Cisco Cisco ONS 15454 SONET Multiservice Provisioning Platform (MSPP) Guía De Diseño

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ML-Series cards support per-VLAN rapid spanning tree (PVRST) and a maximum of 255 rapid 
spanning tree instances. 
 
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by 
determining the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch 
with the highest switch priority (lowest numerical priority value). 
 
In a stable topology with consistent port roles throughout the network, the RSTP ensures that 
every root port and designated port immediately transition to the forwarding state while all 
alternate and backup ports are always in the discarding state (equivalent to blocking in 802.1D). 
The port state controls the operation of the forwarding and learning processes.  Table 5-7 
provides a comparison of 802.1D and RSTP port states. 
 

Enabled Blocking  Discarding 

No 

Enabled Listening  Discarding 

No 

Enabled Learning 

Learning 

Yes 

Enabled Forwarding  Forwarding 

Yes 

Disabled Disabled  Discarding 

No 

 
Rapid Convergence 
 
The RSTP provides for rapid recovery of connectivity following the failure of a ML-Series card, a a 
ML-Series port, or a LAN.  It provides rapid convergence for new root ports and ports connected 
through point-to-point links as follows: 
 

    Root ports:  If the RSTP selects a new root port, it blocks the old root port and 

immediately transitions the new root port to the forwarding state. 

 

    Point-to-point links:  If you connect a port to another port through a point-to-point link and 

the local port becomes a designated port, it negotiates a rapid transition with the other 
port by using the proposal-agreement handshake to ensure a loop-free topology. 

 
Synchronization of Port Roles 
 
When the ML-Series card receives a proposal message on one of its ports and that port is 
selected as the new root port, the RSTP forces all other ports to synchronize with the new root 
information.  The ML-Series card is synchronized with superior root information received on the 
root port if all other ports are synchronized. 
 
If a designated port is in the forwarding state, it transitions to the blocking state when the RSTP 
forces it to synchronize with new root information.  In general, when the RSTP forces a port to 
synchronize with root information and the port does not satisfy any of the above conditions, its 
port state is set to blocking. 
 
After ensuring all of the ports are synchronized, the ML-Series card sends an agreement 
message to the designated switch corresponding to its root port.  When the ML-Series cards 
connected by a point-to-point link are in agreement about their port roles, the RSTP immediately 
transitions the port states to forwarding.