In multi-switching environments, the drive for backup link redundancy has introduced bridging loops into the network as frames are continuously forwarded.
Frames have no bit set for time out values, resulting in switching loops also known as Packet Storms. The accumulation of these frames negatively impacts the network communications eventually leading to resource exhaustion on the switches.
In order to contain switching loops, Switches (also referred to as Bridges and often used interchangeably), employ a protocol like the Rapid Spanning Tree Protocol (RSTP). RSTP has been titled as “A New Solution from An Old Technology” (Wojdak, W., 2003) this is because RSTP as defined in the IEEE 802.1W standard, supersedes the Spanning Tree Protocol laid down in the IEEE 802.1D standard as during the development stage backward compatibility was a must (Lammle, T. and Tedder, W., 2014). The drive in developing RSTP was due to the competition of routed solutions, such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP), and their ability to provide an alternative path, measured in milliseconds as compared to STP’s convergence time of 50 seconds (Odom, W., 2004 p.48)
The Spanning Tree Algorithm (STA) places each switch port into either a forwarding or discarding state normally referred to as a blocking port, thereby reducing the risk of switching loops. The Spanning Tree Algorithm is a distributed algorithm whereby all nodes participating in the same algorithm compute the order in which messages are transmitted and in what order to process the information laid down by the protocol. The final solution is based on the information provided and in the case of the STA the link ports will transition to a discarding or forwarding state.
A logical tree topology is created on the network by RSTP and this is compiled by all switches sending BPDUs version 2 around the network every 2 seconds (Cisco, 2017). Bridge Identifiers (BID) contained as a bit in the BPDU frame are compared and the switch with the lowest mac address numerically will be elected the Root Bridge. The Root Bridge acts as a controller for RSTP, and the non-root bridges determine a single lowest cost path based on bandwidth to the Root Bridge, known as the Root Port.
All ports on a Root Bridge are assigned the Designated Port role whilst port roles between Non-Root Bridges will be either a Designated, Alternate or Backup Port. Port Roles between the Non-Root Bridge segments use the same prerequisites for the Root Bridge election.
Spanning Tree assigns port states to port roles, these states determine whether a port is in the Discarding, Learning or Forwarding state. All ports will initially transition from the discarding to the learning state but the port role dictates whether the port will be in forwarding state when belonging to a Root or Designated Port, and discarding for Alternate or Backup Ports. Frames will be transported around the network in the forwarding state unlike in the discarding state when they are dropped but BPDU’s are forwarded.
Enhancements to RSTP meant that Port Roles and States have been separated, consequently switching ports in the forwarding state are able to determine if they are ssociated to a Root or Designated Port.
Rapid convergence lies in part to a switches ability to accept inferior BPDUs from neighboring switches in the event of a link failure and refrains from transmitting its own BPDU. A blocked backup port will transition to a forwarding state, bypasses the aging timer and forwarding delay whilst ensuring a loop free topology.
The BPDU are considered as keep alive bits and a switch missing three consecutive BPDU’s (Cisco, 2017) will immediately age out the data, allowing for a quicker link failure detection and recovery.
Edge Ports and Link-Type Point-to-Point are used by the Spanning Tree Protocol for improving convergence resulting in reported times of approximately 10 milliseconds for point-to-point links and 6 seconds for the recalculation of Bridge failures (Pallos et al., 2007). Another type of Link is Link-Type Shared which supports legacy half-duplex hub devices but degrades the performance in line with 802.1d criteria.
Edge Ports bypass the discarding and learning states, transitioning directly to the forwarding state, and are defined by the fact that peripherals do not generate BPDUs. On the other side, you have Link-Type Ports which will shift to the forwarding state when a superior BPDU is received, and all other trunking ports will transition to the blocking state.
The default settings may lead to unintended roots due to many organizations using the default settings which inevitably cause the selection of sub-optimal paths, with congested and longer segments being used.
Retired switches are occasionally attached to a production network, resulting in them being elected as the root bridge as it has a lower MAC address and not adhering to the campus network design principals. The consequence is that the inferior switch is now the bottle neck and the original Root Bridge is demoted.
In designing a topology, the recommendation is to employ RSTP on the Access Layers and to implement a routing protocol for the Distribution and Core Layers. In the case of RSTP being implemented across the topology the core switches should remain the primary Root Bridges by modifying the BID, therefore avoiding bottlenecks.
The timer values, outlined in clause 8 of the 802.1W standard (IEEE, 2001) are the same for both protocols allowing RTSP to be deployed along STP. Running these two protocols in parallel is intended as an interim measure whilst migrating the switches to RSTP otherwise the switches default to STP standard.
Since the 802.1W approval by the IEEE board on 14th June 2001, RSTP has been evolving with new standards being published like the introduction of the 802.1s standard which has been designed so that traffic flowing across different vlans can use different paths and the 2012 standard 802.1aq known as Shortest Path Bridging. These enhancements are being driven to improve convergence whilst reducing human error during configuration.
In conclusion, this is an area of continual development, and a patent is now pending for Ethernet Ring Protection Switching Protocol (ERPSP) offering speeds of sub-50 m/s, and so far, deployed on Juniper switches. The patent also includes a migration process from RSTP to ERPSP (NewsRx, 2017), so might this be a possible future replacement for RSTP?
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