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Chapter 7
| Spanning Tree Algorithm
Overview
ports, and disables all other ports. Network packets are therefore only forwarded
between root ports and designated ports, eliminating any possible network loops.
Figure 103: STP Root Ports and Designated Ports
Designated
Root
x
x
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.
x
– 180 –
x
Designated
Port
Designated
Bridge
x
Root
Port
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