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Spanning Tree Protocol
7.2.3 This page will teach students about the ports and devices that are
found in an STP switched network.
When the network has stabilized, it has converged and there is one
spanning-tree per network.
As a result, for every switched network the following elements
exist:
- One root bridge per network
- One root port per non-root
bridge
- One designated port per
segment
- Unused, or non-designated
ports
Root ports and designated ports are used for forwarding (F) data
traffic.
Non-designated ports discard data traffic. These ports are called
blocking (B) or discarding ports.
The next page will discuss the root bridge.
Spanning Tree Protocol
7.2.2 This page will explain how STP can be used to create a loop free
network.
Ethernet bridges and switches can implement the IEEE 802.1d
Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop
free shortest path network.
Shortest path is based on cumulative link costs. Link costs are
based on the speed of the link.
The Spanning-Tree Protocol establishes a root node called the root
bridge. The Spanning-Tree Protocol constructs a topology that has one path for
every node on the network. This tree originates from the root bridge. Redundant
links that are not part of the shortest path tree are blocked.
It is because certain paths are blocked that a loop free topology
is possible. Data frames received on blocked links are dropped.
The Spanning-Tree Protocol requires network devices to exchange
messages to detect bridging loops. Links that will cause a loop are put into a
blocking state.
Switches send messages called the bridge protocol data units
(BPDUs) to allow the formation of a loop free logical topology. BPDUs continue
to be received on blocked ports. This ensures that if an active path or device
fails, a new spanning-tree can be calculated.
BPDUs contain information that allow switches to perform specific
actions:
- Select a single switch that
will act as the root of the spanning-tree.
- Calculate the shortest path
from itself to the root switch.
- Designate one of the
switches as the closest one to the root, for each LAN segment. This switch
is called the designated switch. The designated switch handles all
communication from that LAN segment towards the root bridge.
- Choose one of its ports as
its root port, for each non-root switch. This is the interface that gives
the best path to the root switch.
- Select ports that are part
of the spanning-tree. These ports are called designated ports.
Non-designated ports are blocked.
The Interactive Media Activity will teach students about STP.
The next page will describe the features of a spanning-tree network.
Spanning-Tree Protocol /
Redundant topology and spanning tree
7.2.1 This page will teach students how to create a loop free logical
topology.
Redundant network topologies are designed to ensure that networks
continue to function in the presence of single points of failure. Work is
interrupted less often for users because the network continues to function. Any
interruptions that are caused by a failure should be as short as possible.
Reliability is increased by redundancy. A network that is based on
switches or bridges will introduce redundant links between those switches or
bridges to overcome the failure of a single link. These connections introduce
physical loops into the network. These bridging loops are created so if one
link fails another can take over the function of forwarding traffic.
When the destination of the traffic is unknown to a switch, it
floods traffic out all ports except the port that received the traffic.
Broadcasts and multicasts are also forwarded out every port except the port
that received the traffic. This traffic can be caught in a loop.
In the Layer 2 header, there is no Time To Live (TTL) value. If a
frame is sent into a Layer 2 looped topology of switches, it can loop forever.
This wastes bandwidth and makes the network unusable.
At Layer 3, the TTL is decremented and the packet is discarded
when the TTL reaches 0. This creates a dilemma. A physical topology that
contains switching or bridging loops is necessary for reliability, yet a
switched network cannot have loops.
The solution is to allow physical loops, but create a loop free
logical topology. For this logical topology, traffic destined
for the server farm attached to Cat-5 from any user workstation attached to
Cat-4 will travel through Cat-1 and Cat-2. This will happen even though there
is a direct physical connection between Cat-5 and Cat-4.
The loop free logical topology created is called a tree. This
topology is a star or extended star logical topology. This topology is the
spanning-tree of the network. It is a spanning-tree because all devices in the
network are reachable or spanned.
The algorithm used to create this loop free logical topology is
the spanning-tree algorithm. This algorithm can take a relatively long time to
converge. A new algorithm called the rapid spanning-tree algorithm was
developed to reduce the time for a network to compute a loop free logical
topology.
The next page will discuss STP.
Media access control database instability
7.1.6 This page will explain how incorrect information can be forwarded
in a redundant switched network.
In a redundant switched network it is possible for switches to
learn the wrong information. A switch can incorrectly learn that a MAC address
is on one port, when it is actually on a different port. In this example the MAC address of Router Y
is not in the MAC address table of either switch.
Host X sends a frame directed to Router Y. Switches A and B learn
the MAC address of Host X on port 0.
The frame to Router Y is flooded on port 1 of both switches.
Switches A and B receive this information on port 1 and incorrectly learn the
MAC address of Host X on port 1. When Router Y sends a frame to Host X, Switch
A and Switch B also receive the frame and will send it out port 1. This is
unnecessary, but the switches have incorrectly learned that Host X is on port
1.
In this example the unicast frame from Router Y to Host X will be
caught in a loop.
This page concludes this lesson. The next lesson will describe the
Spanning-Tree Protocol (STP). The first page will discuss physical and logical
loops in a redundant network
Broadcast storms
7.1.4 page will explain the effects of broadcasts and multicasts in a
switched network.
Broadcasts and multicasts can cause problems in a switched
network.
Multicasts are treated as broadcasts by the switches. Broadcast
and multicast frames are flooded out all ports, except the one on which the frame
was received.
If Host X sends a broadcast, like an ARP request for the Layer 2
address of the router, then Switch A will forward the broadcast out all ports.
Switch B is on the same segment and also forwards all broadcasts. Switch B
receives all the broadcasts that Switch A forwarded and Switch A receives all
the broadcasts that Switch B forwarded. Switch A forwards the broadcasts
received from Switch B. Switch B forwards the broadcasts received from Switch
A.
The switches continue to propagate broadcast traffic over and
over. This is called a broadcast storm. This broadcast storm will continue
until one of the switches is disconnected. Since broadcasts require time and
network resources to process, they reduce the flow of user traffic. The network
will appear to be down or extremely slow.
The next page will discuss multiple frame transmissions.
A redundant switched topology may cause broadcast storms, multiple
frame copies, and MAC address table instability problems.
The next page will discuss broadcast storms.
Multiple frame transmissions 7.1.5 page will explain the effects of broadcasts and multicasts in a
switched network.
Broadcasts and multicasts can cause problems in a switched
network.
Multicasts are treated as broadcasts by the switches. Broadcast
and multicast frames are flooded out all ports, except the one on which the frame
was received.
If Host X sends a broadcast, like an ARP request for the Layer 2
address of the router, then Switch A will forward the broadcast out all ports.
Switch B is on the same segment and also forwards all broadcasts. Switch B
receives all the broadcasts that Switch A forwarded and Switch A receives all
the broadcasts that Switch B forwarded. Switch A forwards the broadcasts
received from Switch B. Switch B forwards the broadcasts received from Switch
A.
The switches continue to propagate broadcast traffic over and
over. This is called a broadcast storm. This broadcast storm will continue
until one of the switches is disconnected. Since broadcasts require time and
network resources to process, they reduce the flow of user traffic. The network
will appear to be down or extremely slow.
The next page will discuss multiple frame transmissions.
Redundant Topoligies
7.1.2 This page will explain the concept and benefits of a redundant
topology.
A goal of redundant topologies is to eliminate network outages
caused by a single point of failure. All networks need redundancy for enhanced
reliability.
A network of roads is a global example of a redundant topology. If
one road is closed for repair, there is likely an alternate route to the
destination.
Consider a community separated by a river from the town center. If
there is only one bridge across the river, there is only one way into town. The
topology has no redundancy.
If the bridge is flooded or damaged by an accident, travel to the
town center across the bridge is impossible.
A second bridge across the river creates a redundant topology. The
suburb is not cut off from the town center if one bridge is impassable.
The next page will describe redundant switched topologies.
Redundant switched topologies
7.1.3 This page will explain how switches operate in a redundant
topology.
Networks with redundant paths and devices allow for more network
uptime. Redundant topologies eliminate single points of failure. If a path or
device fails, the redundant path or device can take over the tasks of the
failed path or device.
If Switch A fails, traffic can still flow from Segment 2 to
Segment 1 and to the router through Switch B.
Switches learn the MAC addresses of devices on their ports so that
data can be properly forwarded to the destination. Switches flood frames for
unknown destinations until they learn the MAC addresses of the devices. Broadcasts and multicasts are also flooded.
A redundant switched topology may cause broadcast storms, multiple
frame copies, and MAC address table instability problems.
The next page will discuss broadcast storms.
Redundancy
7.1.1 This page will explain how redundancy can improve network
reliability and performance.
Many companies and organizations increasingly rely on computer
networks for their operations. Access to file servers, databases, the Internet,
intranets, and extranets is critical for successful businesses. If the network
is down, productivity and customer satisfaction decline.
Increasingly, companies require continuous network availability,
or uptime. 100 percent uptime is perhaps impossible, but many organizations try
to achieve 99.999 percent, or five nines, uptime. Extremely reliable networks
are required to achieve this goal. This is interpreted to mean one hour of
downtime, on average, for every 4,000 days, or approximately 5.25 minutes of
downtime per year. To achieve such a goal requires extremely reliable networks.
Network reliability is achieved through reliable equipment and
network designs that are tolerant to failures and faults. Networks should be
designed to reconverge rapidly so that the fault is bypassed.
Figure illustrates redundancy. Assume that a car
must be used to get to work. If the car has a fault that makes it unusable, it
is impossible to use the car to go to work until it is repaired.
On average, if the car is unuseable due to failure one day out of
ten, the car has ninety percent usage. Therefore, reliability is also 90
percent.
A second car will improve matters. There is no need for two cars
just to get to work. However, it does provide redundancy, or backup, in case
the primary vehicle fails. The ability to get to work is no longer dependent on
a single car.
Both cars may become unusable simultaneously, one day in every
100. The second car raises reliability to 99 percent.
The next page will discuss redundant topologies
