algorithm is used to identify the root bridge.
BPDUs are sent out with the Bridge ID (BID). The BID consists
of a bridge priority that defaults to 32768 and the switch base
MAC address. By default BPDUs are sent every two seconds. When
a switch first starts up, it assumes it is the root switch and
sends “inferior” BPDUs. These BPDUs contain the switch MAC
address in both the root and sender BID. All switches see the
BIDs sent. As a switch receives a BPDU with a lower root BID it
replaces that in the BPDUs that are sent out. All bridges see
these and decide that the bridge with the smallest BID value
will be the root bridge. A network administrator may want to
influence the decision by setting the switch priority to a
smaller value than the default, which will make the BID
smaller. This should only be implemented when the traffic flow
on the network is well understood. Lab Activity Lab
Exercise: Selecting the Root Bridge In this lab, the student
will create a basic switch configuration and verify it and
determine which switch is selected as root switch with factory
default settings. Lab Activity e-Lab Activity:
Selecting the Root Bridge In this lab, the following functions
will be performed. Verify configuration of hosts and switch by
testing connectivity. 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 7.2.5 Stages
of spanning-tree port states Time is required for protocol
information to propagate throughout a switched network.
Topology changes in one part of a network are not instantly
known in other parts of the network. There is propagation
delay. A switch should not change a port state from inactive to
active immediately, as this may cause data loops. Each port on
a switch that is using the Spanning-Tree Protocol has one of
five states, as shown in Figure . In the blocking state, ports
can only receive BPDUs. Data frames are discarded and no
addresses can be learned. It may take up to 20 seconds to
change from this state. Ports go from the blocked state to the
listening state. In this state, switches determine if there are
any other paths to the root bridge. The path that is not the
least cost path to the root bridge goes back to the blocked
state. The listening period is called the forward delay and
lasts for 15 seconds. In the listening state, user data is not
being forwarded and MAC addresses are not being learned. BPDUs
are still processed. Ports transition from the listening to the
learning state. In this state user data is not forwarded, but
MAC addresses are learned from any traffic that is seen. The
learning state lasts for 15 seconds and is also called the
forward delay. BPDUs are still processed. A port goes from the
learning state to the forwarding state. In this state user data
is forwarded and MAC addresses continue to be learned. BPDUs
are still processed. A port can be in a disabled state. This
disabled state can occur when an administrator shuts down the
port or the port fails. The time values given for each state
are the default values. These values have been calculated on an
assumption that there will be a maximum of seven switches in
any branch of the spanning tree from the root bridge.
Interactive Media Activity Point and Click: Spanning-Tree
States When the student has completed this activity, the
student will be able to identify the function of spanning-tree
states. 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
7.2.6 Spanning-tree recalculation A switched internetwork
has converged when all the switch and bridge ports are in
either the forwarding or blocked state. Forwarding ports send
and receive data traffic and BPDUs. Blocked ports will only
receive BPDUs. When the network topology changes, switches and
bridges recompute the Spanning Tree and cause a disruption of
user traffic. Convergence on a new spanning-tree topology using
the IEEE 802.1D standard can take up to 50 seconds. This
convergence is made up of the max-age of 20 seconds, plus the
listening forward delay of 15 seconds, and the learning forward
delay of 15 seconds. Lab Activity Lab Exercise:
Spanning-Tree Recalculation In this lab, the student will
create a basic switch configuration and verify it and observe
the behavior of spanning tree algorithm in presence of switched
network topology changes. Lab Activity e-Lab Activity:
Spanning-Tree Recalculation In this lab, the students will
create a basic switch configuration and verify it. 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
7.2.7 Rapid Spanning-Tree Protocol The Rapid Spanning-Tree
Protocol is defined in the IEEE 802.1w LAN standard. The
standard and protocol introduce the following:
- Clarification of port states and roles
- Definition
of a set of link types that can go to forwarding state rapidly
- Concept of allowing switches, in a converged network,
to generate their own BPDUs rather than relaying root bridge
BPDUs
The “blocked” state of a port has been renamed
as the “discarding” state. A role of a discarding port is an
“alternate port”. The discarding port can become the
“designated port” in the event of the failure of the designated
port for the segment. Link types have been defined as
point-to-point, edge-type, and shared. These changes allow
failure of links in switched network to be learned rapidly.
Point-to-point links and edge-type links can go to the
forwarding state immediately. Network convergence does not need
to be any longer than 15 seconds with these changes. The Rapid
Spanning-Tree Protocol, IEEE 802.1w, will eventually replace
the Spanning-Tree Protocol, IEEE 802.1D. Web Links
Understanding Rapid Spanning-Tree Protocol (802.1w)
http://www.cisco.com/warp/
public/473/146.html
Content Summary An understanding of the following key
points should have been achieved: - Redundancy and its
importance in networking
- The key elements of a
redundant networking topology
- Broadcast storms and
their impact on switched networks
- Multiple frame
transmissions and their impact on switched networks
- Causes and results of MAC address database
instability
- The benefits and risks of a redundant
topology
- The role of spanning tree in a redundant-path
switched network
- The key elements of spanning-tree
operation
- The process for root bridge election
- Spanning-tree states
- Spanning-Tree Protocol
compared to Rapid Spanning-Tree Protocol