User guide

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).