# Creating a Copper Ring with RSTP   - **Language:** EN - **Published on:** 2017-10-06 Inhand Networks ISM series managed switches allow for the creation of a fiber ring using the RSTP routing protocol very simply. *Source: Wikipedia.com* In 2001, the IEEE introduced the Rapid Spanning Tree Protocol (RSTP) as 802.1w. RSTP provides significantly faster tree convergence after a topology change by introducing new convergence behaviors and introducing port roles to do so. RSTP was designed to be compatible with the STP standard. While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 times the Hello times (default: 3 times 2 seconds) or in a few milliseconds for a physical link failure. The Hello time is an important and configurable time interval used by RSTP for several purposes; the default value is 2 seconds. ### **Rapid Spanning Tree Operation** RSTP adds new bridge port roles to accelerate convergence after a link failure. The number of states a port can be in has been reduced to three instead of the original five of STP. *RSTP Bridge Port Roles* *Root* - a forwarding port that is the best port from the non-root bridge to the root bridge *Designated* - a forwarding port for each LAN segment *Alternate* - An alternative path to the root bridge. This path is different from using the root port *Backup* - a backup / redundancy path to a segment where another port of the bridge is already connected *Disabled* - Not strictly part of STP, a network administrator can manually disable a port *RSTP Switch Port States* *Discard* - No user data is sent through the port *Learning* - The port is not yet forwarding frames, but is populating the MAC address table *Forward* - The port is fully operational *RSTP Operational Details* - Root switch error detection is performed in 3 Hello times, i.e., 6 seconds if the default hello times have not been changed. - Ports can be configured as *edge* ports if they are connected to a LAN with no other bridges connected. These edge ports transition directly to the forwarding state. RSTP continues to monitor the port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detect edge ports. As soon as the bridge detects a BPDU arriving at an edge port, the port becomes a non-edge port. - RSTP calls the connection between two or more switches a "*link-type*". A port operating in full-duplex mode is assumed to be a point-to-point link, while a half-duplex port (via a hub) is considered a shared port by default. This automatic link type setting can be overridden by explicit configuration. RSTP improves convergence on point-to-point links by reducing the Max-Age time to 3 times the Hello interval, removing the STP listening state, and exchanging a handshake between two switches to quickly transition the port to the forwarding state. RSTP does nothing different from STP on shared links. - Unlike STP, RSTP will respond to BPDUs sent from the root bridge direction. an RSTP bridge will "propose" its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines that this is the primary root information, it sets all other ports to discard. The bridge can send an "agreement" to the first bridge confirming its superior information. The first bridge, upon receiving this agreement, knows that it can quickly transition that port to the forwarding state, bypassing the traditional listening/learning transition. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the main elements that allows RSTP to achieve faster conversion times than STP. - As discussed in the port role details earlier, RSTP maintains backup details related to the discard state of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received in the root port within a determined interval. - RSTP reverts to legacy STP on an interface if an older version of an STP BPDU is detected on that port.  The connection topology between the Inhand switches is that shown in the image above (ISM = managed switch that manages the ring / ISE = unmanaged switch). The configuration of the ISM-2008D switch is as follows: ![Image](https://shop2.marcomweb.it/storage/blocks/images/RvPh1v80kQlV95YZoUehIF62DKEqNtz8L690uycs.png) In this configuration, all ports are excluded from the RSTP protocol except the two that need to form the ring. For these, Point-to-Point mode is explicitly activated. Once this configuration is made, the connection status can be monitored from the following menu: ![Image](https://shop2.marcomweb.it/storage/blocks/images/pGdutjs6H5LYwDV9ym4k3ULMrYBoUCJWNfUgn2RL.png) As can be seen, the first port has a *Designated *role and forwards packets, while the second has a *Backup* role and discards packets. If the ring connection is lost, i.e., any cable in the ring is interrupted, the port configuration changes: ![Image](https://shop2.marcomweb.it/storage/blocks/images/xMbqhTqcqnRVlquAhsWNQ5X1sgHw0dTv6SBxGcyc.png) Both ports in this case have a *Designated* role and forward packets. 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