CCNA Slides Chapter 6
Chapter 6 Catalyst Switch Operations
Objectives
Upon completion of this chapter, you will be able to perform the following tasks:?Describe Layer 2 switching (bridging) operations
?Describe the Catalyst 1900 switch operations ?Describe the Catalyst 1900 switch default configuration
?Configure Catalyst 1900 switch
?Use show commands to verify Catalyst 1900 switch configuration and operations
?Address learning ?Forward/filter decision ?
Loop avoidance Three Switch Functions
How Switches Learn Host
Locations
?Initial MAC address table is empty MAC address table
0260.8c01.11110260.8c01.22220260.8c01.3333
0260.8c01.4444
E0E1E2E3A B
C D
How Switches Learn Hosts
Locations
?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 all
ports except port E0 (unknown unicasts are flooded)
MAC address table 0260.8c01.11110260.8c01.22220260.8c01.3333
0260.8c01.4444
E0: 0260.8c01.1111
E0E1E2E3D
C B
A
How Switches Learn Host
Locations
?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 ports except port E3 (unknown unicasts are flooded)MAC address table
0260.8c01.11110260.8c01.22220260.8c01.33330260.8c01.4444
E0: 0260.8c01.1111
E3: 0260.8c01.4444
E0
E1E2E3D
C A B
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: 0260.8c01.3333
E3: 0260.8c01.44440260.8c01.11110260.8c01.22220260.8c01.33330260.8c01.4444
E0
E1E2E3X X D
C A B
MAC address table
Broadcast and Multicast
Frames
?Station D sends a broadcast or multicast frame
?Broadcast and multicast frames are flooded to
all ports other than the originating port 0260.8c01.11110260.8c01.22220260.8c01.33330260.8c01.4444
E0
E1E2E3D
C A
B
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444MAC address table
Redundant Topology
Server/host X Router Y
Segment 1
Segment 2
?Redundant topology eliminates single points of failure ?Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems
Broadcast Storms
Server/host X Router Y
Segment 1 Broadcast
Switch A Switch B
Segment 2 Host X sends a Broadcast
Broadcast Storms
Server/host X Router Y
Segment 1 Broadcast
Switch A Switch B
Segment 2 Host X sends a Broadcast
Broadcast Storms
Segment 1
Segment 2 Server/host X Router Y
Broadcast
Switches continue to propagate broadcast traffic over and over
Switch A Switch B
Segment 1
Segment 2
Server/host X Router Y
Unicast 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
Segment 1Segment 2
Server/host X
Router Y Unicast
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
?Router Y will receive two copies of the same frame
Unicast Unicast
Segment 1Segment 2
Server/host X
Router Y Unicast
Unicast 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
?Switch A and B learn Host X MAC address on port 0
Port 0
Port 1Port 0Port 1
Segment 1Segment 2
Server/host X
Router Y Unicast
Unicast 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 ?
Switch A and B learn Host X MAC address on port 0?
Frame to Router Y is flooded ?Switch A and B incorrectly learn Host X MAC address on port 1
Port 0Port 1Port 0
Port 1
?Complex topology can cause multiple loops to occur
?
Layer 2 has no mechanism to stop the loop
Server/host
Workstations
Loop
Loop
Loop
Multiple Loop Problems
Broadcast
Solution: Spanning-Tree Protocol
Provides a loop free redundant network topology by placing certain ports in the blocking state Block
x
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 bridge SW X SW Y
100baseT
10baseT
Switch Y Default priority 32768 (8000 hex)MAC 0c0022222222Switch X
Default priority 32768
(8000 hex)
MAC 0c0011111111Spanning-Tree Protocol
Root Bridge Selection
BPDU
BPDU = Bridge protocol data unit
(default = sent every 2 seconds)
Root bridge = Bridge with the lowest bridge ID
Bridge ID = Bridge priority + bridge MAC address
In the example, which switch has the lowest bridge ID?