1 version 3.0 module 7 spanning tree protocol. 2 version 3.0 redundancy redundancy in a network is...
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1Version 3.0
Module 7Spanning Tree
Protocol
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2Version 3.0
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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3Version 3.0
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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4Version 3.0
Reliability and 24x7 network demands have compelled LAN designers to construct multiple paths between user and resource
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5Version 3.0
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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6Version 3.0
Redundant Paths and No Spanning Tree. . .
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00-A1 BB-44
AA-11 CC-23
SATPort 1 Port 200-A1
SATPort 1 Port 200-A1
Port 1 Port 1
Port 2 Port 2
LAN Sw itch 1 LAN Sw itch 2
(1) 00-A1 sends frame to CC-23Sw itch 1 and Sw itch 2 learn about 00-A1
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00-A1 BB-44
AA-11 CC-23
SATPort 1 Port 200-A1
SATPort 1 Port 200-A1
Port 1 Port 1
Port 2 Port 2
LAN Sw itch 1 LAN Sw itch 2
(2) LAN Sw itch 1 Floods packet out Port 2since CC-23 is not know n
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00-A1 BB-44
AA-11 CC-23
SATPort 1 Port 200-A1
SATPort 1 Port 2 00-A1
Port 1 Port 1
Port 2 Port 2
LAN Sw itch 1 LAN Sw itch 2
(3) LAN Sw itch 2 learns (incorrectly) that(Source MAC ) 00-A1 is on Port 2
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10Version 3.0
Or, A Broadcast Storm. . .
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11Version 3.0
Broadcast Storms, like ARP requests
10BaseT Ports (12)
10BaseT Ports (12)
100BaseT Ports
A
Switch A
Switch B
Host A
A
1
1 2
00-90-27-76-96-93
00-90-27-76-5D-FE
Hub
Host B
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12Version 3.0
10BaseT Ports (12)
10BaseT Ports (12)
100BaseT Ports
A
Switch A
Switch B
Host A
A
1
1 2
00-90-27-76-96-93
00-90-27-76-5D-FE
Hub
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.
Host B
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13Version 3.0
10BaseT Ports (12)
10BaseT Ports (12)
100BaseT Ports
A
Switch A
Switch A
Host A
A
1
1 2
00-90-27-76-96-93
00-90-27-76-5D-FE
Hub
Duplicateframe
Duplicateframe
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.
Host B
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14Version 3.0
10BaseT Ports (12)
10BaseT Ports (12)
100BaseT Ports
A
Switch A
Switch B
A
1 2
00-90-27-76-96-93
00-90-27-76-5D-FE
Hub
Duplicate Frame
Duplicate Frame
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!
Host A
Host B
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15Version 3.0
10BaseT Ports (12)
10BaseT Ports (12)
A
Switch A
Switch B
A
1 2
00-90-27-76-96-93
00-90-27-76-5D-FE
Hub
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.
Host A
Host B
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16Version 3.0
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 loop-free topology
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17Version 3.0
Spanning Tree Protocol
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18Version 3.0
Broadcast Frame
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
Spanning Tree Protocol
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19Version 3.0
Root Bridge
RootBridge
Server
Server
= Backup Link
= Forwarding Path
The Spanning-Tree Protocol specifies an algorithm (Spanning-Tree Algorithm) that ultimately creates a logical loop-free topology
A
B
C
H
J
IE
G
FD
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20Version 3.0
• 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.
Spanning Tree Algorithm
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21Version 3.0
Spanning TreeFor every switched network:
•One root bridge
•One root port per non root bridge
•One designated port per segment
•Unused, non-designated ports
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22Version 3.0
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
3 Steps to Spanning Tree
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23Version 3.0
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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24Version 3.0
Bridge Priority
• 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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25Version 3.0
A B
A B
A B
1
1
A
B
C
10BaseT Ports (12)
10BaseT Ports (24)
10BaseT Ports (24)
100BaseT Ports
100BaseT Ports
100BaseT Ports
Bridge Priorities
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26Version 3.0
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)
Switch A: Bridge Priority
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27Version 3.0
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.
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28Version 3.0
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
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29Version 3.0
A B
A B
A B
1
1
A
B
C
10BaseT Ports (12)
10BaseT Ports (24)
10BaseT Ports (24)
100BaseT Ports
100BaseT Ports
Priority: 32768 ID: 00-B0-64-26-6D-00
Priority: 32768 ID: 00-B0-64-58-CB-80
Priority: 32768 ID: 00-B0-64-58-DC-00
Bridge Priorities and Bridge Ids
Which one is the lowest?
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30Version 3.0
A B
A B
1
1
A
B
C
10BaseT Ports (12)
10BaseT Ports (24)
10BaseT Ports (24)
100BaseT Ports
Priority: 32768 ID: 00-B0-64-26-6D-00
Priority: 32768 ID: 00-B0-64-58-CB-80
Priority: 32768 ID: 00-B0-64-58-DC-00
Lowest: A becomes the root bridge
A B
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31Version 3.0
States initially set, later modified by STP
Server ports can be configured to immediately enter STP forward mode
Understanding STP States
• Blocking• Listening• Learning• Forwarding• Disabled
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32Version 3.0
Blocking - No frames forwarded, BPDUs received
Listening - No frames forwarded, listening for frames
Learning - No frames forwarded, but learning MAC addresses
Forwarding – Receiving BPDUs, Forwarding data traffic, receiving data traffic, learns MAC addresses
Disabled - No frames forwarded, no BPDUs heard
Understanding STP States
50 seconds from blocking to forwarding
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33Version 3.0
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
Can go to forward state immediately
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34Version 3.0
Module 7Spanning Tree
Protocol