4.1 Purpose of STP

(C) 2016 Cisco and/or its affiliates.A Layer 2 loop canresult in MAC address table instability, link saturation, and high CPU utilization on
switches and end-devices,resulting in the network becoming unusable.Spanning Tree Protocol (STP)
is a loop-prevention network
protocol that allows for
redundancy while creating a
loop-free Layer 2 topology.Cisco Confidential
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Purpose of STP
STP Recalculation

STP compensates for a failure in
the network by recalculating and
opening up previously blocked
ports.When multiple paths exist between two devices on an Ethernet network, and there is no spanning tree
implementation on the switches, a Layer 2 loop occurs.Without STP enabled, Layer 2 loops can form, causing broadcast,
multicast and unknown unicast frames to loopendlessly.o Spanning tree is enabled, by default, on Cisco switches to prevent Layer 2
loops from occurring.STP logically blocks physical
loops in a Layer 2 network,
preventing frames from circling
the network forever.Layer 2 Ethernet does not include a mechanism to recognize and eliminate endlessly looping frames.Ethernet and Ethernet switches have no comparable mechanism for limiting the number of times a
switch retransmits a Layer 2 frame.STP is based on an algorithm that creates a loop-free topology by selecting a single
root bridge where all other switches determine a single least-cost path.Cisco Confidential
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Purpose of STP
Redundancy in Layer 2 Switched Networks
?Redundancy is an important part of the hierarchical design for eliminating single points of failure
and preventing disruption of network services to users.However, redundant paths in a switched Ethernet network maycause both physical and logical
Layer 2 loops.A loop in an Ethernet LANcan cause continued propagation of Ethernet frames until a link is
disrupted and breaks the loop.Both IPv4 and IPv6 include a mechanism that limits the number of times a Layer 3 networking device can
retransmit a packet.When a loop occurs, the MAC address table on a switch will
constantly change with the updates from the broadcast frames,
which results in MAC database instability.Broadcast storms can be caused by a hardware problem
such as a faulty NIC or from aLayer 2 loop in the network.This topic covers the causes of loops in a Layer 2 network and briefly explains how spanning tree
protocol works.Having alternate physical paths for data to traverse the network makes it possible for users to
access network resources, despite path disruption.Cisco Confidential
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Purpose of STP
Spanning Tree Protocol

?Cisco Confidential
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Purpose of STP
Issues with Redundant Switch Links
?A router will decrement the TTL (Time to Live) in every IPv4 packet, and the Hop Limit field in every
IPv6 packet.STP was developed specifically as a loop prevention mechanism forLayer 2
Ethernet.Cisco Confidential
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Purpose of STP
Layer 2 Loops
?Layer 2
multicasts are typically forwarded the same way as a broadcast by the switch.To prevent these issues from occurring in a redundant network, some type of spanning tree
must be enabled on the switches.Cisco Confidential
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Purpose of STP
The Spanning Tree Algorithm

?Redundant networks require the addition of physical paths, but logical redundancy must also
be part of the design.Ethernet LANs require a loop-free topology with a single path between any two devices.Path redundancy provides multiple network services by eliminating the possibility of a single point of
failure.When these fields are decremented to 0, a router will drop the packet.This can cause high CPU utilization, which makes the switch
unable to forward frames.An unknown unicast frame is when the switch does not have the
destination MAC address in its MAC address table and must
forward the frame out all ports, except the ingress port.Cisco Confidential
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Purpose of STP
Broadcast Storm
?A broadcast storm is an abnormally high number of broadcasts overwhelming the network
during a specific amount of time.Layer 2 broadcasts in a network, such as ARP Requests are very common.ICMPv6
Neighbor Discovery uses Layer 2 multicasts.A host caught in a Layer 2 loop is not accessible to other hosts on the network.Additionally,
due to the constant changes in its MAC address table, the switch does not know out of which
port to forward unicast frames.(C) 2016 Cisco and/or its affiliates.(C) 2016 Cisco and/or its affiliates.(C) 2016 Cisco and/or its affiliates.(C) 2016 Cisco and/or its affiliates.(C) 2016 Cisco and/or its affiliates.(C) 2016 Cisco and/or its affiliates.All rights reserved.All rights reserved.All rights reserved.All rights reserved.All rights reserved.All rights reserved.All rights reserved.???????????????This can bring down a network quickly.???????