Figure 97: stp root ports and designated ports – LevelOne GTL-2691 User Manual

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| Spanning Tree Algorithm

Overview

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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.

Figure 97: STP Root Ports and Designated Ports

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.

MSTP – When using STP or RSTP, it may be difficult to maintain a stable

path between all VLAN members. Frequent changes in the tree structure

can easily isolate some of the group members. MSTP (which is based on

RSTP for fast convergence) is designed to support independent spanning

trees based on VLAN groups. Using multiple spanning trees can provide

multiple forwarding paths and enable load balancing. One or more VLANs

can be grouped into a Multiple Spanning Tree Instance (MSTI). MSTP builds

a separate Multiple Spanning Tree (MST) for each instance to maintain

connectivity among each of the assigned VLAN groups. MSTP then builds a

Internal Spanning Tree (IST) for the Region containing all commonly

configured MSTP bridges.

Figure 98: MSTP Region, Internal Spanning Tree, Multiple Spanning Tree

Designated
Root

Designated
Port

Designated
Bridge

Root
Port