ZyXEL Communications NBG7510 User Manual page 101

Introduction to IEEE 802.1Q Tagged VLAN
A 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 4 bytes longer than an untagged frame and contains 2 bytes of TPID (Tag Protocol
Identifier), residing within the type/length field of the Ethernet frame) and 2 bytes of TCI (Tag Control
Information), starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. If a
frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to
an untagged port. The remaining twelve bits define the VLAN ID, giving a possible maximum number of
4,096 VLANs. Note that user priority and VLAN ID are independent of each other. A frame with VID
(VLAN Identifier) of null (0) is called a priority frame, meaning that only the priority level is significant and
the default VID of the ingress port is given as the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is
used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN
configurations are 4,094.
TPID User Priority
2 Bytes 3 Bits
Multicast
IP packets are transmitted in either one of two ways – Unicast (1 sender – 1 recipient) or Broadcast (1
sender – everybody on the network). Multicast delivers IP packets to a group of hosts on the network –
not everybody and not just 1.
Internet Group Multicast Protocol (IGMP) is a network-layer protocol used to establish membership in a
Multicast group - it is not used to carry user data. IGMP version 2 (RFC 2236) is an improvement over
version 1 (RFC 1112) but IGMP version 1 is still in wide use. If you would like to read more detailed
information about interoperability between IGMP version 2 and version 1, please see sections 4 and 5 of
RFC 2236. The class D IP address is used to identify host groups and can be in the range 224.0.0.0 to
239.255.255.255. The address 224.0.0.0 is not assigned to any group and is used by IP multicast
computers. The address 224.0.0.1 is used for query messages and is assigned to the permanent group of
all IP hosts (including gateways). All hosts must join the 224.0.0.1 group in order to participate in IGMP.
The address 224.0.0.2 is assigned to the multicast routers group.
At start up, the Zyxel Device queries all directly connected networks to gather group membership. After
that, the Zyxel Device periodically updates this information.
DNS Server Address Assignment
Use Domain Name System (DNS) to map a domain name to its corresponding IP address and vice versa,
for instance, the IP address of www.zyxel.com is 204.217.0.2. The DNS server is extremely important
because without it, you must know the IP address of a computer before you can access it.
The Zyxel Device can get the DNS server addresses in the following ways.
1 The ISP tells you the DNS server addresses, usually in the form of an information sheet, when you sign up.
If your ISP gives you DNS server addresses, manually enter them in the DNS server fields.
Chapter 8 Broadband
CFI VLAN ID
1 Bit 12 Bits
NBG7510 User's Guide
101