RSTP

STP/RSTP/MSTP Description

Spanning Tree Protocol (STP) was developed initially as a part of the 802.1D.  The primary goal of this standard is to prevent loops within the network.  In Layer 2 networks, since they represent a broadcast domain, it can be a real problem if a loop occurs because traffic and especially broadcast and multicast, will be looped within the network.  This will often result in all the network bandwidth being consumed by the looped traffic and disrupting the network.  Spanning Tree Protocol was designed to learn the network topology and remove any loops by breaking the network loop when it is detected. 

A secondary benefit came from this by allowing redundant links to be attached to switches via two different paths and letting spanning tree eliminate the loops.  In the case of a failure, STP will re-route around the break by re-enabling the previously blocked path.

This allows for the Layer 2 network to be fairly resilient in the face of network changes and support redundant layer 2 links.   

The key functions of STP are: 

• Discovering the Topology of the network.
• Looking for loops within the Topology.
• Eliminating Loops.
• Opening redundant links in the case of a network failure. 

Spanning Tree Concepts 

Root Bridge - Within the topology, it must resolve to a tree and branch structure.  One switch or bridge must be elected to be the Root Bridge, which is the root of the tree.  The STP protocol will work to ensure that every other bridge only has ONE path to the root bridge. 

The spanning tree will select the root bridge and, typically, you want to bias the protocol to select a particular node within the network to be elected as the root bridge.  Typically, this will be the core router or the router for that segment of the network. 

Bridge ID - Each switch or bridge that is participating in the spanning tree is given a Bridge ID. The spanning tree will select the bridge with the lowest bridge ID as the root bridge.  Typically, the root will have the lowest bridge ID and all the others will be set to the default bridge ID, although some topologies will require more complexity than this. 

Path Cost - STP will attempt to determine the cost of each path from a bridge to the root.  Traffic will be sent along the least cost path to the root and any redundant paths will be blocked. 

Root Port - The port that has been determined to be the least costly path back to the root bridge.  If there is a tie in path cost, then the path to the neighbor that has the lowest bridge ID will be selected as the root. 

Designated Port - Ports that are in the forwarding state and are forwarding traffic away from the root bridge. 

Blocked Port - A blocked port is a port that has been determined to be a redundant path to the root bridge and was not selected as the best path to the root bridge by STP.

STP/RSTP Configuration 

The OLT implements RSTP and has two basic interfaces for RSTP.  The OLT implements RSTP on the uplinks to protect the uplinks to the system.  The OLT also implements RSTP on the ONT ports to ensure that there are no loops on the ports of the ONTs.  Since there are two interfaces, there are two sets of screens for configuring RSTP on the system.

Overview 

Newly created subscriber lines resulting from an Add ONT, card conversion, ONT conversion, or Auto ONT activation operation are automatically assigned the default RSTP profile that is controlled by an operator defined "Line Default Configuration" record. This allows all lines to be created with the same default values.

To aid in bulk configuration changes, Panorama PON copies attributes from a selected Subscriber Line Profile, including the RSTP Profile, to all selected subscriber lines in the data line's view. In addition, any changes to an existing RSTP Profile have those changes reflected immediately on all subscriber lines assigned that profile without additional action by the user.

 Default Profile

Panorama PON initially creates one default RSTP profile. The default profile cannot be edited or deleted. New profiles can be defined and used as needed; the default profile does not have to be used.

However, lines that are associated with a non-default profile are automatically activated. Because a profile can be associated with multiple lines at once, this is an efficient way to activate lines.

RSTP Profile Attributes

The following RSTP profile attributes are available for user creation and editing.

Status

Notifications

• Enable TCN Notifications - Enables notification of TCNs. Select true (default) or false. 
• Enable Root Protected Notification - Enables notifications of Restricted Role Protection events. Select true (default) or false. 
• BPDU Guard Notification - Enables generation of BPDU-GUARD auto-disable detection. Select true (default) or false. 

• Hello Time - Set the port hello interval. Select 1 (default) or 2. 
• Port Priority - Set the relative port priority. The range is 0 to 240, in increments of 16. The default is 128. 
• Path Cost - Sets the per-hop path cost of this path to the root. The range is 1 to 999999. The default is 20000. 
• Admin Point-to-Point MAC - Defines the port as a point-to-point link to another RSTP switch. Select AUTO (default), Off, or
  On.
• Admin Edge Port? - Defines the port as an access port not expected to receive Bridge Protocol Data Unit (BPDU). Select true or false (default).
  • Ports may be configured as edge ports if attached to a LAN that has no other bridges attached. These edge ports transition directly to the forwarding state. RSTP continues to monitor the port for BPDUs in case a bridge is connected. RSTP also automatically detects edge ports. In the absence of detecting BPDUs, the port is marked as an edge, but as soon as the bridge detects a BPDU coming to the edge port, the port becomes a non-edge port.
  • Enable the Admin Edge Port if using RSTP for loop detection. Leave this option disabled if using RSTP for path redundancy.
• Restricted TCN? - Prevents the port from generating Topology Change Notifications (TCNs). Select true (default) or false. 
• Restricted Role - Prevents the port from allowing a RSTP root to be connected to it, Select true (default) or false.