Spanning tree

Overview

This section provides information on the spanning tree protocol and spanning tree groups. Alcatel-Lucent 1665 DMX now supports a faster spanning tree algorithm documented in IEEE 802.1w.

Important!

Alcatel-Lucent 1665 DMX has been designed with multi-vendor interoperobility in mind. As many vendor's equipment (including legacy equipment) may not be able to use IEEE802.1w, Alcatel-Lucent 1665 DMX can automatically fall back on IEEE802.1d in the event that other NEs in the network are running standard spanning tree protocol. However, Alcatel-Lucent 1665 DMX employs IEEE 802.1w as a default spanning tree protocol.

Function of spanning tree

Ethernet requires a network with a tree structure in order to work effectively. Ethernet bridges and switches build tables that define the paths to specific devices. A MAC bridge, for example, will have a filtering database that defines where to send any packets addressed to a specific MAC within the network. The tables are built based on input from devices within the network.

If there is a loop in the network, that is, if there is more than one way to get to a destination device, two problems can result:

Frames may be duplicated in the network. That is, the same frame may be sent to a node via two different paths.
The address information is changed as packets from that device are received across different paths. In a meshed network, the re-configuration of the filtering database can begin to use up all the network resources, so that little bandwidth is left for data traffic.

The spanning tree accomplishes two important functions:

It removes loops from the bridged network
It allows re-configuration if a link fails.

How it works

The spanning tree works as follows:

Initially, every node in the group thinks it is the root node.
As information about the network becomes clear, one node is designated the root node. It is from this node that the distance to any point in the network may be measured.
Configuration BPDUs are sent from each node to determine the most economical route from each node to the root node.
Some ports are blocked so that there are no loops in the network and so that the network provides the most efficient paths from the root to the nodes. This effectively creates a tree structure for the network.
If an active link fails, the network is re configured so that previously blocked links can be used for traffic.

The following figure illustrates the spanning tree:

Figure 5-12: Spanning tree switch

Legend:

Active links	1–2; 2–3; 3–4; 5–6; 6–7; 7–1
Inactive links	4–5
X	Indicates a blocked port (no packets sent or received on this port).

The configuration provides the shortest (most efficient) path from each node to the root. The configuration also provides only one path from the root node to any other node.

Re-configuration

If one of the active links were broken, the network would re-configure to allow traffic to get to the nodes. For example, if the link between 3 and 4 were broken, the link between 5 and 4 would be enabled. When the failure/break is repaired, the network will revert to the original configuration.

Spanning tree on LAN ports

Alcatel-Lucent 1665 DMX supports spanning tree on the LAN (customer facing) ports. STP on LAN ports ensures that single LAN interconnects are protected. Because redundant LAN interconnects create loops, STP manages the loops. STP is supported on the LAN ports of the LNW170 only.

Both LAN ports and VCGs can belong to STP groups. A LAN port can belong to only one STP group at a time. The BDPU VLAN ID is provisionable. VLAN IDs are provisionable on a per-port basis.

LAN based STP works exactly as described in the section above. The figure below depicts STP functioning on the customer side of an Alcatel-Lucent 1665 DMX with LNW170 circuit packs. The nodes marked A and B represent remote Ethernet switches or similar CPEs.

Figure 5-13: Spanning tree on LAN ports

Legend:

Active links	1 to 2; 1 to B; B to A
Inactive links	A to 2
X	Indicates a blocked port (frame forwarding is disabled)

Rapid spanning tree maintenance enhancements

Alcatel-Lucent 1665 DMX supports the following rapid spanning tree maintenance enhancements:

Additional performance monitoring counters for VCG and LAN ports or Link Aggregation Groups (LAGs) to record spanning tree modification status
Spanning tree group Automatic Lock parameter to disable a port which is unstable or rapidly reconfiguring. When the Automatic Lock parameter is enabled and any port in the Spanning Tree Group experiences a threshold-crossing alert (TCA) on the spanning tree state counter, the port is moved to the disabled-failure state preventing traffic flow until released.