Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
7
connected to the central trunk link in the lower part of Figure 77. They are VLAN-unaware and they will
be associated to the VLAN C, because the PVIDs of the VLAN-aware bridges are equal to VLAN C.
Because the VLAN-unaware stations will send only untagged frames, when the VLAN-aware bridge
devices receive these untagged frames they will assign them to VLAN C.
Figure 77 Native VLAN
PVST+
PVST+ provides support for 802.1Q trunks and the mapping of multiple spanning trees to the single
spanning tree of 802.1Q switches.
The PVST+ architecture distinguishes three types of regions:
• A PVST region
• A PVST+ region
• A MST region
Each region consists of a homogenous type of switch. A PVST region can be connected to a PVST+
region by connecting two ISL ports. Similarly, a PVST+ region can be connected to an MST region by
connecting two 802.1Q ports.
At the boundary between a PVST region and a PVST+ region the mapping of spanning trees is
one-to-one. At the boundary between a MST region and a PVST+ region, the ST in the MST region maps
to one PVST in the PVST+ region. The one it maps to is called the common spanning tree (CST). The
default CST is the PVST of VLAN 1 (Native VLAN).
All PVSTs, except for the CST, are tunneled through the MST region. Tunneling means that bridge
protocol data units (BPDUs) are flooded through the MST region along the single spanning tree present
in the MST region.
PVID = C PVID = C
PVID = C
PVID = C
VLAN-unaware
end station
Trunk
link
PVID = A
VLAN A
VLAN-aware
bridge
VLAN-aware
bridge
VLAN B
VLAN A
VLAN B
VLAN C
54710
Access
ports
Access
ports
PVID = B
PVID = A
PVID = B
VLAN-aware
end station
VLAN-unaware
end station
VLAN-unaware
end station
VLAN B
VLAN C