Network Loops and Spanning Tree Protocol (STP)

Spanning-Tree Concepts

• The three-tier model(core, distribution, access) with redundancy attempts to eliminate a single point of failure in a network
• Multiple cabled paths between switches provide physical redundancy in a switched network
• Multiple cabled paths between switches improve reliability and availability of the network
• Multiple cabled paths between switches enable users to access network resources, despite path disruption

MAC Database Instability

• Ethernet frames lack a time to live(TTL) attribute
• Frames continue propagating between switches endlessly or until a link is disrupted/breaks the loop
• This results in MAC database instability, caused by broadcast frame forwarding
• Endless loops can result if there is >1 path for a frame to be fowarded
• When a loop occurs, the MAC address table on a switch can constantly change with the updates from broadcast frames, causing MAC database instability

Broadcast Storms

• A broadcast storm happens when too many broadcast frames are caught in a Layer 2 loop, consuming all available bandwidth
• Broadcast storms are also known as denial of service
• Broadcast storms are inevitable on looped networks
• As network devices send more broadcasts, traffic gets caught within the loop, consuming resources which leads to network failure

Duplicate Unicast Frames

• Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device
• Most upper-layer protocols aren't designed to recognize/cope with duplicate transmissions
• Layer 2 LAN protocols like ethernet lack mechanisms to recognize and eliminate endlessly looping frames

Spanning Tree Algorithm Introduction

• Redundancy is necessary in networks, but can lead to loops and broadcast storms
• Spanning Tree Protocol(STP) ensures there's only 1 logical path between destinations on the network by intentionally blocking redundant paths that could cause a loop
• A port is blocked when user data is prevented from entering or leaving a port, though this excludes Bridge Protocol Data Unit(BPDU) frames used by STP to prevent loops
• Physical paths exist to provide redundancy, but these paths are disabled to prevent loops
• Should failure occur, STP recalculates paths and unblocks necessary ports, activating redundant paths

Spanning Tree Algorithm: Introduction

• The first Spanning Tree Algorithm(STA) step determines which ports to "block" in order to prevent loops
• STA designates a single switch as the root bridge, against which all path calculations are referenced
• All switches in STP send BPDUs(bridge protocol data unit) to determine which switch has the lowest bridge ID (BID)
• The switch with the lowest BID automatically becomes the root bridge

Spanning Tree Algorithm: BPDUs

• A BPDU is a messaging frame exchanged between switches for STP use
• BPDUs contain a Bridge ID(BID) that identifies which switch sent the BPDU
• BID contains the priority value, MAC address of the sending switch, and an optional extended system ID
• Lowest BID is determined as a combination of these three fields

Spanning Tree Algorithm: Port Roles

• After a root bridge is selected, STA calculates the shortest path from it to all switchports in the broadcast domain
• STA considers both path and port costs when determining which path to select as best
• Port costs are determined based on the speed of the link
• Path cost is the sum of all port costs to the root bridge
• STA is used to determine which ports to block and during thids time, traffic cannot be forwarded through network

Spanning Tree Algorithm: Port Roles

• Once STA determines paths to select, it assigns port roles to the participating switch ports
• Port roles describe the relation in a network and whether ports can forward traffic
• Root ports are the switch ports closest to the root bridge in terms of path cost
• Designated ports include all non-root ports still allowed to forward traffic
• The other end of a root port is always a designated port and all ports on the root bridge are designated ports
• Alternate and Backup ports are configured in a blocking state to prevent loops
• Alternate and Backup ports are selected on links where there's no root port with only one end is blocked, allowing faster transition to forwarding when needed
• The original STP used "non-designated" instead of alternate, but alternate is a newer naming convention used by RSTP.

Spanning Tree Algorithm: Root Bridge

• Every STP instance elects one root bridge
• All switches the broadcast domain participate in the election
• After a switch boots, it broadcasts BPDUs every 2 seconds which contain switch BID and the root ID(BID of the root bridge)
• In the beginning, all switches assume they are root bridge
• If the RID from a received BPDU is lower than the RID on the current switch, the switch updates its RID

Spanning Tree Algorithm: Path Cost

• After the root bridge has been selected, the STA needs to determine best path to the root bridge from each destination in the broadcast domain
• Path costs are determined by summing up the individual port costs along the path from the destination to root bridge
• Individual port costs have default values and these values change over time as faster technologies become available

Spanning Tree Algorithm: Root Ports

• Select the port with the lowest overall path cost to the root bridge and each switch can have only one root port
• If multiple paths with the same cost exists, select the port connected to switch advertising the lowest BID( priority, MAC, EID)
• If all paths go through the same switch, the local port receiving the lowest port ID (port priority, port number) is selected
• Customizable port priority is used first and if priority is default, the lowest sending port ID is used

Spanning Tree Algorithm: Root Ports

• The other end of a root port is always designated
• All ports on root bridge are designated
• Steps for selecting a designated port include selecting the port on the switch with the lowest accumulated path cost to root bridge.
• If there is a tie, select the port on the switch with the lowest BID

Bridge Priority

• Defaults to 32,768 and is configurable in increments of 4096
• The lowest priority determines the root bridge

Extended System ID

• Added to support separate STP instances for different VLANs
• Twelve bits are reserved for VLAN ID and the leftmost four bits are used for priority
• Priority plus extended system ID are added together to identify the VLAN; VLAN 1 priority would be 32768+1

MAC Address

• Used as tiebreaker to determine root bridge

List of Spanning Tree Protocols

• STP/IEEE 802.1D-1998 provides original iteration with one STP instance for the whole network
• PVST+ is a Cisco enhancement that provides separate instances for each VLAN
• IEEE 802.1D-2004 is an enhanced version of STP incorporating 802.1w
• Rapid Spanning Tree Protocol(RSTP) or IEEE 802.1w is an evolution of STP providing faster convergence
• Rapid PVST+ is a Cisco enhancement of 802.1w providing a separate instance of RSTP per VLAN
• Multiple Spanning Tree Protocol(MSTP)/IEEE 802.1s is an IEEE standard that maps multiple VLANS to one STP instance

PVST+

• Network can run an independent IEEE 802.1D STP instance for each VLAN
• It's possible to load balance traffic at layer 2 by blocking one trunk port for one VLAN while allowing it for another
• One spanning-tree instance for each VLAN maintained can waste CPU cycles for all switches in the network

PVST+ Operation

• Switch port transitions through five states to learn about the entire STP topology.
• This ensures no loops

Repairing STP Problems

• One way to correct Spanning Tree problems is to manually remove redundant links in the switched network either physically or through configuration, eliminating all loops from the topology.
• Before restoring the redundant links, determine and correct the cause of spanning-tree failure
• Ensure the problem is fixed by carefully monitoring the network

This lesson covers the causes and effects of network loops, including broadcast storms and duplicate unicast frames. It explains the primary function of Spanning Tree Protocol (STP) in preventing loops and maintaining network redundancy. Key concepts include STP port states, BPDU usage, and path calculation.