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Chapter 6Catalyst Switch
Operations
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Address learning Forward/filter decision Loop avoidance
Three Switch Functions
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How Switches Learn HostLocations
Initial MAC address table is empty
MAC address table
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3
A B
C D
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How Switches Learn HostsLocations
Station A sends a frame to Station C Switch caches station A MAC address to port E0 by learning
the source address of data frames
The frame from station A to station C is flooded out to allports except port E0 (unknown unicasts are flooded)
MAC address table
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0: 0260.8c01.1111
E0 E1
E2 E3DC
BA
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How Switches Learn HostLocations
Station D sends a frame to station C Switch caches station D MAC address to port E3 by learning the
source Address of data frames
The frame from station D to station C is flooded out to all portsexcept port E3 (unknown unicasts are flooded)
MAC address table
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0: 0260.8c01.1111E3: 0260.8c01.4444
E0 E1
E2 E3 DC
A B
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How Switches Filter Frames
Station A sends a frame to station C
Destination is known, frame is not flooded
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3
XX
DC
A B
MAC address table
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Broadcast and Multicast Frames
Station D sends a broadcast or multicast frame
Broadcast and multicast frames are flooded toall ports other than the originating port
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3 DC
A B
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
MAC address table
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Broadcast Storms
Segment 1
Segment 2
Server/host X Router Y
Broadcast
Switch A Switch B
Host X sends a Broadcast
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Broadcast Storms
Segment 1
Segment 2
Server/host X Router Y
Broadcast
Switch A Switch B
Host X sends a Broadcast
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Broadcast Storms
Segment 1
Segment 2
Server/host XRouter Y
Broadcast
Switches continue to propagate broadcast traffic overand over
Switch A Switch B
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Multiple Frame Copies
Segment 1
Segment 2
Server/host X Router YUnicast
Switch A Switch B
Host X sends an unicast frame to router Y Router Y MAC address has not been learned by either
switch yet
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Solution: Spanning-Tree Protocol
Provides a loop free redundant network topology byplacing certain ports in the blocking state
Blockx
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Spanning-Tree Operations
One root bridge per network One root port per nonroot bridge One designated port per segment
x
Designated port (F) Root port (F)
Designated port (F) Nondesignated port (B)
Root bridge Nonroot bridgeSW X SW Y
100baseT
10baseT
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Switch YDefault priority 32768(8000 hex)MAC 0c0022222222
Switch XDefault priority 32768(8000 hex)MAC 0c0011111111
Spanning-Tree ProtocolRoot Bridge Selection
BPDU
BPDU = Bridge protocol data unit(default = sent every 2 seconds)
Root bridge = Bridge with the lowest bridge IDBridge ID = Bridge priority + bridge MAC addressIn the example, which switch has the lowest bridge ID?
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Switch YDefault priority 32768MAC 0c0022222222
Switch XDefault priority 32768MAC 0c0011111111
Spanning-Tree ProtocolPort States
Root bridge
x
Port 0
Port 1
Port 0
Port 1
100baseT
10baseT
Designated port (F) Root port (F)
Nondesignated port (B)Designated port (F)
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Spanning-Tree ProtocolPath Cost
Link Speed Cost (reratify IEEE spec) Cost (previous IEEE spec)----------------------------------------------------------------------------------------------------
10 Gbps 2 11 Gbps 4 1100 Mbps 19 1010 Mbps 100 100
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Switch YMAC 0c0022222222Default priority 32768
Switch XMAC 0c0011111111Default priority 32768
Port 0
Port 1
Port 0
Port 1
Switch ZMac 0c0011110000Default priority 32768
Port 0
Can you figure out: What is the root bridge? What are the designated, nondesignated, and root parts? Which are the forwarding and blocking ports?
100baseT
100baseT
Spanning-Tree:
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Switch YMAC 0c0022222222Default priority 32768
Switch XMAC 0c0011111111Default priority 32768
Port 0
Port 1
Port 0
Port 1
Switch ZMac 0c0011110000Default priority 32768
Port 0
Can you figure out: What is the root bridge? What are the designated, nondesignated, and root parts? Which are the forwarding and blocking ports?
100baseT
100baseT
Spanning-Tree:
Designated port (F)
Root port (F)
Nondesignated port (BLK)Designated port (F)
Root port (F)
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Blocking
Listening
Learning
Forwarding
Spanning-Tree Port States
Spanning-tree transitions each portthrough several different state:
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