. A tagged frame is four bytes

longer than an untagged frame and contains two bytes of TPID (Tag Protocol Identifier; residing within the

type/length field of the Ethernet frame) and two bytes of TCI (Tag Control Information; a tagged header starts

after the source address field of the Ethernet frame).

TPID has a defined value of 8100 (hex). The first three bits of the TCI define user priority (giving eight priority

levels). The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. The

remaining twelve bits define the VLAN ID, giving a possible maximum number of 4,096 (2

) VLANs.

TPID

2 bytes

User Priority

3 bits

CFI

1 bit

VLAN ID

12 bits

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 160 161

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ZyXEL Communications 2002 Series Betriebsanweisung Seite 46