CCNA_Spanning Tree protocol

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Nội dung Text: CCNA_Spanning Tree protocol

CCNA – Semester 3 Chapter 5 - Spanning Tree Protocol CCNA Exploration 4.0
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. 2
Role of redundancy 3
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. 4
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. 5
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. 6
Issues with Redundancy : Duplicate Unicast Frames • Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device. 7
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. 8
The Spanning Tree Algorithm 9
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 10
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 the shortest path to the root bridge. Each switch uses the STA to determine which ports to block. Root ports : Switch ports closest to the root bridge. Designated ports : All non-root ports that are still permitted to forward 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) containing the switch BID and the root ID every 2 seconds. 2. Initially, each switch identifies itself as the root bridge after bootup. 3. If the root ID from the BPDU received is lower than the root ID on the receiving switch, the receiving switch updates its root ID identifying the adjacent switch as the root 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 summing up the individual port costs along the path from the destination to the root bridge. • The default port costs are defined by the speed at which the port operates. • Although switch ports have a default port cost associated with them, the port cost is configurable 15
Best Path to the Route Bridge (cont) • Path cost is the sum of all the port costs along the path to the root bridge. • The paths with the lowest path cost become the preferred path, and all other redundant paths are blocked. 16
Port Roles • Root Port : – The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge. • Designated Port : – For root bridges, all switch ports are designated ports. – For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed – Only one designated port is allowed per segment • Non-designated Port ; – The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses • Disabled 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 because it is closest to the root bridge. • When there are two switch ports that have the same lowest path cost to the root bridge, the switch uses the customizable port priority value, or the lowest port ID if both port priority values are the same. • The port ID is the interface ID of the switch port. 18
Port Roles (cont) example 19
Port Roles (cont) example 20