Chapter 8
Layer 2 Switching and Spanning Tree
Protocol (STP)
Before Switching
Network with Switching
Switching Service
Layer 2 switches provides the following:
•
Hardware- based bridging (ASIC)
Switch uses ASIC in building and maintaining their Forward/Filter table.
•
Wire speed
Switch create a private dedicated Collision Domain (for each port) and provide independent
bandwidth on each port so increase the bandwidth for each user (unlike Hub).
•
Low latency
Switches and bridges are faster than Routers because they do not look at network layer header
information.
•
Low cost
Bridges vs. Switches
• Bridges are software base , while switches are hardware base
because they use (ASIC).
• A switch can be viewed as multi port bridge.
• Switches have higher number of ports
than bridges.
• Bridge can have only one spanning tree , while switches can have
many.
Both Bridges and Switches
• Forward layer 2 broadcast.
• Learn MAC address by examining the source address of each
frame received.
• Make forward decision based on layer 2 address.
• Break up collision domain but still have a single broadcast
domain (Switches have 1 broadcast domain BY DEFAULT).
Three switch functions at layer 2
1.
2.
3.
Address Learning
Forward / Filter decision
Loop avoidance
Address Learning.
Notice that a switch or a bridge never learns a BROADCAST or MULTICAST
address because broadcast or multicast never appear as the source address of the
frame.
Address Learning.
Switch#sh mac address-table
Vlan
Mac Address
Type
---- ---------------------1 0005.dccb.d74b
DYNAMIC
1 000a.f467.9e80
DYNAMIC
1 000a.f467.9e8b
DYNAMIC
1 000a.f467.9e8c
DYNAMIC
1 0010.7b7f.c2b0
DYNAMIC
1 0030.80dc.460b
DYNAMIC
1 0030.9492.a5dd
DYNAMIC
1 00d0.58ad.05f4
DYNAMIC
Total Mac Addresses for this criterion: 8
Ports
Fa0/1
Fa0/3
Fa0/4
Fa0/3
Fa0/3
Fa0/3
Fa0/1
Fa0/1
Switch(config)#mac-address-table static aaaa.bbbb.cccc vlan 1 int fa0/5
Redundant link between Switches Problems
1.Broadcast Storm
2. Multiple frame copies
3. Instable MAC Table
•Host_1 sends a unicast frame to Router C
•The MAC address of Router C has not been learned by either switch.
•Switches A and B learn the MAC address of Host_1 on port 0.
•The frame to Router C is flooded.
•Switches A and B incorrectly learn the MAC address of Host_1on port 1.
Switching Modes
•
•
•
Store and Forward.
Cut through (Fast forward ,Real time, Wire speed).
Fragment free (Modified Cut through)
CISCO 1900 Switch Configuration
>enable
#config t
(config)#enable password level 1 cisco
(config)#enable password level 15 cisco1
(config)#enable secret cisco
(config)#hostname 1900A
1900A(config)#ip address 192.168.10.51 255.255.255.0
1900A(config)#ip default-gateway 192.168.10.1
1900A(config)#banner motd @
This is 1900A Switch
@
1900A(config)#int fa0/12
1900A(config)#description Connection_to_HR
1900A(config)#^Z
1900A#sh ip
1900A#delete nvram
CISCO 2950 Switch Configuration
Switch>en
Switch#config t
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#hostname 2950B
2950B(config)#enable secret cisco
2950B(config)#line console 0
2950B(config-line)#password cisco
2950B(config-line)#login
2950B(config)#line vty 0 15
2950B(config-line)#password telnet
2950B(config-line)#banner motd @
Enter the text followed by the '@' to finish
This is 2950B Switch
@
2950B(config)#int f0/4
2950B(config-if)#desc connection to 2950C
2950B(config-if)#int f0/5
2950B(config-if)#desc 2nd connection to 2950C
2950B(config-if)#int vlan 1
2950B(config-if)#ip address 192.168.20.51 255.255.255.0
2950B(config-if)#no shut
2950B(config-if)#exit
2950B(config)#ip default-gateway 192.168.20.1
2950B(config)#exit
2950B#copy run start
Destination filename [startup-config]?
(enter)
Building configuration...
[OK]
CISCO 2950 Switch Configuration
2950B#erase startup
2950B#delete flash:nvram
2950B#reload
2950B#ping 192.168.30.17
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.30.17, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms
Port Security
Switch#config t
Switch(config)#int f0/1
Switch(config-if)#switchport port-security ?
aging
Port-security aging commands
mac-address
Secure mac address
maximum
Max secure addresses
violation
Security violation mode
<cr>
Switch(config-if)#switchport port-security maximum 1
Switch(config-if)# switchport port-security violation ?
protect
Security violation protect mode
restrict
Security violation restrict mode
shutdown
Security violation shutdown mode
Switch(config-if)#switchport port-security violation shutdown
Port Security
Switch(config-if)#switchport port-security mac-address sticky
Switch(config-if)#switchport port-security maximum 2
Switch(config-if)#switchport port-security violation shutdown
Spanning Tree Protocol
Bridge Protocol Data Unit (BPDU)
BID
Cost
Port
BID: Priority, MAC Address.
Cost:
B.W
10Gbps
1Gbps
100Mbps
10Mbps
Cost
2
4
19
Port: Priority, Port no.
1.
2.
3.
4.
Cost
BID
Port Priority
Port No.
100
STP Example 1
STP Example 2
STP Example 3
STP Example 4
STP Example 5
STP Example 6
STP Example 7
Switch#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address
0012.8061.0cc0
Cost
8
Port
25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay
15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address
0012.806b.6880
Hello Time 2 sec Max Age 20 sec Forward Delay
15 sec
Aging Time 300
Interface
Role Sts Cost
Prio.Nbr Type
-------------------------------- -------- --------- --- ---- ---------------Fa0/16
Altn BLK 19
128.16 P2p
Gi0/1
Root FWD 4
128.25 P2p
Switch#show spanning-tree vlan 2
Configuring Catalyst Switches
• Administrative functions (General settings)
• Configuring the IP address and subnet mask
• Setting the IP default gateway
• Setting port security
• Setting PortFast
• Enabling BPDUGuard and BPDUFilter
• Enabling UplinkFast
• Enabling BackboneFast
• Enabling RSTP (802.1w)
• Enabling EtherChannel
• Configuring an STP root switch
• Using the CNA to configure a switch
Spanning Tree PortFast
Switch(config-if)#spanning-tree portfast ?
disable
Disable portfast for this interface
trunk
Enable portfast on the interface even in trunk mode
<cr>
Switch(config-if)#exit
Switch(config)#int range fastEthernet 0/1 - 12
Switch(config-if-range)#spanning-tree portfast
BPDUGuard
Switch(config-if-range)#spanning-tree bpduguard ?
disable
Disable BPDU guard for this interface
enable
Enable BPDU guard for this interface
Switch(config-if-range)#spanning-tree bpduguard enable
BPDUFilter
Switch(config-if-range)#spanning-tree bpdufilter ?
disable
Disable BPDU filtering for this interface
enable
Enable BPDU filtering for this interface
Switch(config-if-range)#spanning-tree bpdufilter enable
UPLinkFast
Switch(config)#spanning-tree uplinkfast
Switch(config)#do show spanning-tree uplinkfast
UplinkFast is enabled
Station update rate set to 150 packets/sec.
UplinkFast statistics
----------------------Number of transitions via uplinkFast (all VLANs)
:1
Number of proxy multicast addresses transmitted (all VLANs) : 8
Name Interface List
-------------------- -----------------------------------VLAN0001 Fa0/1(fwd), Fa0/2
Use it on switches that have blocked port
BackboneFast
Switch(config)#spanning-tree backbonefast
Switch(config)#do show spanning-tree backbonefast
BackboneFast is enabled
BackboneFast statistics
----------------------Number of transition via backboneFast (all VLANs)
Number of inferior BPDUs received (all VLANs)
Number of RLQ request PDUs received (all VLANs)
Number of RLQ response PDUs received (all VLANs)
Number of RLQ request PDUs sent (all VLANs)
Number of RLQ response PDUs sent (all VLANs)
Use it on all switches
:0
:2
:0
:1
:1
:0
STP (802.1d) x RapidSTP (802.1w)
Switch(config)#spanning-tree mode ?
mst
Multiple spanning tree mode
pvst
Per-Vlan spanning tree mode
rapid-pvst
Per-Vlan rapid spanning tree mode
Switch(config)#spanning-tree mode rapid-pvst
EtherChannel
Switch(config)#int port-channel 1
Switch(config-if)#int range f0/1-2
Switch(config-if-range)#switchport mode trunk
1d03h: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 FastEthernet0/2
moved to Forwarding (UplinkFast).
Switch(config-if-range)#switchport nonegotiate
Switch(config-if-range)#channel-group 1 mode desirable
Switch(config-if-range)#do sh int fa0/1 etherchannel
Port state
= Up Sngl-port-Bndl Mstr Not-in-Bndl
Channel group
=1
Mode
= Desirable-Sl
Gcchange
=0
Port-channel
= null
GC
= 0x00010001
Pseudo port-channel = Po1
Port index
=0
Load
= 0x00
Protocol
= PAgP
[output cut]
Selecting Root Bridge
Switch(config)#spanning-tree vlan 1 priority ?
<0-61440> bridge priority in increments of 4096
Switch(config)#spanning-tree vlan 1 root ?
primary
Configure this switch as primary root for this spanning tree
secondary
Configure switch as secondary root
Switch(config)#spanning-tree vlan 1 root primary