Configuring STP

A Bridge allows interconnection of end stations attached to separate LANs (Local Area Networks) and allows the stations to communicate as if they were attached to a single LAN. The Bridge operates below the Media Access Control (MAC) service boundary and is transparent to the protocols operating above this boundary.

In complex networks, a loop may occur when there are two or more paths between two end points. This leads to the duplication of frames, which in turn leads to heavy traffic in the network. To avoid this, STP is used in the PVRST+ 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 Spanning Tree algorithm, PVRST+ provides support for RSTP. The operation of RSTP enables rapid recovery of connectivity following the failure of a Bridge/ Bridge Port or a LAN.

To isolate link fluctuations only to a particular VLAN segment and also to provide Load balancing, IS5Com’s PVRST+ creates a separate Spanning tree for each VLAN. Spanning Tree to VLAN mapping will be configured on a Per VLAN basis.

PVRST+ protocol is an enhancement of RSTP, which works in conjunction with VLAN to provide better control over traffic in the network.

A switch takes the role of either a root or another type of designated switch.

STP assigns port roles to the port of switch as follows to calculate the best loop free path:

• 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 following elements determine the stable and active spanning-tree topology of a switched network.

• Bridge ID (Instance ID, Switch Priority and MAC address)
• Path Cost to the Root Switch maintained for each instance
• Designated Bridge ID (The bridge through which Root Bridge is connected)
• 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 (Bridge Protocol Data Units) are used to communicate and compute the spanning tree topology. These BPDUs contain the following information:

• VLANID to which this BPDU belongs
• 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, it stores the received information for that port. If the port is a root port, it forwards the updated message to all attached LANs for which this switch is the designated bridge.

If the switch receives an inferior configuration BPDU to that currently stored for that port, it discards the BPDU. If the switch is a designated switch for the 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 PVRST+ is in one of the following states for each VLAN Running on the switch.

• Blocking—the interface in this state discards the frames and does not learn the MAC addresses.
• Listening—this is the first state to which a port transit 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.

Each switch has a unique bridge identifier (bridge ID) that determines the selection of the Root Switch. The bridge ID is an 8-byte field that is composed of two subfields as shown in the figure below.

All switches in a Layer 2 network, participating in STP, gather information on other switches in the network through an exchange of data messages called Bridge Protocol Data Units (BPDUs). The exchange of messages results in the following actions:

• Election of a unique Root Switch for each spanning tree instance
• Election of a Designated switch for every switched LAN segment
• Removal of loops in the switched network by blocking Layer 2 interfaces connected to redundant links

The switch with the highest switch priority (the lowest numerical priority value) is elected as the Root Switch. If all switches are configured with the default priority (32768) for VLAN with ID 1 (i.e. Bridge Priority of 32769), the switch with the lowest MAC address becomes the Root Switch. The switch priority value occupies the most significant bits of the bridge ID. The Root Switch is the logical center of the STP topology in a switched network. Redundant paths to the Root are put in STP blocking mode.

BPDUs contain information about the sending switch and its ports. The information includes switch and port MAC addresses, switch priority, port priority, and path cost. The STP uses this information to elect the Root Switch and the root port for the switched network, and the root port and the designated port for each switched segment.

VLAN is a network of computers that behave as if they are connected to the same wire even though they are physically located on different segments of a LAN. VLAN’s are configured through software rather than hardware, which make them extremely flexible. One of the biggest advantages of VLAN’s is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration.