of the Router Y and will therefore flood the frame
out its ports. Switch B also does not know which port Router Y
is on. Switch B then floods the frame it received causing
Router Y to receive multiple copies of the same frame. This is
a cause of unnecessary processing in all devices.
Content
7.1 Redundant Topologies 7.1.6 Media
access control database instability 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 see 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 will
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. Web Links
802.1d Spanning-Tree Protocol http://www.zyxel.com/support/
supportnote/ves1012/app/stp.htm
Content 7.2
Spanning-Tree Protocol 7.2.1 Redundant topology
and spanning tree Redundant networking topologies are designed
to ensure that networks continue to function in the presence of
single points of failure. Users have less chance of
interruption to their work, 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. Switches operate at Layer 2 of
the OSI model and forwarding decisions are made at this layer.
As a result of this process, switched networks must not have
loops. Switches flood traffic out all ports when the traffic is
sent to a destination that is not yet known. Broadcast and
multicast traffic is forwarded out every port, except the port
on which the traffic arrived. This traffic can be caught in a
loop. In the Layer 2 header there is no Time To Live (TTL). 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, 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 is being
introduced to reduce the time for a network to compute a loop
free logical topology. Web Links Understanding
Spanning-Tree Protocol http://www.cisco.com/univercd/cc/
td/doc/product/rtrmgmt/sw_ntman/
cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm
Content
7.2 Spanning-Tree Protocol 7.2.2
Spanning-Tree Protocol 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 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. 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. The message that a switch sends,
allowing the formation of a loop free logical topology, is
called a Bridge Protocol Data Unit (BPDU). 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 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
- Designate one of the
switches as the closest one to the root, for each LAN segment.
This bridge is called the “designated switch”. The designated
switch handles all communication from that LAN 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, the designated ports. Non-designated ports
are blocked.
Interactive Media Activity
Crossword Puzzle: Spanning-Tree States When the student has
completed this activity, the student will be able to identify
the function of spanning-tree states. Interactive Media
Activity Point and Click: Spanning-Tree Protocol After
completing this activity, the student will learn about the
concept of Spanning-Tree Protocol. Web Links
Understanding Spanning-Tree Protocol
http://www.cisco.com/univercd/cc/ td/doc/product/ rtrmgmt/
sw_ntman/ cwsimain/cwsi2/ cwsiug2/vlan2/stpapp.htm
Content 7.2 Spanning-Tree Protocol
7.2.3 Spanning-tree operation 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, 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. Web
Links Understanding Spanning-Tree Protocol
http://www.cisco.com/univercd/cc/
td/doc/product/rtrmgmt/sw_ntman/
cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm
Content
7.2 Spanning-Tree Protocol 7.2.4
Selecting the root bridge The first decision that all switches
in the network make, is to identify the root bridge. The
position of the root bridge in a network will affect the
traffic flow. When a switch is turned on, the spanning-tree