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Page 236 / 998 Scroll up to view Page 231 - 235
VLAN
Overview
iMG/RG Software Reference Manual (Switching)
2-86
•
To allow the frame to indicate the format of MAC Address information carried in MAC user data;
•
To allow VLANs to be supported across different MAC types.
Tagging a frame requires:
•
The addition of a tag header to the frame. This header is inserted immediately following the destination
MAC Address and source MAC Address fields of the frame to be transmitted;
•
Recomputation of the Frame Check Sequence (FCS).
When relaying a tagged frame between 802.3/Ethernet MACs, a switch may adjust the PAD field such that the
minimum size of a transmitted tagged frame is 68 octets.
FIGURE 2-2
Tagged frame format according to IEEE 802.3ac standard
The tag header carries the following information (see
Figure 2-2
):
•
The Tag Protocol Identifier (TPID) carrying an Ethernet Type value (802.1QTagType), which identifies the
frame as a tagged frame. The value of 802.1QTagType is 81-00
Preample
7 octets
Start frame delimiter
1 octet
1 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0
6 octets
Source address
6 octets
Length/type = 802.1QTagType
2 octets
TAG control information
2 octets
MAC control length/type
2 octets
MAC client data
Pad
42-1500 octets
Frame check sequence
4 octets
User priority
VALN identifier VID (12 bit)
CFI
TAG header
Page 237 / 998
Overview
VLAN
2-87
iMG/RG Software Reference Manual (Switching)
•
Tag Control Information (TCI). The TCI field is two octets in length, and contains user priority, CFI and VID
(VLAN Identifier) fields. Figure ... illustrates the structure of the TCI field:
•
User priority. The user priority field is three bits in length, interpreted as a binary number. The user priority
is therefore capable of representing eight priority levels, 0 through 7. This field allows the tagged frame to
carry user priority information across Bridged LANs in which individual LAN segments may be unable to sig-
nal priority.
•
Canonical Format Indicator (CFI). The Canonical Format Indicator (CFI) is a single bit flag value. CFI reset
indicates that all MAC Address information that may be present in the MAC data carried by the frame is in
Canonical format.
•
The meaning of the CFI when set depends upon the variant of the tag header in which it appears.
•
In an Ethernet-encoded tag header, transmitted using 802.3/Ethernet MAC methods, CFI has the following
meanings:
•
When set, indicates that the E-RIF field is present in the tag header, and that the NCFI bit in the RIF
determines whether MAC Address information that may be present in the MAC data carried by the
frame is in Canonical (C) or Non-canonical (N) format;
•
When reset, indicates that the E-RIF field is not present in the tag header, and that all MAC Address
information that may be present in the MAC data carried by the frame is in Canonical format (C).
•
VLAN Identifier (VID). The twelve-bit VLAN Identifier field uniquely identifies the VLAN to which the frame
belongs. The VID is encoded as an unsigned binary number. In Table 8 are described the values of the VID
field that have specific meanings or uses; the remaining values of VID are available for general use as VLAN
identifiers.
A priority-tagged frame is a tagged frame whose tag header contains a VID value equal to the null VLAN ID.
Page 238 / 998
VLAN
VLAN Functional Description
iMG/RG Software Reference Manual (Switching)
2-88
TABLE 2-5
Reserved VID Values
2.4.2
VLAN Functional Description
2.4.2.1 VLAN support on Ethernet interfaces
The Gateway supports up to 16 VLANs (irrespective of whether they are carrying tagged or untagged frames)
from VID=1 up to VID=4094.
If a non-tagged or null-VID tagged packet is received, the ingress port VID is used for look up.
The look up process starts with a VLAN table look up to determine whether the VID is valid.
If the VID is not valid the packet will be dropped and its address will not be learned.
If the VID is valid, FID is retrieved for further look up.
FID + DA is used to determine the destination port. FID + SA is used for learning purposes.
2.4.2.1.1 VLAN definition and port tagging
By default the Gateway starts with only one VLAN defined with name default and VID=1.
All the system ports are members of the default VLAN.
Creating and configuring a new VLAN is a two-step process:
•
A VLAN is created by specifying a name for the VLAN and its VID value.
•
The ports are added to the VLAN. When a port is added it's necessary to specify the frame format in which
packets associated with that VLAN will be transmitted from that port: untagged or tagged.
Note that a physical port can be a member of one or more VLANs.
VID Value
(Hexadecimal)
Meaning/Use
0
The null VLAN ID. Indicates that the tag header contains only user
priority information; no VLAN identifier is present in the frame. This
VID value shall not be configured as a PVID, configured in any Filter-
ing Database entry, or used in any Management operation.
1
The default PVID value used for classifying frames on ingress through
a switch port. The PVID value can be changed by management on a
per-port basis.
FFF
Reserved for implementation use. This VID value shall not be config-
ured as a PVID, configured in any Filtering Database entry, used in
any Management operation, or transmitted in a tag header.
Page 239 / 998
VLAN Functional Description
VLAN
2-89
iMG/RG Software Reference Manual (Switching)
•
A port can be member of two or more VLANs only if it is tagged on all the VLANs or it is untagged on one
VLAN only and tagged on all the other VLANs. A port cannot be member of two or more VLANs as
untagged port.
To change the tagged/untagged frame format of a port for a specific VLAN it's necessary remove the port from
the VLAN and then re-add the port to the VLAN, specifying the required frame format.
When a port is removed from a VLAN and the same port is not a member of any other VLAN, the port is auto-
matically added to the default VLAN with the untagged attribute.
2.4.2.2 VLAN support on ADSL interface
The ADSL Residential Gateways extend the support on tagged frames from the Ethernet ports to the ADSL
port.
Specifically, only on ADSL connections that use RFC1483 encapsulation method, it’s possible assign a connec-
tion to manage tagged traffic for one or more VLANs and simultaneously manage also untagged frames for one
VLAN only.
2.4.2.2.1 Untagged RFC1483 connections
To assign an RFC1483 to manage untagged frames for one VLAN, use the command RFC1483 SET TRANS-
PORT FRAME UNTAGGED.
•
All the incoming untagged frames that from the ADSL port arrive to the residential gateway on the PVC
channel specific for the RFC1483 transport, are forwarded internally to the bridge software as tagged
frames with the VLAN identifier equal to the VID value of VLAN specified.
•
If the same RFC1483 transport has not been assigned to manage any tagged frame, any tagged incoming
frames are silently discharged.
•
All the outgoing tagged frames that from the bridge software must be sent outside on the ADSL port, are fil-
tered to discharge not valid tagged frames:
•
If the frame VID value in the 802.1Q header equals the VID value of VLAN specified, the 802.1Q header is
removed and the frame is sent as untagged frame, otherwise the frame is silently discharged.
2.4.2.2.2 Tagged RFC1483 connections
To assign an RFC1483 to manage tagged frames for one VLAN, use the command rfc1483 set transport frame
tagged.
All the incoming tagged frames that from the ADSL port arrive to the residential gateway on the PVC channel
specific for the RFC1483 transport and having the VID value equal to the VID value of VLAN specified, are sim-
ply forwarded internally to the bridge software as tagged frames maintaining the same VLAN identifier.
Page 240 / 998
VLAN
VLAN Functional Description
iMG/RG Software Reference Manual (Switching)
2-90
All the incoming tagged frames that from the ADSL port arrive to the residential gateway on the PVC channel
specific for the RFC1483 transport and having the VID value different to the VID value of VLAN specified are
silently discharged.
Note that it’s possible assign the same RFC1483 transport to manage tagged frames for more than one VLAN
simply entering multiple times the command RFC1483 SET TRANSPORT FRAME TAGGED for each VLAN to
be configured.
All the incoming untagged frames that from the ADSL port arrive to the residential gateway on the PVC chan-
nel specific for the RFC1483 transport, are silently discharged if the RFC1483 transport has not being assigned
any VLAN as untagged transport.
•
All the outgoing tagged frames that from the bridge software must be sent outside on the ADSL port, are
filtered to discharge not valid tagged frames:
If the frame VID value in the 802.1Q header equals the VID value of VLAN specified, the frame is sent as tagged
frame maintaining the same VLAN identifier; otherwise the frame is silently discharged.
2.4.2.3 VLAN versus IP interface
One of the major constraints when using VLANs is that packets exchanged between hosts that are members of
the same VLAN cannot be received by hosts that are members of a different VLAN.
The
Gateway solves this limitation by offering a packet routing service between different VLANs.
The routing of packets between VLANs is based on the classical layer 3 routing method as, for example, a typi-
cal router performs between IP interfaces.
Based on this approach, there is the requirement that each VLAN that you wish to be involved in the routing of
packets must have an associated IP interface.
In this way, the Layer 3 routing process is able to treat VLAN IP interfaces as though they were distinct Ether-
net ports, and route rules apply as they would for a multi-port router.
Each primary IP interface uses the VLAN data transport services (frame tagging and untagging and related layer
2 forwarding) as though it were an Ethernet port.
For the system point of view, when a VLAN is used to support an IP interface, the VLAN becomes a transport
device supporting Ethernet traffic (see
Figure 2-3
).

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