Alcatel OS-LS-6224 User Manual page 214

3
Configuring the Switch
STA uses a distributed algorithm to select a bridging device (STA-compliant switch,
bridge or router) that serves as the root of the spanning tree network. It selects a
root port on each bridging device (except for the root device) which incurs the lowest
path cost when forwarding a packet from that device to the root device. Then it
selects a designated bridging device from each LAN which incurs the lowest path
cost when forwarding a packet from that LAN to the root device. All ports connected
to designated bridging devices are assigned as designated ports. After determining
the 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.
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 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 one tenth of the time required by 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.
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 (an extension of RSTP) is designed to support
independent spanning trees based on VLAN groups. Once you specify the VLANs to
include in a Multiple Spanning Tree Instance (MSTI), the protocol will automatically
build an MSTI tree to maintain connectivity among each of the VLANs. MSTP
maintains contact with the global network because each instance is treated as an
RSTP node in the Common Spanning Tree (CST).
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