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Cisco 3 – Switching STP
Chapter 8
Spanning Tree Protocol (STP)
Having redundancy in a network is important, and allows the network to be
fault tolerant. That is, when faults occur the networking is not shut
downed.
However, redundant topologies that are based on on switches and bridges
are susceptible to:
1.
Broadcast storms
2.
Multiple frame transmissions, and
3.
MAC database instability
NOTE: Cisco interchanges bridges and switches to mean switches
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What is redundancy? Ideally for business, it is 24/7 uptime. Though this is
some what impossible, so business are looking for “5 nines”, 99.999%.
Which would be 5.25 minutes of downtime per year!
The goal of redundant topologies is to eliminate network outages that are
caused by a single point of failure.
So if you have redundant switches for reliability, then until the switches
learn the MAC address of all devices, this topology will cause:
Perrine. J
•
Broadcast storms
•
Multiple frame copies, and
•
MAC address table instability problems
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So when broadcast occurs, sending of multicasts and broadcast messages
will flood the network. The frame is sent out all ports except the one on
which the frame was received.
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Server/host
switch A
switch B
When server/host sends a message such as an ARP, switch A will broadcast it out to
every port. When switch B gets the ARP, it will broadcast it out every port; then switch A
will broadcast it again out every port; switch B will then broadcast it out every port; etc.,
etc. This is called a broadcast storm.
The switches continue to propagate broadcast traffic, and hence are so busy
processing the broadcasts that user traffic is unlikely to flow.
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Multiple Frame Transmission
Most protocols are designed to ignore or cope with duplicate transmissions.
In general, protocols that make use of a sequence numbering mechanism
assume that many transmissions have failed and that the sequence number
has recycled.
Other protocols attempt to hand the duplicate transmission to the appropriate
upper-layer protocol, with unpredictable results.
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Multiple Frame Transmission
Server/host X
Router Y
segment 1
switch A
switch B
segment 2
• When host X sends a unicast frame to router Y, one copy is received over the segment 1, while
switch A receives a copy & puts it into its buffers.
• if switch A examines the destination address field in the frame and finds no entry in the MAC
address table for router Y, it floods the frame on all ports except for the originating port.
• when switch B receives a copy of the frame through switch A on segment 2, it also forwards a
copy of the frame onto segment 1 if there is not entry in the MAC address table for router Y.
• router Y receives a copy of the same frame for the second time!
A loop avoidance solution would eliminate the problem of duplicate transmissions by logically
breaking the loop & preventing one of the four interfaces from transmitting frames during normal
operation.
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MAC Database Instability
Server/host X
Router Y
segment 1
PORT 0
PORT 0
switch A
switch B
PORT 1
PORT 1
segment 2
Switch B installs a mapping between the MAC address of Host X and Port 0, which connects to
segment 1 when the first frame arrives.
Sometime later, when the copy of the frame that was transmitted through switch A arrives at port
1 of switch B, switch B must remove the first entry and install one that incorrectly maps the MAC
address of station X to the Port 1 that connects to segment 2.
Depending on the internal architecture of the switch, it might or might not cope well
with rapid changes in its MAC database.
So a loop-avoidance solution would eliminate the problem of the switch not coping
well with the rapid change of the MAC address.
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Spanning Tree Overview
Solution to this problem is solved by the Spanning Tree Protocol (STP).
It is a Layer 2 link-management protocol that is used to maintain a loop-free
network.
Digital Equipment Corporation (Digital) originally developed STP. The IEEE 802
committee subsequently revised the Digital spanning tree algorithm and published
it in the IEEE 802.1d specification.
The purpose of STP is to maintain a loop-free network topology. A loop-free
topology is accomplished when the switch or bridge recognizes a loop in the
topology and logically blocks one or more redundant ports automatically.
Remember the states:
• Blocking
• Listening
• Learning
• Forwarding
• Disabled
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Switched networks provide the benefits of
• smaller collision domains,
• microsegmentation
• full duplex operation
But simply, the benefit of the switch is performance.
When a component of the active topology fails, you must determine a new loopfree topology. You need to recalculate or converge on a new loop-free topology as
quickly as possible to reduce the time that end stations lack access to network
resources.
STP, defined by IEEE 802.1d, is too slow in converging on a new topology for
today’s network.
A new standard, IEEE 802.1w (RSTP) ,Rapid STP, has been defined to over
come know limitations.
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Spanning Tree (STP) - Terms
Root Bridge:
The root bridge is determined by combining the priority of the bridge and the
MAC address. (If two bridges/switches have the same priority value, then the
MAC address is used to determine which one has the lowest ID.)
There is only one (1) root bridge per network
Designated ports:
Ports on a root bridge are called designated ports
Each segment will have one designated port selected
Non-root Bridges:
Other bridges/switches in your network are called non-root bridges
Root port
Is the port with lowest cost (determined by a link’s bandwidth) to the root
bridge
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segment 1- 100BASET
MAC: 0c00c8111111
Priority: 32768
Root port PORT 0
PORT 0
switch A
switch B
PORT 1
Which switch is the root bridge?
MAC: 0c00c8222222
Priority: 32768
PORT 1
segment 2 – 10BASET
switch A ( same priority, but lowest MAC address)
Hence PORT 0 & PORT 1 on switch A are the “designated port”.
Which switch is/are non-root bridges?
Which ports are “root ports”?
switch B
PORT 0 of switch B
Because the connection from switch B to the root switch (A) is from PORT
0 using a 100Mbps link and has the best cost.
Designated ports on other switches?
Each segment has one designated port. Since PORT 0 & PORT 1 of switch
A are “designated ports” (because it is the root bridge) there are no other
“designated ports.”
STP will block PORT 1 of switch B to make it ‘loop-free’.
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Spanning Tree Operation
The following will be true for every switched network:
• one root bridge exists per network
• one root port exists per non-root bridge
• one designated port exists per segment
• undesignated ports are unused.
Root ports and designated ports are used for Forwarding (F) data traffic.
Undesignated ports discard data traffic. These are called Blocking (B) or
discarding ports.
Every non-root bridge must select one root port.
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Switches flood traffic out all ports when it is for a destination that is not yet known.
Broadcast and multicast traffic is forwarded out every port other than the port on
which the traffic arrives (floods).
This traffic is caught in a loop, because the Layer 2 header has not time to live
(TTL). Layer 3 devices do have TTL.
A physical topology that contains switching or bridging loops is necessary for
redundancy and reliability. Hence one must allow physical loops but create a
loop-free logical topology!
The loop-free logical topology that is created is called a tree.
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Spanning Tree Algorithm
STP establishes a root node, called the root bridge, and constructs a topology
that has one path for reaching every network node. The resulting tree originates
from the root bridge.
Redundant links that are not part of the shortest path tree are blocked.
A loop-free topology is possible because certain paths are blocked. Data frames
that are received on blocked links are dropped.
STP requires network devices to exchange messages to detect bridging loops.
The messages that switches send that allow the forming of a loop-free logical
topology are called bridge protocol data units (BPDUs).
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Spanning Tree Algorithm
The BPDUs contain enough information so that all switches can do the following:
• select a single switch that will act as the root of the spanning tree.
• calculate the shortest path from itself to the root switch
• for each LAN segment, designate one of the switches as the closest one to
the root. This bridge is called the designated switch. The designated switch
handles all communication from that LAN toward the root bridge.
• each non-root switch chooses one of its ports as its root port. This is the
interface that gives the best path to the root switch.
• select ports that are part of the spanning tree, the designated ports. Nondesignated ports are blocked.
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Spanning Tree Algorithm
When the spanning tree is creating a loop-free logical topology, it always uses the
same 4-step decision sequences. As every BPDU arrives, it is checked against
this 4-step sequence to see if it has a lower value than the existing BPDU that is
saved for that port.
1.
Lowest root bridge ID (BID)
2.
Lowest path cost to root bridge
3.
Lowest sender bridge ID
4.
Lowest port ID
When a bridge first becomes active, all of its ports are sending BPDUs every 2
seconds (default).
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When STP is enabled, every bridge in the network goes through the blocking and
transitory states of listening and learning at power up.
If ports are properly configured, they then stabilize to the forwarding or blocking
state.
Forwarding ports provide the lowest-cost path to the root bridge.
Two transitional states, listening & learning, occur when a bridge recognizes a
change in the network topology.
When the bridge first boots up, it thinks it is the root bridge and transitions to the
listening state.
When a port is in the transitional listening state, it can send & receive BPDUs to
determine the active topology. At this point, no user data is being sent.
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Ports that remain as designated or root ports after 15 seconds (forward delay)
transition to the learning state. When the port is in a learning state, it can populate
its MAC address table with MAC addresses that are heard on its port, but does
not forward user frames.
Ports that are not the designated or root ports transition back to the blocking state.
The normal time that it takes for a port to transition from the blocking state to the
forwarding state is 30 to 50 seconds.
The time that it takes for a port to transition from the listening state to the learning
state or from the learning state to the forwarding state is called the forward delay
(default value 15 seconds.)
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Note:
Each segment in a bridged network has one designated port. This port functions
as the single bridge port that both sends and receives traffic to and from that
segment and the root bridge.
The idea behind this is that if only one port handles traffic for each link, all the
loops have been broken.
Note:
When STP is faced with a tie, it goes with the lowest BID.
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Spanning Tree Path Cost
Spanning-tree path cost is an accumulated total path cost based on the
bandwidth of all the links in the path.
With the introduction of 10 Gigabit Ethernet, IEEE had to revise its path cost.
Link Speed
Cost(revised)
Cost(previous)
10 Gbps
2
1
1 Gbps
4
1
100 Mbps
19
10
10 Mbps
100
100
Lower the cost better the path is.
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Spanning Tree Timers
Timer
Purpose
Default
Hello Time
Time between sending of configuration
2 seconds
BPDUs by the root bridge.
Forward Delay
Duration of listening & learning states
15 seconds
Max Age
Time BPDU stored
20 seconds
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Spanning Tree Convergence
Convergence in STP means a state in which all the switch and bridge ports have
transitioned to either the forwarding or the blocking state.
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Rapid Spanning Tree Protocol
(RSTP)
Rapid Spanning Tree Protocol (RSTP), IEEE 802.1 w is designed to
significantly speed the recalculation of the spanning tree when the network
topology changes.
The rapid transition is the most important feature introduced with RSTP. Before
802.1w, the spanning-tree algorithm waited passively for the network to converge
before transitioning a port to the forwarding state
The new RSTP actively confirms that a port can safely transition to forwarding
without relying on a timer configuration.
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Rapid Spanning Tree Protocol
(RSTP)
To achieve fast convergence on a port, the protocol relies on two new variables:
•
Edge port
•
Link type (point-to-point)
Edge ports are those that are directly connected to end stations.
Though RSTP is only able to achieve rapid transition to forwarding on edge ports
and on point-to-point links.
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