Switching Basics and Intermediate Routing CCNA 3 Chapter 7

Spanning Tree Protocol Introduction Redundancy is desirable in a network –Helps minimize network downtime –Downside: increased likelihood of Layer 2 or Layer 3 loops Spanning Tree Protocol (STP) was invented to address issues caused by physical redundancy in a switched topology –Two major solutions: IEEE 802.1d: original standard, five states IEEE 802.1w: enhancements, becoming the standard

Redundant Topologies Introduction Redundancy is critical in a network –Allows a network to be fault tolerant –A network without redundancy can suffer downtime from the failure of a single link, port, or device –Goal is to balance the cost of redundancy with the need for network availability Switched networks have some drawbacks: –Broadcast storms –Multiple frame transmissions –MAC address database instability

Redundant Topologies Introduction Switched networks have benefits: –Smaller collision domains –Microsegmentation –Full duplex operation –Better network performance Redundancy protects against lost connectivity because of a failed individual component –Can result in physical topologies with loops –Physical layer loops can cause serious problems in switched networks

Redundant Topologies Redundancy If the network is down, productivity and customer satisfaction decline Companies require continuous network availability, or uptime –100% uptime is nearly impossible –“Five nines” uptime (99.999%) is the goal of many organizations –Means one hour of downtime for every 4000 days (5.25 minutes of downtime a year)

Redundant Topologies Redundancy Network reliability is achieved through reliable equipment and network designs that are tolerant to failures and faults –Networks should reconverge rapidly to bypass the fault Goal of redundant topologies is to eliminate outages caused by a single point of failure

Redundant Topologies Redundant Switched Topologies Problems that can occur with redundant links and devices in switched or bridged networks: –Broadcast storms: without a loop-avoidance process in place, each switch or bridge broadcasts endlessly –Multiple frame transmission: multiple copies of unicast frames can be delivered to destination stations; can cause unrecoverable errors –MAC address instability: results from copies of the same frame being received on different ports of the switch; data forwarding can be impaired

Redundant Topologies Redundant Switched Topologies A Redundant Switched Topology Can Be a Source of Layer 2 Problems

Redundant Topologies Redundant Switched Topologies Layer 2 LAN protocols, such as Ethernet, lack a mechanism to recognize and eliminate endlessly looping frames –Some Layer 3 protocols utilize a Time to Live (TTL) mechanism that limits how many times a packet can be retransmitted by a Layer 3 networking device –Layer 2 devices lack such a capability, so a loop-avoidance mechanism is required

Redundant Topologies Broadcast Storms Broadcasts and multicasts can cause problems in a switched network –Without specialized switch configurations, switches treat multicasts the same as broadcasts –Broadcast and multicast frames are flooded out all ports except the one on which the frame was received –Broadcast storms are not as prevalent due to the move to Layer 3 switching

Redundant Topologies Broadcast Storms Broadcast Storm

Redundant Topologies Broadcast Storms How a broadcast storm can occur in the previous slide: –Host X sends a broadcast frame, such as an ARP; Switch A receives the frame –Switch A examines the Destination Address field in the frame and determines the frame must be flooded to segment 2 –When the copy of the frame arrives at Switch B, the process repeats and a copy of the frame is transmitted to the Ethernet, segment 1 near Switch B –Because the original copy of the frame also arrives at Switch B via the top Ethernet, the frames travel around the loop in both directions, even after the destination has received a copy of the frame

Redundant Topologies Broadcast Storms A broadcast storm can disrupt normal traffic flow –Every device on the switched or bridged network must process the frames because they are broadcasts Takes CPU cycles –A loop-avoidance mechanism (spanning tree) eliminates this problem by preventing one of the four interfaces from transmitting frames during normal operation, thus breaking the loop

Redundant Topologies Multiple Frame Transmissions Multiple copies of the same frame can arrive at the intended host –Can cause problems with the receiving protocol as most protocols do not cope with or recognize duplicate transmissions Protocols that use a sequence numbering mechanism assume that many transmissions have failed and that the protocol is recycling numbers Other protocols attempt to hand the duplicate transmission to the appropriate upper-layer protocol, with unpredictable results

Redundant Topologies Multiple Frame Transmissions Multiple Frame Transmissions Can Occur in a Redundant Switched Network

Redundant Topologies Multiple Frame Transmissions How multiple copies of frames can arrive at the intended host in previous slide: –Host X sends a unicast frame to Router Y; one copy is received over Ethernet segment 1; at the same time Switch A receives a copy of the frame –Switch A examines the Destination Address field in the frame, finds no entry in its table, and floods the frame –Switch B receives the frame and forwards it to segment 1 if the table has no entry for Router Y –Router Y receives a second copy of the frame

Redundant Topologies MAC Database Instability MAC database instability results when multiple copies of a frame arrive on different ports of a switch Depending on the internal architecture of the switch, it might or might not cope well with rapid changes in its MAC database STP eliminates this problem by preventing one of the interfaces from transmitting frames during normal operation

Redundant Topologies MAC Database Instability MAC Database Instability Can Also Occur in Redundant Switched Networks

Spanning Tree Protocol STP Background Spanning Tree Protocol (STP) was originally developed by Digital Equipment Corporation –The IEEE 802 committee revised the DEC spanning- tree algorithm in the IEEE 802.1d specification IEEE 802.1d is used by Cisco switches STP is enabled by default on Catalyst switches –Purpose of STP is to maintain a loop-free network topology STP continually probes the network so in can detect the addition or failure of a link

Spanning Tree Protocol STP Background STP Intelligently Blocks Selected Ports to Logically Solve Problems That Physical Loops Cause

Spanning Tree Protocol Spanning Tree Operation Convergence in STP is a state in which all switch and bridge ports have transitioned into a forwarding or blocking state –Necessary for normal network operations –Amount of time for convergence is a key issue; fast convergence time is desirable –30 to 50 seconds with IEEE 802.1d STP uses two key concepts when converging a loop-free logical topology –Bridge ID –Path cost

Spanning Tree Protocol Spanning Tree Operation Spanning-tree path cost: based on cumulative link costs –Link costs are based on the speed of the link Spanning-Tree Path Costs for the Revised and Previous IEEE Specification

Spanning Tree Protocol Spanning Tree Operation Various Spanning-Tree Parameters Include Designated Ports, Nondesignated Ports, and Root Ports

Spanning Tree Protocol Spanning Tree Operation STP performs three steps when it initially converges on a logically loop-free topology: –Elects one root bridge: on the root bridge, all ports are designated ports that are normally in the forwarding state that can send and receive traffic –Selects the root port on the nonroot bridge: STP establishes one root port on the nonroot bridge (any bridge that is not the root bridge) Root ports are normally in the forwarding state

Spanning Tree Protocol Spanning Tree Operation STP performs three steps when it initially converges on a logically loop-free topology (continued): –Selects the designated port on each segment: only one designated port is selected on each segment The designated port has the lowest-cost path to the root bridge Designated ports are normally in the forwarding state Nondesignated ports are normally in the blocking state to logically break the loop topology

Spanning Tree Protocol Spanning Tree Operation As a result, for every switched network, these elements exist: –One root bridge per network –One root port per nonroot bridge –One designated port per segment –Unused, or nondesignated ports Root ports and designated ports are used for forwarding data traffic Nondesignated ports discard all data traffic and are called blocking or discarding ports

Spanning Tree Protocol Selecting the Root Bridge The root bridge is the bridge with the lowest bridge ID –The bridge ID (BID) includes the priority and MAC address of the bridge –Switches and bridges that run the spanning- tree algorithm exchange configuration messages every 2 seconds by default –They use a multicast frame called the bridge protocol data unit (BPDU)

Spanning Tree Protocol Selecting the Root Bridge Bridge ID Determines the Root Bridge

Spanning Tree Protocol Selecting the Root Bridge Each bridge must have a unique BID assigned –The default in IEEE 802.1d is 32,768 Binary ; hex 0x8000 Is the midrange value The root bridge is the bridge with the lowest BID; it is a combination of bridge priority and MAC address values –Setting the switch priority smaller makes the BID smaller

Spanning Tree Protocol Selecting the Root Bridge Root Bridge Selection Relies on BPDUs

Spanning Tree Protocol Spanning Tree Port States With STP, ports transition through four states at power- up: –Blocking –Listening –Learning –Forwarding Ports then stabilize to forwarding or blocking states Forwarding ports provide the lowest cost path to the root bridge During a topology change, ports temporarily go through listening and learning states

Spanning Tree Protocol Spanning Tree Port States STP Flow Chart

Spanning Tree Protocol Spanning Tree Port States Initially, all bridge ports start in the blocking state, listening for BPDUs –When a bridge first boots up, it thinks it is the root bridge, so it transitions to the listening state –An absence of BPDUs for a certain period of time is called the max_age Default setting of 20 seconds –If a port is in the blocking state and does not receive a BPDU within the max_age, it transitions from the blocking state to the listening state –When in the listening state, it can determine the active topology

Spanning Tree Protocol Spanning Tree Port States During the listening state, no user data is passed through the switch port –The bridge selects the root bridge –The bridge selects the root ports on the nonroot bridges –The bridge selects designated ports on each segment The time it takes for a port to transition from listening to learning or learning to forwarding is called the forward delay; has a default value of 15 seconds

Spanning Tree Protocol Spanning Tree Port States The learning state reduces the amount of flooding required when data forwarding begins –If a port is still a designated or root port at the end of the learning state, the port transitions to the forwarding state It can send and receive user data –Ports that are not designated or root ports transition back to the blocking state

Spanning Tree Protocol Spanning Tree Port States A port normally transitions from the learning state to the forwarding state in 30 to 50 seconds If a Cisco switch port is connected only to end-user stations (not to another switch or bridge), a feature called PortFast can be enabled –Automatically transitions from blocking to forwarding

Spanning Tree Protocol Spanning Tree Port States Nondesignated Ports Are Blocking and Others Are Forwarding

Spanning Tree Protocol Spanning Tree Port States Spanning-Tree Operation with Three Switches

Spanning Tree Protocol Spanning-Tree Recalculation When a network topology changes, switches must recompute STP –Disrupts user traffic A switched network has converged when all switch and bridge ports are in either forwarding or blocking states –Forwarding ports send and receive data traffic and BPDUs –Blocking ports receive only BPDUs

Spanning Tree Protocol Spanning-Tree Recalculation STP Has Converged

Spanning Tree Protocol Spanning-Tree Recalculation Port 1/2 Fails, Resulting in STP Recalculation

Spanning Tree Protocol Spanning-Tree Recalculation STP Reconverges

Spanning Tree Protocol Rapid Spanning-Tree Protocol Rapid Spanning Tree Protocol (RSTP) significantly reduces the time to reconverge the active topology when physical or configuration changes occur –Defines additional port RSTP port roles Alternate Backup –Defines port states as discarding, learning, or forwarding

Spanning Tree Protocol Rapid Spanning-Tree Protocol RSTP Defines Five Port Roles (Backup Not Shown)

Spanning Tree Protocol Rapid Spanning-Tree Protocol RSTP provides rapid connectivity following the failure of a switch, a switch port, or a LAN –A new root port and the designated port on the other side of the bridge transition to forwarding through an explicit handshake –RSTP allows switch port configuration so that the ports can transition to forwarding directly when the switch reinitializes

Spanning Tree Protocol Rapid Spanning-Tree Protocol RSTP (IEEE 802.1w) supercedes STP while remaining compatible with STP RSTP port roles: –Root: a forwarding port elected for the spanning tree topology –Designated: a forwarding port elected on every LAN segment –Alternate: an alternate path to the root bridge –Backup: a backup path that provides a redundant but less desirable path –Disabled: a port with no role in spanning tree

Spanning Tree Protocol Rapid Spanning-Tree Protocol RSTP has a different set of port states –The RSTP port state controls the forwarding and learning processes and provides the values of discarding, learning and forwarding RSTP Port States

Spanning Tree Protocol Rapid Spanning-Tree Protocol In a stable topology, RSTP ensures that every root port and designated port transitions to forwarding –All alternate and backup ports are always in the discarding state STP waits passively for topology changes to occur; RSTP actively confirms a port can transition safely without relying on a timer configuration, uses edge ports and point-to-point links –Results in faster convergence

Spanning Tree Protocol Rapid Spanning-Tree Protocol RSTP Incorporates the Concepts of Edge Ports and Point-to-Point Links

Spanning Tree Protocol Rapid Spanning-Tree Protocol With edge ports, no ports directly connected to end stations can create bridging loops –Edge ports go directly to forwarding, skipping listening and learning states RSTP can achieve rapid transition to forwarding only on edge ports, new root ports and point-to-point links: –Edge ports: immediately transitions to forwarding, same as a PortFast port –Root ports: if RSTP elects a new root port, it blocks the old one and transitions the new one to forwarding –Point-to-point links: if one port connects to another through a p-to-p link and it becomes a designated port, a rapid transition is negotiated with the other port

Spanning Tree Protocol Rapid Spanning-Tree Protocol The link-type variable is automatically derived from the duplex mode of the port –A port operating in full-duplex mode is point- to-point –A port operating in half-duplex mode is considered shared by default –The automatic link-type setting can be overridden with an explicit configuration

Spanning Tree Protocol Summary Redundancy is the duplication of components that allows continued functionality despite the failure of an individual component –In a network, this means having a backup method to connect all devices –Network downtime is decreased because single points of failure are reduced or eliminated

Spanning Tree Protocol Summary A redundant switched topology might cause: –Broadcast storms Caused by multiple hosts sending and receiving broadcast messages Network appears to be down or extremely slow –Multiple frame transmission A router receives multiple copies of a frame from multiple switches because of an unknown MAC address –MAC address table instability If a switch incorrectly learns the MAC address of a device on a port, it can cause a loop situation

Spanning Tree Protocol Summary Switches operate at OSI Layer 2 –Decisions are made at this level –No TTL value is decremented Physical network topologies need switching or bridging loops to provide reliability, but a switched network cannot have loops –Solution: allow physical loops but create a loop-free logical topology

Spanning Tree Protocol Summary The loop-free topology is called a spanning tree –Star or extended star that spans the network –All devices are reachable –The algorithm that creates the loop-free logical topology is the spanning-tree algorithm STP establishes a root node, called the root bridge

Spanning Tree Protocol Summary STP constructs a topology that has one node for every device on the network –Results in a tree that originates from the root bridge –Redundant links that are not part of the shortest path tree are blocked –A loop-free logical topology is possible because certain paths are blocked –Data frames received on blocked links are dropped

Spanning Tree Protocol Summary Switches send messages called bridge protocol data units (BPDUs) –Allow a loop-free logical topology to be formed –Blocked ports continue to receive BPDUs –BPDUs contain information that allows switches to: Select a single switch that will act as the root Calculate the shortest path to the root switch Designate one of the switches as the designated switch Choose one of its ports as the root port, for each nonroot switch Select the ports (designated ports) that are part of the spanning tree

Spanning Tree Protocol Summary The IEEE 802.1w standard defines RSTP –Clarifies port states and roles –Defines a set of link types –Allows switches in a converged network to generate BPDUs rather than use the root bridge’s BPDUs –The STP blocking state of a port is renamed as the discarding state –The role of a discarding port is that of an alternate port –The discarding port can become the designated port if the designated port of the segment fails