Introducing Spanning Tree

Industrial Connectivity
Introducing Spanning Tree
Outline
• Overview
• Redundant Switched and Bridged Topologies
• Broadcast Storms
• Multiple Frame Copies
• MAC Database Instability
• Summary
PROPRIETARY
1
Redundant Topology
• Redundant topology eliminates single points of failure.
• Redundant topology causes broadcast storms, multiple frame
copies and MAC address table instability problems.
PROPRIETARY
2
Broadcast Storms
• Host X sends a broadcast.
• Switches continue to propagate broadcast
traffic over and over.
PROPRIETARY
3
Multiple Frame Copies
• Host X sends a unicast frame to router Y.
• The MAC address of router Y has not been learned
by either switch.
• Router Y will receive two copies of the same frame.
PROPRIETARY
4
MAC Database Instability
• Host X sends a unicast frame to router Y.
• The MAC address of router Y has not been learned by either switch.
• Switches A and B learn the MAC address of host X on port 0.
• The frame to router Y is flooded.
• Switches A and B incorrectly learn the MAC address of host X on port 1.
PROPRIETARY
5
Summary
• Bridged and switched networks are commonly designed
with redundant links and devices, which can introduce
problems, such as broadcast storms, multiple frame
transmission, and MAC database instability.
• A broadcast storm is created when each switch on a
redundant network floods broadcast frames endlessly.
• Multiple frame transmissions occur when multiple copies
of the same frame arrive at the intended host, potentially
causing problems with the receiving protocol.
• MAC database instability occurs when multiple copies of
a frame arrive on different ports of a switch.
PROPRIETARY
6
Outline
• Overview
• Spanning Tree Protocol
• Spanning Tree Operation
• Root Bridge Selection
• Spanning Tree Port States
• Spanning Tree Path Costs
• Spanning Tree Recalculation
• Rapid Spanning Tree Protocol
• Summary
PROPRIETARY
7
Spanning Tree Protocol
Provides a loop-free redundant network topology by
placing certain ports in the blocking state
PROPRIETARY
8
Spanning Tree Operation
• One root bridge per network
• One root port per nonroot bridge
• One designated port per segment
• Nondesignated ports are unused
PROPRIETARY
9
Root Bridge Selection
• BPDU = Bridge Protocol Data Unit
(default = sent every two seconds)
• Root bridge = Bridge with the lowest bridge ID
• Bridge ID =
In this example, which switch has the lowest bridge ID?
PROPRIETARY
10
Spanning Tree Port States
Spanning tree transits each port through several
different states:
PROPRIETARY
11
Spanning Tree Port States
PROPRIETARY
12
Spanning Tree Operation
PROPRIETARY
13
Spanning Tree Path Costs
Data Rate
STP Cost (802.1t-2001)
4 Mbit/s
5,000,000
10 Mbit/s
2,000,000
16 Mbit/s
1,250,000
100 Mbit/s
200,000
1 Gbit/s
20,000
10 Gbit/s
2,000
PROPRIETARY
14
Spanning Tree Recalculation
PROPRIETARY
15
Spanning Tree Convergence
• Convergence occurs when all the switch and bridge
ports have transitioned to either the forwarding or the
blocking state.
• When the network topology changes, switches and
bridges must recompute STP, which disrupts user
traffic.
PROPRIETARY
Rapid Spanning Tree Protocol
PROPRIETARY
17
Rapid Transition to Forwarding
PROPRIETARY
18
Summary
• STP is a bridge-to-bridge protocol used to maintain a
loop-free network.
• To maintain a loop-free network topology , STP
establishes a root bridge, a root port, and designated
ports.
• With STP, the root bridge has the lowest BID, which is
made up of the bridge priority and the MAC address.
PROPRIETARY
19
Summary
• When STP is enabled, every bridge in the network goes
through the blocking state and the transitory states of
listening and learning at power up. If properly configured,
the ports then stabilize to the forwarding or blocking
state.
• If the network topology changes, STP maintains
connectivity by transitioning some blocked ports to the
forwarding state.
• RSTP significantly speeds the recalculation of the
spanning tree when the network topology changes.
PROPRIETARY
20
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