ch6.pdf

E1-E2 Consumer Mobility RSTP MSTP PVSTP PVRSTP 6 RSTP, MSTP, PVSTP, PVRSTP 6.1 LEARNING OBJECTIVE This chapter will make you understand about concepts of various forms of spanning tree protocols which can be implemented in...

E1-E2 Consumer Mobility RSTP MSTP PVSTP PVRSTP 6 RSTP, MSTP, PVSTP, PVRSTP 6.1 LEARNING OBJECTIVE This chapter will make you understand about concepts of various forms of spanning tree protocols which can be implemented in a switched network. 6.2 INTRODUCTION In a local area network, containing the redundant path, if the flow of traffic is not carefully monitored and controlled, the data can be caught in a loop that circles around network segments, affecting performance and bringing traffic to a near halt. If STP is enabled in the LAN bridges or switches (when used in the context of STP, the term bridge can also refer to a network switch), it will monitor all network links, identifies redundant connections and disables the ports that can lead to looping. The original spanning tree protocol and algorithm were invented in 1985 by Radia Perlman when she was working at Digital Equipment Corporation.Spanning tree protocols were later standardized by the Institute of Electrical and Electronics Engineers (IEEE) as IEEE 802.1D. Since then, the protocol has evolved in a number of ways, and new variations have been introduced. Spanning trees uses an algorithm to search for the redundant links in the LAN and select the best paths. It is mainly used to put all links in either forwarding or blocking. When the best path is selected, all the links without a redundant link is likely to be in the forwarding state. The redundant links that were not as good as the selected links would be in blocking state. Spanning Tree never uses multiple links to the same destination i.e. there is no load-sharing feature with Spanning Tree protocol. In a network that contains redundant paths, bridges need to continually understand the topology of the network to control the flow of traffic and prevent looping. To do this, they exchange bridge protocol data units (BPDUs) via an extended LAN that uses a spanning tree protocol. BPDUs are data messages that provide the bridges with network information that's used to carry out STP operations. 6.3 STP PORT STATES When STP is enabled on a network bridge, each port is set to one of five states to control frame forwarding: 1. Disabled. The ports are totally disabled i.e. ports does not participate in frame forwarding or STP operations. 2. Blocking. The port does not participate in frame forwarding and discards frames received from the attached network segment. However, the port continues to listen for and process BPDUs. 3. Listening. From the blocking state, the port transitions to the listening state. The port discards frames from the attached network segment or forwarded from another port. However, it receives BPDUs and redirects them to the switch module for processing. E1-E2 Version 3.0 April 2021 Page 55 of 103 For Restricted Circulation E1-E2 Consumer Mobility RSTP MSTP PVSTP PVRSTP 4. Learning. The port moves from the listening state to the learning state. It listens for and processes BPDUs but discards frames from the attached network segment or forwarded from another port. It also starts updating the address table with the information it's learned. In addition, it processes user frames but does not forward those frames. 5. Forwarding. The port moves from the learning state to the forwarding state and starts forwarding frames across the network segments. This includes frames from the attached network segment and those forwarded from another port. The port also continues to receive and process BPDUs, and the address table continues to be updated. STP moves from the blocking state through the forwarding state in relatively short order, usually between 15 to 20 seconds for each state. Every port starts in the blocking state. If it's been disabled, the port enters directly into the blocking state upon being enabled. STP balances the states across ports to avoid bridge looping, while still making redundancy possible. 6.4 RSTP Rapid Spanning Tree Protocol (IEEE 802.1D ) is a network protocol that is an improvement over Spanning Tree Protocol that provides high availability of network and loop-free topology within Ethernet networks. A primary advantage to RSTP networks is that they offer high availability. When a network failure does occur, devices are able to continue communicating across the network as data can be rerouted around the failure. Critical systems depend on a high level of resiliency to faults and hardware failures and RSTP provides a key improvement over traditional network architectures by minimizing downtime. RSTP prevents network loops when using multiple switches by blocking redundant paths on a network. In essence, the protocol is a set of rules by which switches on the network determine the most efficient way to send broadcasts across the network by establishing a root bridge and blocking specific ports with the purpose of preventing network loops. The RSTP algorithm follows these general steps: 1. Determine the root bridge: The switch with the lowest bridge priority will be the root bridge. If there is a tie, the switch with the lowest MAC address is selected. 2. All interfaces on the root bridge are placed in a Forwarding state: In a forwarding state, the port will send and receive data and learn MAC addresses on the network. 3. All other switches on the network (non-root) select a root port: The root port is the best path to the root bridge based on its port cost. The lower the port speed, the higher the cost. Therefore, the fastest port is chosen as the root port based on its low port cost. There is only one root port per non-root switch. 4. Designated ports are selected: These ports are permitted to forward traffic and are also selected based on port cost. All root bridge ports are designated ports. 5. All other ports are in a discarding (blocking) state: These ports are not passing data to other switches on the network and not updating MAC address tables. E1-E2 Version 3.0 April 2021 Page 56 of 103 For Restricted Circulation E1-E2 Consumer Mobility RSTP MSTP PVSTP PVRSTP 6.5 RSTP PORT ROLES The port that receives Bridge Protocol Data Units (BPDUs) on Root Port the root bridge. These are messages that are exchanged between switches on the network. The port selected in each local area network (LAN) segment Designated Port which provides the lowest root path cost. The higher the link speed, the lower the cost value. If a port is not to be used as a designated port, it will become Alternate Port an alternate port. This is a backup for the root port in case of failure and is blocked during typical operation of the root port. This is the backup for the designated port. If the root port fails, the backup port becomes the new designated port. The backup Backup Port port is normally blocked with the designated port is operating correctly. 6.6 RSTP STATES RSTP can be described by three port states: Discarding, Learning, and Forwarding. The Disabled, Blocking, and Listening states described by STP have been combined into the Discarding state in RSTP. The functionality is similar. In RSTP, the Alternate port and Backup port are allowed to directly enter the forwarding state which allows the convergence time of the network to be reduced significantly. In STP, the port must wait for the network to converge (40-50 seconds) before entering the forwarding state. This is a major advantage of RSTP over STP. STP RSTP Description Disabled The port only listens for BDPUs. There is Blocking Discarding no forwarding, processing, or learning of MAC addresses. Listening The port sends and receives BPDUs and Learning Learning also learns MAC addresses. Forwarding does not occur in this state. The port can send and receive data, learn Forwarding Forwarding MAC addresses, and forward data to its destination 6.7 MSTP MSTP protocol creates multiple spanning trees (instances) for each Virtual LAN (VLAN) on a single physical network. This allows for each VLAN to have a configured root bridge and forwarding topology.RSTP provides rapid convergence of the spanning tree. MSTP, which uses RSTP to provide rapid convergence, enables VLANs to be grouped into a spanning-tree instance, provides for multiple forwarding paths for data traffic, and enables load balancing. MSTP gives way better scaling as bridges have less computations to perform. MSTP permits numerous VLANs to share STP instances. MSTP is standardized by the IEEE within the 802.1Q-2014 report. E1-E2 Version 3.0 April 2021 Page 57 of 103 For Restricted Circulation E1-E2 Consumer Mobility RSTP MSTP PVSTP PVRSTP 6.8 PVSTP Per-VLAN Spanning Tree (PVST) is a CISCO proprietary version of STP and maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other VLANs on another trunk without causing a Spanning Tree loop. 6.9 PVRSTP PVRSTP is based on the IEEE 8012.1w standard. It supports fast convergence IEEE 802.1D. PVRSTP is compatible with IEEE 802.1D spanning tree. PVRSTP sends BPDUs on all ports, instead of only the root bridge sending BPDUs, and supports the discarding, learning, and forwarding states.When the mode is changed to PVRSTP, version 0 STP BPDUs are no longer transmitted and version 2 PVRSTP BPDUs that carry per-VLAN information are transmitted on the VLANs enabled for spanning-tree. If a version 0 BPDU is seen, PVRSTP reverts to sending version 0 BPDUs.Per VLAN Rapid Spanning Tree Protocol (PVRSTP) embeds support for PVSTP FastBackbone and FastUplink. There is no provision to enable or disable these features in PVRSTP 6.10 CONCLUSIONS Spanning tree protocols help in preventing the loop in a switched network containing the redundant links. In STP, the port must wait for the network to converge (40-50 seconds) before entering the forwarding state. In RSTP, the Alternate port and Backup port are allowed to directly enter the forwarding state which allows the convergence time of the network to be reduced significantly. MSTP protocol creates multiple spanning trees (instances) for each Virtual LAN (VLAN) on a single physical network. This allows for each VLAN to have a configured root bridge and forwarding topology. Per-VLAN Spanning Tree (PVST) is a CISCO proprietary version of STP and maintains a spanning tree instance for each VLAN configured in the network. E1-E2 Version 3.0 April 2021 Page 58 of 103 For Restricted Circulation