Falcon R-Class | User Guide
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The first RSTP device, upon receiving this agreement, knows now that it can rapidly change that port
to the forwarding state.
Similar proposal agreement handshake messages propagate within the network, restoring the
connectivity very quickly after a topology change, bypassing the traditional listening/learning state
transition process.
Therefore, a cascading effect is created away from the RSTP root where each designated port
proposes to its neighbors to determine if a rapid transition is possible. In this way RSTP achieves
faster convergence times than STP.
RSTP device port roles:
Root
–
A forwarding port that is the best port from no root-bridge to Root bridge.
Designated
–
A forwarding port for every LAN segment.
Alternate
–
An alternate port to the root bridge.
Disabled
–
A network administrator can manually disable a port.
Backup
–
provides an alternate designated port.
4.15.1.2
Understanding MSTP
RSTP does not solve the problem inherent in STP: all VLANs within a LAN must share the same
spanning tree topology. An STP or RSTP network has only one spanning tree instance for the entire
network and includes all VLANs in the network. Falcon switches utilize the Multiple Spanning Tree
protocol (MSTP, 802.1s) to ensure that only one active path exists between any two nodes in a
spanning tree instance. An instance includes a unique set of VLANs, belongs to a specific spanning
tree region and creates a separate per instance forwarding topology.
A region may comprise multiple spanning tree instances (each with a different set of VLANs). Each
spanning tree instance is independent of other instances. Each region can support up to 16 spanning
tree instances.
MSTP region: a group of interconnected switches that share the same attributes is defined as an MST
region. An MST region includes multiple spanning tree instances (MSTI) which provide different paths
for different VLAN. Each MSTI can have its own independent topology. Note that MSTP recognizes
an STP or RSTP LAN as a distinct spanning tree region.
A region can include two types of STP instances:
•
Internal Spanning Tree Instance (IST instance). This is the default spanning tree instance in any
MST region. IST provides the root switch for the region and by default comprises all VLANs in the
region except those VLANs assigned to MSTI.
•
Multiple Spanning Tree Instance (MSTI). This type of configurable STP instance includes assigned
VLANs which operate as part of the same single spanning tree topology. IST instance is defined
as Instance 0 whereas all other MST instances are numbered from 1 to 15.
•
All MST instances within the same region share the same protocol timers, each MST instance has
its own topology Parameters, such root switch ID, root path cost and additional selected
Parameters.
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