Blade ICE G8124 Manuale Utente

BMD00220, October 2010

Chapter 8: Spanning Tree Protocols  

  111

Using STP, network devices detect and eliminate logical loops in a bridged or switched network. 
When multiple paths exist, Spanning Tree configures the network so that a switch uses only the 
most efficient path. If that path fails, Spanning Tree automatically sets up another active path on the 
network to sustain network operations.

BLADEOS STP/PVST+ mode implements IEEE 802.1D (1998) Spanning Tree Protocol (STP) 
with enhancements that allow each VLAN to be assigned to one of 127 available STGs. 
STP/PVST+ uses IEEE 802.1Q for tagging STP data on a per-VLAN basis, and is compatible with 
Cisco PVST+ mode. For Cisco R-PVST/R-PVST+ compatibility, see 

The relationship between ports, trunk groups, VLANs, and Spanning Trees is shown in 

.

STP/PVRST+ mode employs a sequence of port states in the process: Listening, Learning, and 
Forwarding or Blocking. This process can result in inherent delays for resolving network paths.

To mitigate delays, you can use Port Fast Forwarding (see 

) to 

permit a port that participates in STP/PVST+ to bypass the Listening and Learning states and enter 
directly into the Forwarding state. While in the Forwarding state, the port listens to the BPDUs to 
learn if there is a loop and, if dictated by normal STG behavior (following priorities, and so on), the 
port transitions into the Blocking state.

This feature permits the G8124 to interoperate well within Rapid Spanning Tree networks.

Ports, Trunk Groups, and VLANs

Port

Trunk group, 
or one or more VLANs

Trunk group

One or more VLANs

VLAN (non-default)

One VLAN per STG,
or in enhanced modes:

RSTP: All VLANs are in STG 1

PVRST: One VLAN per STG

MSTP: Multiple VLANs per STG