Extreme Networks ExtremeWare XOS 10.1, Manual

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148 ExtremeWare XOS 10.1 Concepts Guide
Spanning Tree Protocol (STP)
Per-VLAN Spanning Tree
Switching products that implement Per-VLAN Spanning Tree (PVST) have been in existence for many
years and are widely deployed. To support STP configurations that use PVST, ExtremeWare XOS has an
operational mode called PVST+.
NOTE
In this document, PVST and PVST+ are used interchangeably. PVST+ is an enhanced version of PVST
that is interoperable with 802.1Q STP. The following discussions are in regard to PVST+, if not
specifically mentioned.
STPD VLAN Mapping
Each VLAN participating in PVST+ must be in a separate STPD and the VLAN number must be the
same as the STPD identifier (StpdID). As a result, PVST+ protected VLANs cannot be partitioned.
This fact does not exclude other non-PVST+ protected VLANs from being grouped into the same STPD.
A protected PVST+ VLAN can be joined by multiple non-PVST+ protected VLANs to be in the same
STP domain.
Native VLAN
In PVST+, the native VLAN must be peered with default VLAN on Extreme devices, as both are the
only VLAN allowed to send and receive untagged packets on the physical port.
Third-party PVST+ devices send VLAN 1 packets in a special manner. ExtremeWare XOS does not
support PVST+ for VLAN 1. Therefore, when the switch receives a packet for VLAN 1, the packet is
dropped.
When a PVST+ instance is disabled, the fact that PVST+ uses a different packet format raises an issue. If
the STPD also contains ports not in PVST+ mode, the flooded packet has an incompatible format with
those ports. The packet is not recognized by the devices connected to those ports.
Rapid Spanning Tree Protocol
The Rapid Spanning Tree Protocol (RSTP; 802.1w) provides an enhanced spanning tree algorithm that
improves the convergence speed of bridged networks. RSTP takes advantage of point-to-point links in
the network and actively confirms that a port can safely transition to the forwarding state without
relying on any timer configurations. If a network topology change or failure occurs, RSTP rapidly
recovers network connectivity by confirming the change locally before propagating that change to other
devices across the network. For broadcast links, there is no difference in convergence time between STP
and RSTP.
RSTP supersedes legacy STP protocols, supports the existing STP parameters and configurations, and
allows for seamless interoperability with legacy STP.