Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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202 ExtremeWare 7.2e Installation and User 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 has an
operational mode called PVST+. Summit 400 has limited support for PVST+ and only operates in
compatibility mode.
As stated before, a VLAN can span multiple STPDs. However, on the Summit 400, there is a hardware
limitation that restricts each physical port to a single STPD. If the Summit 400 port is already a member
of an STPD, then that port cannot be in another VLAN that is in a different STPD, or not in a STPD at
all.
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+ VLANs can not be partitioned.
This fact does not exclude other non-PVST+ VLANs from being grouped into the same STPD. A PVST+
VLAN can be joined by multiple non-PVST+ 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 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. Therefore,
ExtremeWare has the following limitation:
• If an STPD contains both PVST+ and non-PVST+ ports, the STPD must not be disabled. Otherwise,
the BPDUs are flooded in the format of the incoming STP port.
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