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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 45: 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 must 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 46: MSTP Region, Internal Spanning Tree, Multiple Spanning Tree
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