Specifications

STP Rules and Restrictions
ExtremeWare XOS 10.1 Command Reference Guide 369
Encapsulation Modes
You can configure ports within an STPD to accept specific BPDU encapsulations. This STP port
encapsulation is separate from the STP mode of operation. For example, you can configure a port to
accept the PVST+ BPDU encapsulation while running in 802.1D mode.
An STP port has three encapsulation modes:
• 802.1d mode
This mode is used for backward compatibility with previous STP versions and for compatibility with
third-party switches using IEEE standard 802.1d. BPDUs are sent untagged in 1D mode. Because of
this, any given physical interface can have only one STPD running in 1D mode.
• Extreme Multiple Instance Spanning Tree Protocol (EMISTP) mode
EMISTP mode is an extension of STP that allows a physical port to belong to multiple STPDs by
assigning the port to multiple VLANs. EMISTP adds significant flexibility to STP network design.
BPDUs are sent with an 802.1Q tag having an STPD instance Identifier (StpdID) in the VLANid field.
• PVST+ mode
This mode implements PVST+ in compatibility with third-party switches running this version of STP.
The STPDs running in this mode have a one-to-one relationship with VLANs, and send and process
packets in PVST+ format.
These encapsulation modes are for STP ports, not for physical ports. When a physical port belongs to
multiple STPDs, it is associated with multiple STP ports. It is possible for the physical port to run in
different modes for different domains to which it belongs.
STP Rules and Restrictions
This section summarizes the rules and restrictions for configuring STP.
• The carrier VLAN must span all of the ports of the STPD.
• The StpdID must be the VLANid of one of its member VLANs, and that VLAN cannot be
partitioned.
• A default VLAN can not be partitioned. If a VLAN traverses multiple STP domains, the VLAN must
be tagged.
• An STPD can carry, at most, one VLAN running in PVST+ mode, and its StpdID must be identical
with that VLANid. In addition, the PVST+ VLAN cannot be partitioned.
• The default VLAN of a PVST+ port must be identical with the native VLAN on the PVST+ device
connected to that port.
• If a port supports 802.1d-STPD, then the port must be configured with a default VLAN. If not, the
BPDUs for that STPD are not flooded when the STPD is disabled.
• If an STPD contains both PVST+ and non-PVST+ ports, it must be enabled. If it is disabled, the
BPDUs are flooded in the format of the incoming STP port, which may be incompatible with those of
the connected devices.
• 802.1d ports must be untagged; EMISTP/PVST+ ports must be tagged.
• An STPD with multiple VLANs must contain only VLANs that belong to the same virtual router
instance.