Spanning Tree Protocol (STP)


            Spanning Tree Protocol (STP) is described as a Layer 2 link management protocol which provides path redundancy while preventing loops in the network. In order for a Layer 2 Ethernet network to function effectively, there must be only one active path existing between any two stations (Balchunas, 2014). Many active paths at the end of the station leads to a loops in the network. As a result, the existence of a loop in the network, the end station is likely to receive duplicate messages. Also, switches is likely to learn end-station MAC addresses in the case of many Layer 2 interface. These developments makes the network to be unstable.

The purpose of the STP

            The purpose of the spanning tree protocol (STP) is to prevent the broadcast storm caused by switching loops. Initially, spanning tree protocols (STP) was defined as IEEE 802.1D. The introduction of the switching loop in the network, a destructive broadcast storm forms within the shortest time possible. The development of the storm occurs when there are an endless forwarding through the loop (Tetz, 2015). If this condition persists, the storm will eventually choke the network hence obstructing other network traffic. When the STP is deployed on the switches, it identifies the topology or map of the entire switching network. the STP detect the existence of any loops and block or disable them, hence eliminating all the loops in many ports as necessary in the entire topology.

            Studies have indicated that an eliminated port can re-emerge if another port goes down. As a result, the section purpose of the STP is to maintain redundancy and fault-tolerance. However, it is important to understand that STP only support load balancing when EtherChannel is deployed because ports are blocked in order to get rid of loops in the switching network (Wilkins, 2011). The STP switches exchange Bridge Protocol Data Units (BPDU’s) to build the topology database. BPDU’s are forwarded out all ports every two seconds, to a dedicated MAC multicast address of 0180.c200.0000.

The function of STP

            STP serve several functions in switching network. These include:

  • Detecting and preventing Layer 2 bridging loops from forming. It is important to note that parallel path can exist, but only one path is permitted to forward frames in order to eliminate the cases of choking the network. This is one of the main functions of the STP in the switching network.
  • It ensure that switching network is designed in accordance to the IEEE 802.1D bridge protocol standard.
  • The spanning tree serves as enhancement to 802.1w, hence providing more rapid convergence during topology changes than with traditional spanning tree.
  • Serves as one of the requirements for the cisco switches. Cisco switches operates STP per VLAN with PVST (Balchunas, 2014). It is also important to note that trunking is needed to run RPVST.
  • STP also function as a proprietary extension that permits switches to interoperate between CST and PVST. PVST bridge protocol data unit (BPDU) which are tunneled over an 802.1Q trunk. Catalyst switches run PVST plus by default.

All these functions are accomplished through four processes:

Root bridge election: The STP select the bridge with the lowest ID to become the root of the spanning tree. A bridge ID (BID) composed of two-byte priority and a six-byte MAC address. By default the priority ranges from 0 to 65535 (Tetz, 2015). The BID consist of two components in the original 8022.1D standard: 16-bit bridge priority and 48-bit MAC address.

Identifying root port: The root port with the lowest root path cost is identified to be connected to the root bridge. It is important to understand that each switch have one root port. The root bridge cannot serve as a root port since the root port acts as a point to the root bridge.

Identifying designated port: For each network segment a single designated port is identified. The designated port is responsible for forwarding BPDUs and frames to that segment. When two ports become eligible to the designated port, then there is a possibility of loop developing. As a result one of the ports is placed in a blocking state in order to eliminate the loop.

Bridging loops are removed: Switch ports that are neither designated ports nor root ports are placed in the blocking state, thus eliminating the bridge loop.

The benefits of STP

            The key benefits of STP are that:

  • BPDUs are generated by every switch and sent out at the hello interval
  • Switches no longer require artificial forwarding delay timer

STP configuration

            It is important to bear in mind that STP is always enable by default on all cisco switches for all ports and VLANs. For instance, PVST+ is the default STP mode on most modern Cisco switches, hence permitting each VLAN to run a separate STP (Wilkins, 2011). However, STP can be disable but it must be done with caution since it would lead to broadcast storm. The process of disabling STP in the entire VLAN involves typing the following command:

Switch (config) # no spanning-tree vlan 101

While the range of VLAN may be specified as follows:

Switch (config) # no spanning-tree vlan 1-4094

The process of disabling STP per-port basis for a specific VLAN can be done in the following manner:

Switch (config) # interface gi2/23

Switch (config) # no spanning-tree vlan 101

To configure priority on per-VLAN basis:

Switch A (config) # spanning-tree vlan 101 priority 8192

To configure root bridge for a specific VLAN:

Switch A (config) # spanning-tree vlan 101 root primary

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