Multiple Stp - ZyXEL Communications MSC1024G Series User Manual

Chapter 15 Switch Screens
In STP, once a stable network topology has been established, all devices listen for Hello
BPDUs transmitted from the root bridge. If an STP-aware device does not get a Hello BPDU
after a predefined interval (Max Age), the device assumes that the link to the root bridge is
down. This device then initiates negotiations with other devices to reconfigure the network to
re-establish a valid network topology.
RSTP assigns three port states to eliminate packet looping while STP assigns five (see
following table). A device port is not allowed to go directly from blocking state to forwarding
state so as to eliminate transient loops.
Table 248 RSTP and STP Port States
RSTP PORT
STATE
Discarding
Discarding
Discarding
Learning
Forwarding

15.9.2 Multiple STP

Multiple Spanning Tree Protocol (IEEE 802.1s) is backward compatible with STP/RSTP and
addresses the limitations of existing spanning tree protocols (STP and RSTP) in networks to
include the following features:
• One Common and Internal Spanning Tree (CIST) that represents the entire network's
connectivity.
• Grouping of multiple bridges (or switching devices) into regions that appear as one single
bridge on the network.
• Multiple Spanning Tree Instances (Mists) that allow multiple VLANs to use the same
spanning tree.
• Load-balancing is possible as traffic from different VLANs can use distinct paths in a
region.
15.9.2.1 MSTP Network Example
The following figure shows a network example where two VLANs are configured on the two
switches. If the switches are using STP or RSTP, the link for VLAN 2 will be blocked as STP
and RSTP allow only one link in the network and block the redundant link.
482
STP PORT
DESCRIPTION
STATE
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and
processed.
Listening All BPDUs are received and processed.
Learning All BPDUs are received and processed. Information frames are
submitted to the learning process but not forwarded.
Forwarding All BPDUs are received and processed. All information frames are
received and forwarded.
MSC1000G/1024G/1224G Series User's Guide