Module 7
Spanning Tree
Protocol
Version 3.0
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Redundancy
• Redundancy in a network is needed in case there
is loss of connectivity in one segment.
• But redundancy in itself presents problems –
loops.
• 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.
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Redundancy
• Companies want 100% uptime, but 99.999%
(5 nines) is the goal.
• Remember the goal is reliability without
faults. Fault tolerance is achieved by
redundancy.
• Example of having 1 car versus 2 cars – 1
is always available – redundancy
• So companies should:
– eliminate single points of failure and
– design alternate routes to a destination
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Reliability and 24x7 network
demands have compelled LAN
designers to construct multiple
paths between user and resource
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Redundant Switched Topologies
• Again, if one path fails, the other path or
device can take over.
• This is good, but there is a downside that has
to be accounted for:
– Broadcast storms
– Multiple (or duplicate) frame copies
– MAC address table instabilities
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Redundant Paths and No
Spanning Tree. . .
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(1) 00-A1 sends frame to CC-23
Switch 1 and Switch 2 learn about 00-A1
00-A1
BB-44
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2
00-A1
Port 1
Port 1
LAN Switch 1
LAN Switch 2
Port 2
Port 2
AA-11
CC-23
(2) LAN Switch 1 Floods packet out Port 2
since CC-23 is not known
00-A1
BB-44
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2
00-A1
Port 1
Port 1
LAN Switch 1
LAN Switch 2
Port 2
Port 2
AA-11
CC-23
(3) LAN Switch 2 learns (incorrectly) that
(Source MAC ) 00-A1 is on Port 2
00-A1
BB-44
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2
00-A1
Port 1
Port 1
LAN Switch 1
LAN Switch 2
Port 2
Port 2
AA-11
CC-23
Or, A Broadcast
Storm. . .
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Broadcast Storms, like ARP requests
1
10BaseT Ports (12)
Switch A
Host A
Hub
A
00-90-27-76-96-93
A
10BaseT Ports (12)
Switch B
1 2
100BaseT Ports
Host B
00-90-27-76-5D-FE
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Because it is a Layer 2 broadcast frame, both switches,
Switch A and Switch B, flood the frame out all ports,
including their port A’s.
1
10BaseT Ports (12)
Switch A
Host A
Hub
A
00-90-27-76-96-93
A
10BaseT Ports (12)
Switch B
1 2
100BaseT Ports
Host B
00-90-27-76-5D-FE
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Both switches receive the same broadcast, but on a
different port. Doing what switches do, both switches flood
the duplicate broadcast frame out their other ports.
1
10BaseT Ports (12)
Switch A
Duplicate
frame
Duplicate
frame
A
Host A
Hub
00-90-27-76-96-93
A
10BaseT Ports (12)
Switch A
1 2
100BaseT Ports
Host B
00-90-27-76-5D-FE
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Here we go again, with the switches flooding the same
broadcast again out its other ports. This results in
duplicate frames, known as a broadcast storm!
10BaseT Ports (12)
Switch A
Host A
Hub
A
Duplicate
Frame
Duplicate
Frame
00-90-27-76-96-93
A
10BaseT Ports (12)
Switch B
1 2
100BaseT Ports
Host B
00-90-27-76-5D-FE
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Layer 2 broadcasts not only take up network bandwidth,
but must be processed by each host. This can severely
impact a network, to the point of making it unusable.
10BaseT Ports (12)
Switch A
Host A
Hub
A
00-90-27-76-96-93
A
10BaseT Ports (12)
Switch B
1 2
Host B
00-90-27-76-5D-FE
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Redundant Topology
• The traffic that switches flood out all ports can be
caught in a loop, because in the Layer 2 header
there is no TTL.
• (Remember that in Layer 3 the TTL is
decremented and the packet is discarded when
the TTL reaches 0)
• You need switching (bridging) for reliability, but
now the problem of loops – a switched network
cannot have loops if it is to do what it is
supposed to do.
• Solution? Allow physical loops, but create a loopfree topology
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Spanning Tree
Protocol
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Spanning Tree Protocol
Standby Link
•
•
•
•
•
•
•
Switches forward broadcast frames
Prevents loops
Loops can cause broadcast storms and duplicate frames
Allows redundant links
Prunes topology to a minimal spanning tree
Resilient to topology changes and device failures
Main function of the Spanning Tree Protocol (STP) is to allow redundant
switched/bridged paths without suffering the effects of loops in the
network
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Root Bridge
B
H
C
J
A
E
I
Server
D
F
Root
Bridge
Server
= Forwarding Path
= Backup Link
G
The Spanning-Tree Protocol specifies an algorithm (SpanningTree Algorithm) that ultimately creates a logical loop-free
topology
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Spanning Tree Algorithm
•
The STA is used to calculate a loop-free logical
topology.
•
Spanning-tree frames called bridge protocol data units
(BPDUs) are sent and received by all switches in the
network at regular intervals and are used to determine
the spanning tree topology.
•
These BPDUs are used to determine the shortest path
to the root bridge, and which ports will forward frames
as part of the spanning tree – BPDUs sent out every 2
seconds
•
A separate instance of STP runs within each
configured VLAN.
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Spanning Tree
For every switched
network:
•One root bridge
•One root port per
non root bridge
•One designated
port per segment
•Unused, nondesignated ports
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3 Steps to Spanning Tree
Step 1: Electing a Root Bridge
• Bridge Priority
• Bridge ID
• Root Bridge
Step 2: Electing Root Ports
• Path Cost or Port Cost
• Root Path Cost
• Root Port
Step 3: Electing Designated Ports
• Path Cost or Port Cost
• Root Path Cost
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Step 1: Electing a Root Bridge
•
The first step is for switches to select a
Root Bridge.
•
The root bridge is the bridge from which
all other paths are decided.
•
Only one switch can be the root bridge.
Election of a root bridge is decided by:
1. Lowest Bridge Priority
2. Lowest Bridge ID (tie-breaker)
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Bridge Priority
Version 3.0
•
This is a numerical value.
•
The switch with the with the lowest bridge
priority is the root bridge.
•
The switches use BPDU’s to accomplish
this.
•
All switches consider themselves as the
root bridge until they find out otherwise.
•
All Cisco Catalyst switches have the
default Bridge priority of 32768.
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Bridge Priorities
A
1
AB
10BaseT Ports (12)
B
10BaseT Ports (24)
C
1
10BaseT Ports (24)
Version 3.0
100BaseT
Ports
AB
100BaseT
Ports
AB
100BaseT
Ports
25
Switch A: Bridge Priority
Catalyst 1900 - Spanning Tree Configuration - Option 1
----------------------- Information -----------------------------------[V] VLANs assigned to option
1-1005
----------------------- Settings --------------------------------------[B] Bridge priority
32768 (8000 hex)
[M] Max age when operating as root
20 second(s)
[H] Hello time when operating as root
2 second(s)
[F] Forward delay when operating as root
15 second(s)
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In case of a tie, the Bridge ID is used…
Bridge ID
•
The Bridge ID is the MAC address assigned
to the individual switch.
•
The lower Bridge ID (MAC address) is the
tiebreaker.
•
Because MAC addresses are unique, this
ensures that only one bridge will have the
lowest value.
•
NOTE: There are other tie breakers, if these
values are not unique, but we will not cover
those situations.
Version 3.0
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Catalyst 1900 Management Console
Copyright (c) Cisco Systems, Inc. 1993-1998
All rights reserved.
Enterprise Edition Software
Ethernet Address:
00-B0-64-26-6D-00
PCA Number:
73-3122-04
PCA Serial Number: FAB03503222
Model Number:
WS-C1912-EN
System Serial Number: FAB0351U08M
Power Supply S/N:
PHI033301VQ
PCB Serial Number: FAB03503222,73-3122-04
Version 3.0
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Bridge Priorities and Bridge Ids
Which one is the lowest?
A
1
Priority: 32768
ID: 00-B0-64-26-6D-00
AB
10BaseT Ports (12)
B
Priority: 32768
100BaseT
Ports
AB
ID: 00-B0-64-58-CB-80
10BaseT Ports (24)
C
1
Priority: 32768
ID: 00-B0-64-58-DC-00
100BaseT
Ports
AB
10BaseT Ports (24)
Version 3.0
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Lowest: A becomes the root bridge
A
1
Priority: 32768
ID: 00-B0-64-26-6D-00
AB
10BaseT Ports (12)
B
Priority: 32768
100BaseT
Ports
ID: 00-B0-64-58-CB-80
AB
10BaseT Ports (24)
C
1
Priority: 32768
ID: 00-B0-64-58-DC-00
10BaseT Ports (24)
Version 3.0
AB
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Understanding STP States
States initially set, later modified by STP
•
•
•
•
•
Blocking
Listening
Learning
Forwarding
Disabled
Server ports can be configured to
immediately enter STP forward mode
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Understanding STP States
 Blocking - No frames forwarded, BPDUs
received
 Listening - No frames forwarded,
listening for frames
 Learning - No frames forwarded, but
learning MAC addresses
50 seconds
from
blocking to
forwarding
Version 3.0
 Forwarding – Receiving BPDUs,
Forwarding data traffic, receiving data
traffic, learns MAC addresses
 Disabled - No frames forwarded, no
BPDUs heard
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Rapid Spanning Tree Protocol
•
•
•
•
IEEE 802.1w
Will eventually replace 802.1d
Port states and roles will be clarified
A set of link types will be defined that will allow going to a
forwarding stage quicker
• All switches will generate their own BPDUs instead of relying on
the root bridge.
• Link types would be:
– Point to point
– Edge-type
– Shared
Version 3.0
Can go to forward state
immediately
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Module 7
Spanning Tree
Protocol
Version 3.0
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