Web Management Guide-R07

Table Of Contents
Chapter 7
| Spanning Tree Algorithm
Overview
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Figure 94: 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 – 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 95: MSTP Region, Internal Spanning Tree, Multiple Spanning Tree
An MST Region consists of a group of interconnected bridges that have the same
MST Configuration Identifiers (including the Region Name, Revision Level and
Configuration Digest – see “Configuring Multiple Spanning Trees” on page 199). An
MST Region may contain multiple MSTP Instances. An Internal Spanning Tree (IST)
is used to connect all the MSTP switches within an MST region. A Common
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x
Region R
IST
(for this Region)
MST 1
MST 2