Chapter 11 Spanning Tree Protocol
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11.1.3 STP Port States
STP assigns five port states to eliminate packet looping. A bridge port is not
allowed to go directly from blocking state to forwarding state so as to eliminate
transient loops.
11.1.4 Multiple STP
Multiple Spanning Tree Protocol (IEEE 802.1s) is backwards 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.
• A VLAN can be mapped to a specific Multiple Spanning Tree Instance (MSTI).
MSTI allows multiple VLANs to use the same spanning tree.
• Load-balancing is possible as traffic from different VLANs can use distinct paths
in a region.
11.1.4.1 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
Table 26 STP Port States
PORT
STATE
DESCRIPTION
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and processed.
Listening All BPDUs are received and processed.
Note: The listening state does not exist in RSTP.
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.
