Spanning Tree ProtocolImplementing Cisco Networks

CCNA Project Report

OutlineIntroductionWhy Use STP?Spanning Tree TermsSpanning Tree Operation (STA)STP Port StatesSTP TimersSTP Versions

CCNA Project Report*

CCNA Project Report

IntroductionLayer 2 Bridge-to-Bridge ProtocolLoop Avoidance MechanismDeveloped by Radia PerlmanDefined in IEEE 802.1d standardEnabled by default on Cisco Catalyst switchesUses the Spanning Tree AlgorithmCreates Loop-free Logical TopologyCCNA Project Report*

CCNA Project Report

Why Use STP?The Need for STPIn a layer 2 switched network, while multiple links betweendevices provide protection against single points of failure,they also introduce physical loops in the network.Redundant links can cause a lot of problems, including:Broadcast stormsMultiple copies of Ethernet frameThrashing of the MAC table

STP prevents physical networking loops from occurringby identifying the redundant links in the network, and blockingsome of them to create a loop-free logical topology.

CCNA Project Report*

CCNA Project Report

Spanning Tree TermsRoot Bridge the focal point of the network that influences all STP decisionsRoot Port the port (on a nonroot bridge) used to reach the root bridgeDesignated Port the port with the least advertised path cost to the root bridge

CCNA Project Report*

CCNA Project Report

Spanning Tree Terms (contd.)Bridge ID Priority + MAC Address, used to identify each switch in the network, default priority = 32,768BPDU Bridge Protocol Data Unit, used by switches to exchange STP information among themselvesPath/Port Cost determined by the bandwidth of a link

CCNA Project Report*

CCNA Project Report

BPDU Frame FormatCCNA Project Report*

CCNA Project Report

Spanning Tree Operation (STA)Step 1: Electing a Root BridgeBridge Priority, Bridge ID, Root BridgeStep 2: Electing Root PortsPath/Port Cost, Root Path Cost, Root PortStep 3: Electing Designated PortsPath/Port Cost, Root Path CostCCNA Project Report*

CCNA Project Report

STP Port StatesDisabled Administratively downBlocking Receives BPDUs onlyListening Building active topologyLearning Building bridging tableForwarding Building MAC table, sending/receiving user dataCCNA Project Report*

CCNA Project Report

STP TimersHello Time how often BPDUs are sent (default = 2 seconds)Max Age maximum time to retain the received BPDU information (default = 20 seconds)Forward Delay time to wait before transitioning from listening to learning state, and from learning to forwarding state (default = 15 seconds)CCNA Project Report*

CCNA Project Report

STP VersionsSpanning Tree Protocol (IEEE 802.1d)Per VLAN Spanning Tree (PVST+)Rapid Spanning Tree Protocol, or RSTP (IEEE 802.1w)

CCNA Project Report*

CCNA Project Report

Thank YouPresented by:Premdeep S. GillRoll# LU-0453-10Batch# 100223CCNA Project Report*

CCNA Project Report

*STP is a layer 2 protocol that is used to maintain a loop-free switched network. Its main task is to stop network loops from occurring on the layer 2 network.STP uses the Spanning Tree Algorithm (STA) to first create a topology database and then search out and disable redundant links.STP creates a tree structure of loop-free leaves and branches that spans the entire Layer 2 network.*Layer 2 broadcast frames, such as Ethernet frames, do not have a TTL (Time To Live) field like the IP header. Once an Ethernet frame starts to loop, it will probably continue until someone shuts off one of the switches or breaks a link.Switching loops can occur any time there is a redundant path in the network. The switches will flip flop the MAC table entry for a connected device, resulting in a phenomenon known as Thrashing of the MAC Table, creating extremely high CPU utilization.* - One Root Bridge Per Broadcast Domain - One Root Port Per Non-root Bridge - One Designated Port Per Segment- The switch with the lowest BID becomes the Root Bridge, which acts as the focal point of the network. All other switches in the network will be Non-root bridges, and will select a single path to the root bridge. The port with the best path (least path/port cost) to the root bridge is called the Root Port.- Bridge Identifier (BID) is used to identify each bridge/switch in the network. BID is 8 bytes long Bridge Priority (2 bytes), Base MAC Address (6 bytes). It is used in determining the center of the network, in respect to STP, known as the Root Bridge.- Designated Port has the lowest advertised cost to reach the root bridge. It is always in forwarding mode.*- Bridge Identifier (BID) is used to identify each bridge/switch in the network. BID is 8 bytes long Bridge Priority (2 bytes), Base MAC Address (6 bytes). It is used in determining the center of the network, in respect to STP, known as the Root Bridge.- Bridge Protocol Data Unit (BPDU) are layer 2 frames exchanged between switches to determine the root bridge and subsequently the root and designated ports.- STP path costs are incremented as BPDUs are received on a port, not as they are sent out a port.*If the Path Cost and Bridge IDs are equal (as in the case of parallel links), the switch goes to the Port Priority as a tiebreaker.Lowest port priority wins (all ports are set to 32).Port Priority can be set in the range 0 63.If all ports have the same priority, the port with the lowest Port Number forwards frames.

*Root Path Cost is the cumulative cost of all links to the root bridge. This is the value transmitted in the BPDU, indicated by the Cost of Path field. It is calculated by adding the receiving ports Path Cost to the value contained in the BPDU.Each segment in a bridged network has one Designated Port, chosen based on cumulative Root Path Cost to the Root Bridge. The switch containing the designated port is referred to as the Designated Bridge for that segment.All other ports, which are not Root Ports or Designated Ports, become Non-Designated Ports, which are put in blocking mode.*Disabled No frames forwarded, no BPDUs heardBlocking No frames forwarded, BPDUs heardListening No frames forwarded, listening for framesLearning No frames forwarded, learning addressesForwarding Frames forwarded, learning addressesWhen a switch is first powered on, all the ports are in blocking mode.PortFast can be used for ports connected to end devices such as PCs or servers.*Max Age Timer: Time to wait before transitioning to learning state if three BPDUs are missed (default = 20 seconds)*STP 802.1d Common Spanning Tree (CST), Mono Spanning Tree (MST)Cisco Enhancements (First Evolution) Portfast, Uplink fast, Backbone fastCisco Enhancements (Second Evolution) PVST (ISL), PVST+ (ISL+802.1q), additional BPDU Guard & Root Guard, includes previous enhancementsCisco MISTP Uses PVST+, includes previous enhancements, Catalyst 4000/6000RSTP 802.1w Edge Fast (Cisco Portfast), Uplink Fast RSTP (Cisco Uplink fast), Backbone Fast Engine (Cisco Backbone fast)MST (Multiple Spanning Tree) 802.1s, uses RSTP**