Chapter 11 Spanning Tree Protocol

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.

Table 22 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.

11.1.4 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.

•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.

11.1.4.1MSTP 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.

 

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