Download - Ccna3 3.0-07 Spanning Tree Protocol
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
1/34
1Version 3.0
Module 7Spanning Tree
Protocol
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
2/34
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 theactive loop free topology do not participate in the
forwarding of data frames.
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
3/34
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 byredundancy.
Example of having 1 car versus 2 cars1is always availableredundancy
So companies should:
elim inate sing le po ints o f fai lure and
design alternate rou tes to a dest inat ion
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
4/34
4Version 3.0
Reliability and 24x7 network
demands have compelled LAN
designers to construct multiple
paths between user and resource
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
5/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
6/34
6Version 3.0
Redundant Paths and No
Spanning Tree. . .
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
7/34
00-A1 BB-44
AA-11 CC-23
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2
00-A1
Port 1 Port 1
Port 2 Port 2
LAN Switch 1 LAN Switch 2
(1) 00-A1 sends frame to CC-23
Switch 1 and Switch 2 learn about 00-A1
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
8/34
00-A1 BB-44
AA-11 CC-23
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2
00-A1
Port 1 Port 1
Port 2 Port 2
LAN Switch 1 LAN Switch 2
(2) LAN Switch 1 Floods packet out Port 2
since CC-23 is not known
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
9/34
00-A1 BB-44
AA-11 CC-23
SAT
Port 1 Port 2
00-A1
SAT
Port 1 Port 2 00-A1
Port 1 Port 1
Port 2 Port 2
LAN Switch 1 LAN Switch 2
(3) LAN Switch 2 learns (incorrectly) that
(Source MAC ) 00-A1 is on Port 2
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
10/3410
Version 3.0
Or, A Broadcast
Storm. . .
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
11/3411
Version 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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
12/3412
Version 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 As.
Host B
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
13/3413
Version 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
Duplicate
frameDuplicate
frame
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
14/34
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
DuplicateFrame
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
15/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
16/34
16Version 3.0
Redundant Topology
The traffic that switches flood out all ports can becaught in a loop, because in the Layer 2 headerthere is no TTL.
(Remember that in Layer 3 the TTL is
decremented and the packet is discarded whenthe TTL reaches 0)
You need switching (bridging) for reliability, butnow the problem of loopsa switched network
cannot have loops if it is to do what it issupposed to do.
Solution? Allow physical loops, but create a loop-free topology
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
17/34
17Version 3.0
Spanning Tree
Protocol
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
18/34
18Version 3.0
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
19/34
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 logicalloop-free
topology
A
B
C
H
J
IE
G
FD
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
20/34
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 thenetwork at regular intervals and are used to determine
the spanning tree topology.
These BPDUs are used to determ ine the sho rtest path
to the root br idge, and wh ich po rts wi l l forward framesas part of the spanning tree BPDUs sent out every 2
seconds
A separate instance of STP runs within each
configured VLAN.
Spanning Tree Algorithm
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
21/34
21Version 3.0
Spanning Tree
For every switchednetwork:
One root bridge
One root port per
non root bridgeOne designated
port per segment
Unused, non-
designated ports
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
22/34
22Version 3.0
Step 1: Electing a Root Bridge
Bridge Priority
Bridge ID
Root BridgeStep 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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
23/34
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)
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
24/34
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 BPDUs to accomplishthis.
All switches consider themselves as the
root bridge until they find out otherwise.
All Cisco Catalyst switches have the
default Bridge priority of 32768.
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
25/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
26/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
27/34
27Version 3.0
In case of a tie, the Bridge ID is used
Bridge ID The Bridge ID is the MAC addressassigned
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.
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
28/34
28Version 3.0
Catalyst 1900 Management ConsoleCopyright (c) Cisco Systems, Inc. 1993-1998
All rights reserved.Enterprise Edition Software
Ethernet Address: 00-B0-64-26-6D-00
PCA Number: 73-3122-04PCA Serial Number: FAB03503222Model Number: WS-C1912-EN
System Serial Number: FAB0351U08MPower Supply S/N: PHI033301VQPCB Serial Number: FAB03503222,73-3122-04
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
29/34
29Version 3.0
A B
A B
A B
1
1
A
B
C
10BaseT Ports (12)
10BaseT Ports (24)
10BaseT Ports (24)
100BaseTPorts
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?
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
30/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
31/34
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
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
32/34
32Version 3.0
Blocking- No frames forwarded, BPDUsreceived
Listening- No frames forwarded,
listening for frames
Learning- No frames forwarded, but
learning MAC addresses
ForwardingReceiving BPDUs,
Forwarding data traffic, receiving datatraffic, learns MAC addresses
Disabled- No frames forwarded, no
BPDUs heard
Understanding STP States
50 seconds
from
blocking to
forwarding
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
33/34
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 stateimmediately
-
8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol
34/34
Module 7Spanning Tree
Protocol