STP Overview

The bridge allows interconnection of end stations attached to separate LANs and enables the end stations to communicate as if they were attached to a single LAN. The bridge operates below the MAC service boundary and is transparent to the protocols operating above this boundary.

In complex networks, a loop can occur when there are two or more paths between two end points. This situation leads to the duplication of frames, which in turn leads to heavy traffic in the network. To avoid this situation, STP is used in the SEFOS software. STP forms a logical, loop-free topology from the physical topology and forwards the frames without duplication. To avoid prolonged stabilization time following a reconfiguration event in the Spanning Tree algorithm, SEFOS provides support for RSTP. RSTP provides for rapid recovery of connectivity following the failure of a bridge/bridge port or a LAN.

To isolate link fluctuations specific to a particular VLAN segments and to provide for load balancing, SEFOS provides support for multiple spanning trees. These spanning trees can be configured on a per-VLAN basis, or multiple VLANs can be mapped to the same spanning tree.

A switch can take the role of either a root or a designated switch.

STP calculates the best loop free path by assigning port roles to the port of switch as follows:

• Root - The port that offers the lowest cost path towards the root bridge.

• Designated - A forwarding port elected for every switched LAN segment.

• Alternate - A blocked port providing an alternate path to the root bridge of the spanning tree.

• Backup - A blocked port that acts as a backup for the path provided by a designated port.

The stable, active STP of a switched network is determined by the following elements:

• Bridge ID (switch priority and MAC address)

• Path cost to the root switch

• Port identifier (port priority and the port number)

When switches in a network come up, each switch assumes itself to be the root bridge and starts sending configuration messages through all its ports. BPDUs are used to communicate and compute the spanning tree topology. These BPDUs contain the following information:

• Unique bridge ID of the switch that has been identified as the root

• The spanning-tree path cost to the root

• The bridge ID of the sending switch

• Message age

• The identifier of the sending interface (port priority and port number)

• Values for the hello, forward-delay, and max-age protocol timers

When a switch receives a superior configuration BPDU on a port, the switch stores the received information for that port. If the port is a root port, the switch forwards the updated message to all the attached LANs for which this switch is the designated bridge.

If the switch receives a BPDU with a configuration inferior to the BPDU currently stored for that port, the switch discards the BPDU. If the switch is a designated switch for that LAN from which the inferior information was received, then the switch sends up-to-date information stored for that port, thus discarding inferior information and propagating superior information in the network. Each layer 2 interface in the switch running STP can be in one of the following states:

• Blocking - The interface in this state discards the frames and does not learn the MAC addresses.

• Listening - This is the first state that a port can transition to after blocking. The interface enters this state when spanning tree decides that the interface must participate in frame forwarding.

• Learning - An interface enters this state from listening state. In this state, the interface gets ready to participate in frame forwarding and learns MAC addresses from the packet received.

• Forwarding - In this state, the interface receives and forwards frames received on that port or forwards frames switched from another port. This transition from blocking to forwarding takes 30 seconds.