Spanning Tree Algorithm
101
Instruction Manual - NXA-ENET8-POE+
Spanning Tree Algorithm
This chapter describes the following basic topics:
Loopback Detection
- Configures detection and response to loopback BPDUs.
Global Settings for STA
- Configures global bridge settings for STP, RSTP and MSTP.
Interface Settings for STA
- Configures interface settings for STA, including priority, path cost, link type, and designation
as an edge port.
Global Settings for MSTP
- Sets the VLANs and associated priority assigned to an MST instance
Interface Settings for MSTP
- Configures interface settings for MSTP, including priority and path cost.
Overview
The Spanning Tree Algorithm (STA) can be used to detect and disable network loops, and to provide backup links between switches,
bridges or routers. This allows the switch to interact with other bridging devices (that is, an STA-compliant switch, bridge or router)
in your network to ensure that only one route exists between any two stations on the network, and provide backup links which
automatically take over when a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
STP - Spanning Tree Protocol (IEEE 802.1D)
RSTP - Rapid Spanning Tree Protocol (IEEE 802.1w)
MSTP - Multiple Spanning Tree Protocol (IEEE 802.1s)
STP
- STP uses a distributed algorithm to select a bridging device (STP-compliant switch, bridge or router) that serves as the root
of the spanning tree network. It selects a root port on each bridging device (except for the root device) which incurs the lowest
path cost when forwarding a packet from that device to the root device. Then it selects a designated bridging device from each LAN
which incurs the lowest path cost when forwarding a packet from that LAN to the root device. All ports connected to designated
bridging devices are assigned as designated ports. After determining the lowest cost spanning tree, it enables all root ports and
designated ports, and disables all other ports. Network packets are therefore only forwarded between root ports and designated
ports, eliminating any possible network loops.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol Data Units) transmitted
from the Root Bridge. If a bridge does not get a Hello BPDU after a predefined interval (Maximum Age), the bridge assumes that the
link to the Root Bridge is down. This bridge will then initiate negotiations with other bridges to reconfigure the network to
reestablish a valid network topology.
RSTP
- RSTP is designed as a general replacement for the slower, legacy STP. RSTP is also incorporated into MSTP. RSTP achieves
much faster reconfiguration (i.e., around 1 to 3 seconds, compared to 30 seconds or more for STP) by reducing the number of
state changes before active ports start learning, predefining an alternate route that can be used when a node or port fails, and
retaining the forwarding database for ports insensitive to changes in the tree structure when reconfiguration occurs.
FIG. 109
STP Root Ports and Designated Ports
Designated
Root
Designated
Bridge
Designated
Port
Root
Port