Spanning Tree Protocol
ExtremeWare XOS 11.3 Concepts Guide
426
An STP port has three possible encapsulation modes:
â—Ź 802.1D mode
Use this mode for backwards compatibility with previous STP versions and for compatibility with
third-party switches using IEEE standard 802.1D. BPDUs are sent untagged in 802.1D mode. Because
of this, any given physical interface can have only one STPD running in 802.1D mode.
â—Ź Extreme Multiple Instance Spanning Tree Protocol (EMISTP) mode
EMISTP mode is proprietary to Extreme Networks and 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.
â—Ź Per VLAN Spanning Tree (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.
To configure the BPDU encapsulation mode for one or more STP ports, use the following command:
configure stpd <stpd_name> ports mode [dot1d | emistp | pvst-plus] <port_list>
To configure the default BPDU encapsulation mode on a per STPD basis, use the following command:
configure stpd <stpd_name> default-encapsulation [dot1d | emistp | pvst-plus]
Instead of accepting the default encapsulation modes of dot1d for the default STPD s0 and emistp for
all other STPDs, this command allows you to specify the type of BPDU encapsulation to use for all
ports added to the STPD (if not otherwise specified).
STPD Identifier
An StpdID is used to identify each STP domain. You assign the StpdID when configuring the domain,
and that carrier VLAN of that STPD cannot belong to another STPD.
An StpdID must be identical to the VLANid of the carrier VLAN in that STP domain.
NOTE
If an STPD contains at least one port not in 802.1D mode, you must configure the STPD with an StpdID.
STP States
Each port that belongs to a member VLAN participating in STP exists in one of the following states:
â—Ź Blocking
A port in the blocking state does not accept ingress traffic, perform traffic forwarding, or learn MAC
source addresses. The port does receive STP BPDUs. During STP initialization, the switch always
enters the blocking state.
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