Alcatel-Lucent 6850-48 Network Guide
Spanning Tree Overview
Configuring Spanning Tree Parameters
page 13-6
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
Spanning Tree Overview
Alcatel-Lucent switches support the use of the 802.1D Spanning Tree Algorithm and Protocol (STP), the
802.1w Rapid Spanning Tree Algorithm and Protocol (RSTP), the 802.1Q 2005 Multiple Spanning Tree
Protocol (MSTP), and the Ring Rapid Spanning Tree Protocol (RRSTP).
802.1w Rapid Spanning Tree Algorithm and Protocol (RSTP), the 802.1Q 2005 Multiple Spanning Tree
Protocol (MSTP), and the Ring Rapid Spanning Tree Protocol (RRSTP).
RSTP expedites topology changes by allowing blocked ports to transition directly into a forwarding state,
bypassing listening and learning states. This provides rapid reconfiguration of the Spanning Tree in the
event of a network path or device failure.
bypassing listening and learning states. This provides rapid reconfiguration of the Spanning Tree in the
event of a network path or device failure.
The 802.1w standard is an amendment to the 802.1D document, thus RSTP is based on STP. Regardless of
which one of these two protocols a switch or VLAN is running, it can successfully interoperate with other
switches or VLANs.
which one of these two protocols a switch or VLAN is running, it can successfully interoperate with other
switches or VLANs.
802.1Q 2005 is a new version of MSTP that combines the 802.1D 2004 and 802.1S protocols. This imple-
mentation of 802.1Q 2005 also includes improvements to edge port configuration and provides administra-
tive control to restrict port role assignment and the propagation of topology change information through
bridge ports.
mentation of 802.1Q 2005 also includes improvements to edge port configuration and provides administra-
tive control to restrict port role assignment and the propagation of topology change information through
bridge ports.
MSTP is an enhancement to the 802.1Q Common Spanning Tree (CST), which is provided when an
Alcatel-Lucent switch is running in the flat Spanning Tree operating mode. The flat mode applies a single
spanning tree instance across all VLAN port connections on a switch. MSTP allows the configuration of
Multiple Spanning Tree Instances (MSTIs) in addition to the CST instance. Each MSTI is mapped to a set
of VLANs. As a result, flat mode can now support the forwarding of VLAN traffic over separate data
paths.
Alcatel-Lucent switch is running in the flat Spanning Tree operating mode. The flat mode applies a single
spanning tree instance across all VLAN port connections on a switch. MSTP allows the configuration of
Multiple Spanning Tree Instances (MSTIs) in addition to the CST instance. Each MSTI is mapped to a set
of VLANs. As a result, flat mode can now support the forwarding of VLAN traffic over separate data
paths.
RRSTP is faster than MSTP. It is used in a ring topology where bridges are connected in a point to point
manner. This protocol identifies the bridge hosting the alternate (ALT) port in lesser convergence time.
This ALT port is changed to the forwarding state immediately without altering the MSTP state to enable
the data path. The RRSTP frame travels from the point of failure to the bridge hosting the ALT port in
both the directions. The MAC addresses matching the ports in the ring are flushed to make the data path
convergence much faster than normal MSTP.
manner. This protocol identifies the bridge hosting the alternate (ALT) port in lesser convergence time.
This ALT port is changed to the forwarding state immediately without altering the MSTP state to enable
the data path. The RRSTP frame travels from the point of failure to the bridge hosting the ALT port in
both the directions. The MAC addresses matching the ports in the ring are flushed to make the data path
convergence much faster than normal MSTP.
This section provides a Spanning Tree overview based on RSTP operation and terminology. Although
MSTP is based on RSTP, see
MSTP is based on RSTP, see
for specific
.
How the Spanning Tree Topology is Calculated
The tree consists of links and bridges that provide a single data path that spans the bridged network. At the
base of the tree is a root bridge. One bridge is elected by all the bridges participating in the network to
serve as the root of the tree. After the root bridge is identified, STP calculates the best path that leads from
each bridge back to the root and blocks any connections that would cause a network loop.
base of the tree is a root bridge. One bridge is elected by all the bridges participating in the network to
serve as the root of the tree. After the root bridge is identified, STP calculates the best path that leads from
each bridge back to the root and blocks any connections that would cause a network loop.
To determine the best path to the root, STP uses the path cost value, which is associated with every port on
each bridge in the network. This value is a configurable weighted measure that indicates the contribution
of the port connection to the entire path leading from the bridge to the root.
each bridge in the network. This value is a configurable weighted measure that indicates the contribution
of the port connection to the entire path leading from the bridge to the root.
In addition, a root path cost value is associated with every bridge. This value is the sum of the path costs
for the port that receives frames on the best path to the root (this value is zero for the root bridge). The
bridge with the lowest root path cost becomes the designated bridge for the LAN, as it provides the short-
est path to the root for all bridges connected to the LAN.
for the port that receives frames on the best path to the root (this value is zero for the root bridge). The
bridge with the lowest root path cost becomes the designated bridge for the LAN, as it provides the short-
est path to the root for all bridges connected to the LAN.