Content Overview Redundancy in a
network is extremely important because redundancy allows
networks to be fault tolerant. Redundant topologies protect
against network downtime due to a failure of a single link,
port, or networking device. Network engineers are often
required to make difficult decisions, balancing the cost of
redundancy with the need for network availability. Redundant
topologies based on switches and bridges are susceptible to
broadcast storms, multiple frame transmissions, and MAC address
database instability. Therefore network redundancy requires
careful planning and monitoring to function properly. Switched
networks provide the benefits of smaller collision domains,
microsegmentation, and full duplex operation. Switched networks
provide better performance. Redundancy in a network is required
to protect against loss of connectivity due to the failure of
an individual component. Providing this redundancy, however,
often results in physical topologies with loops. Physical layer
loops can cause serious problems in switched networks.
Broadcast storms, multiple frame transmissions, and media
access control database instability can make such networks
unusable. The Spanning-Tree Protocol is used in switched
networks to create a loop free logical topology from a physical
topology that has loops. Links, ports, and switches that are
not part of the active loop free topology do not participate in
the forwarding of data frames. The Spanning-Tree Protocol is a
powerful tool that gives network administrators the security of
a redundant topology without the risk of problems caused by
switching loops. Students completing this module should be able
to: - Define redundancy and its importance in
networking
- Describe the key elements of a redundant
networking topology
- Define broadcast storms and
describe their impact on switched networks
- Define
multiple frame transmissions and describe their impact on
switched networks
- Identify causes and results of MAC
address database instability
- Identify the benefits and
risks of a redundant topology
- Describe the role of
spanning tree in a redundant-path switched network
- Identify the key elements of spanning tree operation
- Describe the process for root bridge election
- List
the spanning-tree states in order
- Compare
Spanning-Tree Protocol and Rapid Spanning-Tree Protocol
Content 7.1 Redundant Topologies
7.1.1 Redundancy 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 is lost and customers are
dissatisfied.Companies are increasingly looking for 24 hour,
seven day a week uptime for their computer networks. Achieving
100% uptime is perhaps impossible but securing a 99.999% or
five nines uptime is a goal that organizations set. This is
interpreted to mean one day of downtime, on average, for every
30 years, or one hour of downtime, on average, for every 4000
days, or 5.25 minutes of downtime per year. Achieving such a
goal requires extremely reliable networks. Reliability in
networks is achieved by reliable equipment and by designing
networks that are tolerant to failures and faults. The network
is designed to reconverge rapidly so that the fault is
bypassed. Fault tolerance is achieved by redundancy. Redundancy
means to be in excess or exceeding what is usual and natural.
How does redundancy help achieve reliability? Assume that the
only way to get to work is by a car. If the car develops a
fault that makes it unusable, going to work will be impossible
until it is repaired and returned. If the car fails and is
unavailable, on average one day in ten then there is 90% usage.
Going to work is possible nine days in every ten. Reliability
is therefore 90%. Buying another car will improve matters.
There is no need for two cars just to get to work. However, it
does provide redundancy (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. Purchasing a second redundant car has
improved reliability to 99%. Web Links Redundancy and
Broadcast Storms http://www.howstuffworks.com/ lan-switch5.htm
Content 7.1 Redundant Topologies
7.1.2 Redundant topologies 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 an
outlying 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. Building 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.
Web Links Understanding Spanning-Tree Protocol – the
Fundamental Bridging Algorithm http://www.oreillynet.com/pub/a/
network/2001/03/30/ net_2nd_lang.html
Content 7.1
Redundant Topologies 7.1.3 Redundant switched
topologies 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. If port 1
fails on Switch A then traffic can still flow through port 1 on
Switch B. Switches learn the MAC addresses of devices on their
ports so that data can be properly forwarded to the
destination. Switches will 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. Web
Links Understanding Spanning-Tree Protocol – the
Fundamental Bridging Algorithm http://www.oreillynet.com/pub/a/
network/2001/03/30/ net_2nd_lang.html
Content 7.1
Redundant Topologies 7.1.4 Broadcast storms
Broadcasts and multicasts can cause problems in a switched
network. Multicasts are treated as broadcasts by the switches.
Broadcasts and multicasts 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, being on the same segment, also forwards all
broadcasts. Switch B sees all the broadcasts that Switch A
forwarded and Switch A sees all the broadcasts that Switch B
forwarded. Switch A sees the broadcasts and forwards them.
Switch B sees the broadcasts and forwards them. 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. The switches and end
devices will be so busy processing the broadcasts that user
traffic is unlikely to flow. The network will appear to be down
or extremely slow. Web Links Shining the Lights on
Broadcast Storms http://www.networkuptime.com/tips/lights/
Content 7.1 Redundant Topologies
7.1.5 Multiple frame transmissions In a redundant switched
network it is possible for an end device to receive multiple
frames. Assume that the MAC address of Router Y has been timed
out by both switches. Also assume that Host X still has the MAC
address of Router Y in its ARP cache and sends a unicast frame
to Router Y. The router receives the frame because it is on the
same segment as Host X. Switch A does not have the MAC address