Implementing IBM System Networking 10Gb Ethernet Switches

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 59

The details of the operations of STP are not covered in this book. What is important to

remember is that STP works in Layer 2 by detecting forwarding loops and logically disabling

the link that is part of the loop. STP operates by transmitting and receiving Bridge Protocol

Data Units (BPDUs).

For more information about STP, see the originalIEEE 802.1d specification at the

following website:

http://standards.ieee.org/getieee802/download/802.1D-2004.pdf

There are multiple flavors of STP, as described in the following sections.

2.2.1 Rapid Spanning Tree Protocol

Rapid Spanning Tree Protocol (RSTP) (802.1w) provides rapid convergence of the Spanning

Tree and provides the fast reconfiguration critical for networks that carry delay-sensitive

traffic, such as voice and video. RSTP significantly reduces the time to reconfigure the active

topology of the network when changes occur to the physical topology or its

configuration parameters.

RSTP is compatible with devices that run IEEE 802.1d STP. If the switch detects IEEE

802.1d BPDUs, it responds with IEEE 802.1d-compatible data units.

2.2.2 Per-VLAN Rapid Spanning Tree Protocol (PVRST)

Per-VLAN Rapid Spanning Tree Protocol (PVRST) is based on IEEE 802.1w RSTP. Like

RSTP, PVRST mode provides rapid Spanning Tree convergence. However, similar to the

way standard STP allows per-VLAN spanning-tree instance on a switch. Each VLAN has its

own Spanning Tree instance and tree, so that the VLANs can use different paths. The

drawback is that the number of Spanning Tree instances can grow significantly and affect

processing time on the switches in networks with many VLANs.

2.2.3 Multiple Spanning Tree Protocol

Multiple Spanning Tree Protocol (MSTP) extends Rapid Spanning Tree Protocol (RSTP),

allowing multiple Spanning Tree instances, which may each include multiple VLANs.

In MSTP mode, IBM System Networking switches support up to 32 instances of Spanning

Tree. MSTP allows frames assigned to different VLANs to follow separate paths, with each

path based on an independent Spanning Tree instance. This approach provides multiple

forwarding paths for data traffic, enabling load-balancing, and reducing the number of

Spanning Tree instances required to support many VLANs.