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Stp premdeep gill-lu045310-14aug12
The document outlines the Spanning Tree Protocol (STP) in Cisco networks, detailing its purpose as a loop avoidance mechanism in layer 2 switched networks. It covers essential concepts such as root bridges, port states, timers, and STP versions, explaining how STP works by blocking redundant links to maintain a loop-free topology. The report emphasizes the importance of STP in preventing issues like broadcast storms and MAC table thrashing through the effective management of network topology.
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Stp premdeep gill-lu045310-14aug12
- 1. Spanning Tree Protocol ImplementingCisco Networks
- 2. Outline Introduction WhyUse STP? Spanning Tree Terms Spanning Tree Operation (STA) STP Port States STP Timers STP Versions CCNA Project Report 2
- 3. Introduction Layer 2Bridge-to-Bridge Protocol Loop Avoidance Mechanism Developed by Radia Perlman Defined in IEEE 802.1d standard Enabled by default on Cisco Catalyst switches Uses the Spanning Tree Algorithm Creates Loop-free Logical Topology CCNA Project Report 3
- 4. Why Use STP? TheNeed for STP In a layer 2 switched network, while multiple links between devices 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 storms • Multiple copies of Ethernet frame • Thrashing of the MAC table STP prevents physical networking loops from occurring by identifying the redundant links in the network, and blocking some of them to create a loop-free logical topology. CCNA Project Report 4
- 5. Spanning Tree Terms Root Bridge – the focal point of the network that influences all STP decisions Root Port – the port (on a nonroot bridge) used to reach the root bridge Designated Port – the port with the least advertised path cost to the root bridge CCNA Project Report 5
- 6. Spanning Tree Terms(contd.) Bridge ID – Priority + MAC Address, used to identify each switch in the network, default priority = 32,768 BPDU – Bridge Protocol Data Unit, used by switches to exchange STP information among themselves Path/Port Cost – determined by the bandwidth of a link CCNA Project Report 6
- 7. Spanning Tree Operation(STA) Step 1: Electing a Root Bridge • Bridge Priority, Bridge ID, Root Bridge Step 2: Electing Root Ports • Path/Port Cost, Root Path Cost, Root Port Step 3: Electing Designated Ports • Path/Port Cost, Root Path Cost CCNA Project Report 8
- 8. STP Port States Disabled – Administratively down Blocking – Receives BPDUs only Listening – Building active topology Learning – Building bridging table Forwarding – Building MAC table, sending/receiving user data CCNA Project Report 9
- 9. STP Timers HelloTime – 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 10
- 10. STP Versions SpanningTree Protocol (IEEE 802.1d) Per VLAN Spanning Tree (PVST+) Rapid Spanning Tree Protocol, or RSTP (IEEE 802.1w) CCNA Project Report 11
- 11. Thank You Presented by: PremdeepS. Gill Roll# LU-0453-10 Batch# 100223 CCNA Project Report 12
Editor's Notes
- #4 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.
- #5 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.
- #6 - 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.
- #7 - 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.
- #8 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.
- #9 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 port’s 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.
- #10 Disabled – No frames forwarded, no BPDUs heard Blocking – No frames forwarded, BPDUs heard Listening – No frames forwarded, listening for frames Learning – No frames forwarded, learning addresses Forwarding – Frames forwarded, learning addresses When 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.
- #11 Max Age Timer: Time to wait before transitioning to learning state if three BPDUs are missed (default = 20 seconds)
- #12 STP 802.1d – Common Spanning Tree (CST), Mono Spanning Tree (MST) Cisco Enhancements (First Evolution) – Portfast, Uplink fast, Backbone fast Cisco Enhancements (Second Evolution) – PVST (ISL), PVST+ (ISL+802.1q), additional BPDU Guard & Root Guard, includes previous enhancements Cisco MISTP – Uses PVST+, includes previous enhancements, Catalyst 4000/6000 RSTP 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