Chapter 5: Spanning Tree Protocol pdf

CCNA – Semester 3 Chapter 5 - Spanning Tree Protocol CCNA Exploration 4.0 2 Objectives • Explain the role of redundancy in a converged network • Summarize how STP works to eliminate Layer 2 loops in a converged network • Explain how the STP algorithm uses three steps to converge on a loop-free topology • Implement rapid per VLAN spanning tree (rapid PVST+) in a LAN to prevent loops between redundant switches. 3 Role of redundancy 4 Redundancy in a hierarchical network • The hierarchical design model addresses issues found in the flat model network topologies. One of the issues is redundancy. • Having multiple paths for data to traverse the network allows for a single path to be disrupted without impacting the connectivity of devices on the network. 5 Issues with Redundancy : Layer 2 Loop • When multiple paths exist between two devices on the network, a Layer 2 loop can occur. • Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. So, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly. 6 Issues with Redundancy : Broadcast Storm • A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. • As a result, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication. 7 Issues with Redundancy : Duplicate Unicast Frames • Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device. 8 Real-world redundancy issues • Network loops that are a result of accidental duplicate connections in the wiring closets are a common occurrence. • The example displays a loop that occurs if a switch is connected to two different switches on a network that are both also interconnected. The impact of this type of loop is much greater because it affects more switches directly. 9 The Spanning Tree Algorithm 10 Spanning Tree Protocol • STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop. • STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed blocking state ports • A port is considered blocked when network traffic is prevented from entering or leaving that port [...]... The disabled port is a switch port that is administratively shut down A disabled port does not function in the spanning- tree process 17 Port Roles (cont) • • • • When determining the root port on a switch, the switch compares the path costs on all switch ports participating in the spanning tree The switch port with the lowest overall path cost to the root is automatically assigned the root port role... of the spanning tree The root bridge then broadcasts the information into the whole network 31 Cisco and STP Variants 32 Cisco and STP Variants 33 PVST + • • Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network With PVST+, more than one trunk can block for a VLAN and load sharing can be implemented In a Cisco PVST+ environment, you can tune the spanningtree parameters... bridge 4 The switch then forwards new BPDU frames with the lower root ID to the other adjacent switches 5 Eventually, the switch with the lowest BID ends up being identified as the root bridge for the spanning- tree instance 12 Root Bridge & Election Process 13 Root Bridge & Election Process (cont) • BID Structure • Root Bridge 14 Best Path to the Route Bridge • • • The path information is determined by.. .Spanning Tree Algorithm (STA) • • The STA designates a single switch as the root bridge and uses it as the reference point for all path calculations After the root bridge has been determined, the STA calculates... configures its switch port to be non-designated and, therefore, in the blocking state to prevent the loop from occurring 21 Port Roles (cont) example 22 BPDU • • STP determines a root bridge for the spanning- tree instance by exchanging BPDUs BPDU Fields 23 BPDU (cont) • • By default, BPDU frames are sent every 2 seconds after a switch is booted When adjacent switches receive a BPDU frame, they compare... fields: bridge priority, extended system ID, and MAC address Each field is used during the root bridge election 25 BID (cont) 26 Port States and BPDU Timers • To facilitate the learning of the logical spanning tree, each switch port transitions through five possible port states and three BPDU timers 27 Port States and BPDU Timers (cont) • • • • • Blocking - The port is a non-designated port and does not... the MAC address table Forwarding - The port is considered part of the active topology and forwards frames and also sends and receives BPDU frames Disabled - The Layer 2 port does not participate in spanning tree and does not forward frames 28 Port States and BPDU Timers (cont) • Cisco PortFast Technology – When a switch port configured with PortFast is configured as an access port, that port transitions... traffic on the network Non-designated ports : All ports configured to be in a blocking state to prevent loops 11 Root Bridge & Election Process • The root bridge serves as a reference point for all spanning- tree calculations to determine which redundant paths to block • An election process determines which switch becomes the root bridge 1 After a switch boots, it sends out BPDU frames (more detail later) . loop is much greater because it affects more switches directly. 9 The Spanning Tree Algorithm 10 Spanning Tree Protocol • STP ensures that there is only one logical path between all destinations. CCNA – Semester 3 Chapter 5 - Spanning Tree Protocol CCNA Exploration 4.0 2 Objectives • Explain the role of redundancy. in the spanning- tree process 18 Port Roles (cont) • When determining the root port on a switch, the switch compares the path costs on all switch ports participating in the spanning tree.