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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 5 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 3 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 4 Purpose of STP Spanning Tree Protocol

?Cisco Confidential 6 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 7 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 9 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 8 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.???????

4.1 Purpose of STP

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Purpose of STP
Redundancy in Layer 2 Switched Networks
➢ This topic covers the causes of loops in a Layer 2 network and briefly explains how spanning tree
protocol works.

➢ Redundancy is an important part of the hierarchical design for eliminating single points of failure
and preventing disruption of network services to users.

➢ Redundant networks require the addition of physical paths, but logical redundancy must also
be part of the design.

➢ Having alternate physical paths for data to traverse the network makes it possible for users to
access network resources, despite path disruption.

➢ However, redundant paths in a switched Ethernet network maycause both physical and logical
Layer 2 loops.
➢ Ethernet LANs require a loop-free topology with a single path between any two devices.

➢ A loop in an Ethernet LANcan cause continued propagation of Ethernet frames until a link is
disrupted and breaks the loop.
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Purpose of STP
Spanning Tree Protocol

➢ Spanning Tree Protocol (STP)
is a loop-prevention network
protocol that allows for
redundancy while creating a
loop-free Layer 2 topology.
➢ STP logically blocks physical
loops in a Layer 2 network,
preventing frames from circling
the network forever.

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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.

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Purpose of STP
Issues with Redundant Switch Links
➢ Path redundancy provides multiple network services by eliminating the possibility of a single point of
failure.
➢ 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.
➢ 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.
➢ Layer 2 Ethernet does not include a mechanism to recognize and eliminate endlessly looping frames.
➢ Both IPv4 and IPv6 include a mechanism that limits the number of times a Layer 3 networking device can
retransmit a packet.
➢ A router will decrement the TTL (Time to Live) in every IPv4 packet, and the Hop Limit field in every
IPv6 packet. When these fields are decremented to 0, a router will drop the packet.
➢ Ethernet and Ethernet switches have no comparable mechanism for limiting the number of times a
switch retransmits a Layer 2 frame.

➢ STP was developed specifically as a loop prevention mechanism forLayer 2
Ethernet.
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Purpose of STP
Layer 2 Loops
➢ Without STP enabled, Layer 2 loops can form, causing broadcast,
multicast and unknown unicast frames to loopendlessly.
➢ This can bring down a network quickly.
➢ 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.
➢ 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.

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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. Broadcast storms can be caused by a hardware problem
such as a faulty NIC or from aLayer 2 loop in the network.
➢ Layer 2 broadcasts in a network, such as ARP Requests are very common. Layer 2
multicasts are typically forwarded the same way as a broadcast by the switch. 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.
➢ To prevent these issues from occurring in a redundant network, some type of spanning tree
must be enabled on the switches.

• Spanning tree is enabled, by default, on Cisco switches to prevent Layer 2
loops from occurring.

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Purpose of STP
The Spanning Tree Algorithm

➢ 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.

➢ STP prevents loops from occurring by configuring a loop-free path through the
network using strategically placed "blocking-state" ports.

➢ The switches running STP are able to compensate for failures by dynamically
unblocking the previously blocked ports and permitting traffic to traverse the alternate
paths.

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Purpose of STP
The Spanning Tree Algorithm (Cont.)
How does the STA create a loop-free topology?

1. Selecting a Root Bridge: This bridge (switch) is the reference point for the entire network to build
   a spanning tree around.


2. Block Redundant Paths: 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. When a port is
   blocked, user data is prevented from entering or leaving that port.


3. Create a Loop-Free Topology: A blocked port has the effect of making that link a non-
   forwarding link between the two switches. This creates a topology where each switch has only a
   single path to the root bridge, similar to branches on a tree that connect to the root of the tree.


4. Recalculate in case of Link Failure: The physical paths still exist to provide redundancy, but
   these paths are disabled to prevent the loops from occurring. If the path is ever needed to
   compensate for a network cable or switch failure, STP recalculates the paths and unblocks the
   necessary ports to allow the redundant path to become active. STP recalculations can also
   occur any time a new switch or new inter-switch link is added to the network.

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4.2 STP Operations

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STP Operations
Steps to a Loop-Free Topology
Using the STA, STP builds a loop-free topology in a four-step process:

1. Elect the root bridge.


2. Elect the root ports.


3. Elect designated ports.


4. Elect alternate (blocked) ports.

➢ During STA and STP functions, switches use Bridge Protocol Data Units (BPDUs) to share
information about themselves and their connections.
➢ BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports.
➢ Each BPDU contains a bridge ID (BID) that identifies which switch sent the BPDU.
➢ The BID is involved in making many of the STA decisions including root bridge and port roles.
➢ The BID contains a priority value, the MAC address of the switch, and an extended system ID.
➢ The lowest BID value is determined by the combination of these three fields.

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STP Operations
Steps to a Loop-Free Topology (Cont.)
➢ Bridge Priority:
✓The default priority value for all Cisco switches is the decimal value 32768.
✓The range is 0 to 61440 in increments of 4096.
✓A lower bridge priority is preferable.
✓A bridge priority of 0 takes precedence over all other bridge priorities.
➢ Extended System ID:
✓The extended system ID value is a decimal value added to the bridge priority value
in the BID to identify the VLAN for this BPDU.
➢ MAC address:
✓When two switches are configured with the same priority and have the same
extended system ID, the switch having the MAC address with the lowest value,
expressed in hexadecimal, will have the lower BID.
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STP Operations

1. Elect the Root Bridge
   ➢ The STA designates a single switch as the root bridge
   and uses it as the reference point for all path
   calculations.
   ➢ Switches exchange BPDUs to build the loop-free
   topology beginning with selecting the root bridge.
   ➢ All switches in the broadcast domain participate in the
   election process. After a switch boots, it begins to
   send out BPDU frames every two seconds.
   ➢ These BPDU frames contain the BID of the sending
   switch and the BID of the root bridge, known as the
   Root ID.
   ➢ The switch with the lowest BID will become the root
   bridge.
   ➢ At first, all switches declare themselves as the root
   bridge with their own BID set as the Root ID.
   Eventually, the switches learn through the exchange of
   BPDUs which switch has the lowest BID and will agree
   on one root bridge.
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   STP Operations
   Impact of Default BIDs
   ➢ Because the default BID is 32768, it is possible for two or
   more switches to have the same priority. In this scenario,
   where the priorities are the same, the switch with the
   lowest MAC address will become the root bridge. The
   administrator should configure the desired root bridge
   switch with a lower priority.
   ➢ In the figure, all switches are configured with the same
   priority of 32769. Here the MAC address becomes the
   deciding factor as to which switch becomes the root
   bridge. The switch with the lowest hexadecimal MAC
   address value is the preferred root bridge. In this
   example, S2 has the lowest value for its MAC address
   and is elected as the root bridge for that spanning
   tree instance.
   ➢ Note: The priority of all the switches is 32769. The value
   is based on the 32768 default bridge priority and the
   extended system ID (VLAN 1 assignment) associated
   with each switch (32768+1).
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   STP Operations
   Determine the Root Path Cost
   ➢ When the root bridge has been elected for a given spanning tree instance, the STA starts determining the
   best paths to the root bridge from all destinations in the broadcast domain.
   ➢ The path information, known as the internal root path cost, is determined by the sum of all the
   individual port costs along the path from the switch to the root bridge.
   ➢ When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal
   root path cost.
   ➢ The default port costs are defined by the speed at which the port operates.
   ➢ The table shows the default port costs suggested by IEEE. Cisco switches by default use the values as
   defined by the IEEE 802.1D standard, also known as the short path cost, for both STP and RSTP.
   ➢ Although switch ports have a default port cost associated with them, the port cost is configurable.
   ➢ The ability to configure individual port costs gives the administrator the flexibility to manually control the
   spanning tree paths to the root bridge.

STP Cost: IEEE RSTP Cost: IEEE
Link Speed
802.1D-1998 802.1w-2004
10 Gbps 2 2,000
1 Gbps 4 20,000
100 Mbps 19 200,000
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10 Mbps 100 2,000,000
STP Operations
2. Elect the Root Ports
➢ After the root bridge has been determined, the STA
algorithm is used to select the root port.
➢ Every non-root switch will select one root port.
➢ The root port is the port closest to the root bridge in
terms of overall cost to the root bridge.
➢ Paths with the lowest cost become preferred, and all
other redundant paths are blocked.
➢ In the example, the internal root path cost from S2 to the
root bridge S1 over path 1 is 19 while the internal root
path cost over path 2 is 38. Because path 1 has a lower
overall path cost to the root bridge, it is the preferred path
and F0/1 becomes the root port on S2.

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STP Operations
3. Elect Designated Ports
➢ Every segment between two switches will have one
designated port.
➢ The designated port is a port on the segment that has the
internal root path cost to the root bridge.
➢ In other words, the designated port has the best path to
receive traffic leading to the root bridge.
➢ What is not a root port or a designated port becomes an
alternate or blocked port.
➢ All ports on the root bridge are designated ports.
➢ If one end of a segment is a root port, the other end is a
designated port.
➢ All ports attached to end devices are designated ports.
➢ On segments between two switches where neither of the
switches is the root bridge, the port on the switch with the
least-cost path to the root bridge is a designated port.

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STP Operations
4. Elect Alternate (Blocked) Ports
➢ If a port is not a root port or a designated
port, then it becomes an alternate (or
backup) port.
➢ Alternate ports are in discarding or blocking
state to prevent loops.
➢ In the figure, the STA has configured port
F0/2 on S3 in the alternate role.
➢ Port F0/2 on S3 is in the blocking state and
will not forward Ethernet frames.
➢ All other inter-switch ports are in forwarding
state.
➢ This is the loop-prevention part of STP.

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STP Operations
Elect a Root Port from Multiple Equal-Cost Paths
When a switch has multiple equal-cost paths to the root bridge, the switch will
determine a port using the following criteria:
➢ Lowest sender BID
➢ Lowest sender port priority
➢ Lowest sender port ID

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STP Operations
Elect a Root Port from Multiple Equal-Cost Paths (Cont.)
Lowest Sender BID: This topology has four switches with switch S1 as the root bridge. Port F0/1 on
switch S3 and port F0/3 on switch S4 have been selected as root ports because they have the root path
cost to the root bridge for their respective switches. S2 has two ports, F0/1 and F0/2 with equal cost paths
to the root bridge. The bridge IDs of S3 and S4, will be used to break the tie. This is known as the
sender’s BID. S3 has a BID of 32769.5555.5555.5555 and S4 has a BID of 32769.1111.1111.1111.
Because S4 has a lower BID, the F0/1 port of S2, which is the port connected to S4, will be the root
port.

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STP Operations
Elect a Root Port from Multiple Equal-Cost Paths (Cont.)
Lowest Sender Port Priority: This topology has two switches which are connected with two
equal-cost paths between them. S1 is the root bridge, so both of its ports are designated
ports.
S4 has two ports with equal-cost paths to the root bridge. Because both ports are connected
to the same switch, the sender’s BID (S1) is equal. So the first step is a tie. Next, is the
sender’s (S1) port priority. The default port priority is 128, so both ports on S1 have the same
port priority. This is also a tie. However, if either port on S1 was configured with a lower port
priority, S4 would put its adjacent port in forwarding state. The other port on S4 would be a
blocking state.

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STP Operations
Elect a Root Port from Multiple Equal-Cost Paths (Cont.)
• Lowest Sender Port ID: The last tie-breaker is the lowest sender’s port ID. Switch S4 has
received BPDUs from port F0/1 and port F0/2 on S1. The decision is based on the sender’s
port ID, not the receiver’s port ID. Because the port ID of F0/1 on S1 is lower than port F0/2,
the port F0/6 on switch S4 will be the root port. This is the port on S4 that is connected to the
F0/1 port on S1.
• Port F0/5 on S4 will become an alternate port and placed in the blocking state.

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STP Operations
STP Timers and Port States
STP convergence requires three timers, as follows:
• Hello Timer -The hello time is the interval between BPDUs. The default is 2 seconds but can be
modified to between 1 and 10 seconds.
• Forward Delay Timer -The forward delay is the time that is spent in the listening and learning
state. The default is 15 seconds but can be modified to between 4 and 30 seconds.
• Max Age Timer -The max age is the maximum length of time that a switch waits before attempting
to change the STP topology. The default is 20 seconds but can be modified to between 6 and 40
seconds.
Note: The default times can be changed on the root bridge, which dictates the value of these timers for
the STP domain.

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STP Operations
STP Timers and Port States (Cont.)
STP facilitates the logical loop-free path throughout the broadcast domain. The spanning tree is determined through the
information learned by the exchange of the BPDU frames between the interconnected switches. If a switch port
transitions directly from the blocking state to the forwarding state without information about the full topology during the
transition, the port can temporarily create a data loop. For this reason, STP has five ports states, four of which are
operational port states as shown in the figure. The disabled state is considered non-operational.

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STP Operations
Operational Details of Each Port State

The table summarizes the operational details of each port state

Forwarding Data
Port State BPDU MAC Address Table
Frames
Blocking Receive only No update No

Listening Receive and send No update No

Learning Receive and send Updating table No

Forwarding Receive and send Updating table Yes

Disabled None sent or received No update No

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STP Operations
Per-VLAN Spanning Tree

➢ STP can be configured to operate in an environment with multiple VLANs. In
Per-VLAN Spanning Tree (PVST) versions of STP, there is a root bridge
elected for each spanning tree instance.
➢ This makes it possible to have different root bridges for different sets of
VLANs.
➢ STP operates a separate instance of STP for each individual VLAN.
➢ If all ports on all switches are members of VLAN 1, then there is only one
spanning tree instance.