CCNA3_M7_Spanning-Tree_Protocol (1130768)

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CCNA – Semester3Module 7Spanning Tree ProtocolObjectives• Redundant topologies• Spanning-Tree protocolRedundant TopologiesNetwork reliability• Many companies and organizations increasingly rely oncomputer networks for their operations.• Achieving 100% uptime is perhaps impossible butsecuring a 99.999% or five nines uptime is a goal thatorganizations set.• Reliability in networks is achieved by reliable equipmentand by designing networks that are tolerant to failuresand faults.• Fault tolerance is achieved by redundancy. Redundancymeans to be in excess or exceeding what is usual andnatural.Redundant switched topologies•••Networks with redundantpaths and devices allow formore network uptime.Redundant topologieseliminate single points offailure.A redundant switchedtopology may cause problems:– broadcast storms– multiple frame copies– MAC address table instabilityBroadcast storms••Broadcasts and multicasts can cause problems in a switchednetwork.The network will appear to be down or extremely slow.Multiple frame transmissions• In a redundant switched network it is possible for an enddevice to receive multiple frames.• This is a cause of unnecessary processing in alldevices.Media access control database instability••In a redundant switched network it is possible for switches to learnthe wrong information.A switch can incorrectly learn that a MAC address is on one port,when it is actually on a different port.Spanning-Tree ProtocolBridging loop••In the Layer 2 header there is no Time To Live (TTL).

If a frame issent into a Layer 2 looped topology of switches, it can loop forever.This wastes bandwidth and makes the network unusable.Loop free topology• The loop free logicaltopology created iscalled a tree.• This topology is a staror extended star logicaltopology, the spanningtree of the network.• It is a spanning treebecause all devices inthe network arereachable or spanned.Redundant topology and spanning tree• The algorithm used to create this loop freelogical topology is the spanning-tree algorithm.• This algorithm can take a relatively long time toconverge.• A new algorithm called the rapid spanning-treealgorithm is being introduced to reduce the timefor a network to compute a loop free logicaltopology.IEEE 802.1d• Ethernet bridges and switches can implement the IEEE802.1D Spanning-Tree Protocol and use the spanningtree algorithm to construct a loop free shortest pathnetwork.• Shortest path is based on cumulative link costs.

Linkcosts are based on the speed of the link.Spanning-Tree Protocol•••The Spanning-Tree Protocol establishes a root node, called the rootbridge.The Spanning-Tree Protocol constructs a topology that has onepath for reaching every network node.The resulting tree originates from the root bridge. Redundant linksthat are not part of the shortest path tree are blocked.BPDU• The Spanning-Tree Protocol requires network devices toexchange messages to detect bridging loops. Links thatwill cause a loop are put into a blocking state.• The message that a switch sends, allowing theformation of a loop free logical topology, is called aBridge Protocol Data Unit (BPDU). BPDUs continue to bereceived on blocked ports.• This ensures that if an active path or device fails, a newspanning tree can be calculated.BPDUBPDU functions•••••••Create one spanning tree for a networkSelect the root switch of the spanning treeCalculate the shortest path to the root switchChoose designated switch.Choose a root port, for each non-root switch.Select designated ports per segment.Non-designated ports are blocked.Spanning-TreeSelecting the root bridge• BPDUs are sent out with the Bridge ID (BID).• The BID consists of a bridge priority that defaults to32768 and the switch base MAC address.• Change the root switch by setting the switch priority to asmaller value than the default• By default BPDUs are sent every two seconds.Stages of spanning-tree port statesSTP States• Blocking - No frames forwarded, BPDUs heard, takes 20seconds• Listening - No frames forwarded, listening for frames,determine if there are any other paths to the root bridge,takes 15 seconds• Learning - No frames forwarded, learning addresses• Forwarding - Frames forwarded, learning addresses• Disabled - No frames forwarded, no BPDUs heard,shutdown or fail.Spanning-tree recalculation• A switched internetwork has converged when allthe functional switch ports are in either theforwarding or blocked state.• When the network topology changes, switchesrecompute the Spanning Tree and cause adisruption of user traffic.• Convergence on a new spanning-tree topologyusing the IEEE 802.1D standard can take up to50 seconds.Rapid Spanning-Tree Protocol• The Rapid Spanning-Tree Protocol is defined inthe IEEE 802.1w LAN standard.

The standardand protocol introduce the following:– Clarification of port states and roles– Definition of a set of link types that can go to forwardingstate rapidly– Concept of allowing switches, in a converged network, togenerate their own BPDUs rather than relaying rootbridge BPDUsRSTP States• The “blocked” state of a port has been renamedas the “discarding” state. A role of a discardingport is an “alternate port”.• The discarding port can become the“designated port” in the event of the failure ofthe designated port for the segment.RSTP Port DesignationRSTP Link Type• Link types have been defined as point-to-point,edge-type, and shared.

These changes allowfailure of links in switched network to be learnedrapidly.• Point-to-point links and edge-type links can goto the forwarding state immediately.• Network convergence does not need to be anylonger than 15 seconds with these changes.RSTP Link TypeSummary• The key elements of a redundant networkingtopology• The benefits and risks of a redundanttopology• The role of spanning tree in a redundantpath switched network• The key elements of spanning-treeoperation• The process for root bridge election• Spanning-tree states• Rapid Spanning-Tree ProtocolLab TopologyCCNA3 – Module7.

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