PT Activity 5.5.2: Challenge Spanning
Tree Protocol
Topology Diagram
Addressing Table
Device
|
Interface
|
IP Address
|
Subnet Mask
|
Default Gateway
|
S1
|
VLAN 99
|
172.17.99.11
|
255.255.255.0
|
N/A
|
S2
|
VLAN 99
|
172.17.99.12
|
255.255.255.0
|
N/A
|
S3
|
VLAN 99
|
172.17.99.13
|
255.255.255.0
|
N/A
|
PC1
|
NIC
|
172.17.10.21
|
255.255.255.0
|
172.17.10.12
|
PC2
|
NIC
|
172.17.20.22
|
255.255.255.0
|
172.17.20.12
|
PC3
|
NIC
|
172.17.30.23
|
255.255.255.0
|
172.17.30.12
|
Port
Assignments – S2
Ports
|
Assignment
|
Network
|
Fa0/1
- 0/5
|
802.1q Trunks (Native VLAN 99)
|
172.17.99.0 /24
|
Fa0/6 - 0/10
|
VLAN 30 – Guest(Default)
|
172.17.30.0 /24
|
Fa0/11 - 0/17
|
VLAN 10 – Faculty/Staff
|
172.17.10.0 /24
|
Fa0/18 - 0/24
|
VLAN 20 - Students
|
172.17.20.0 /24
|
All contents are Copyright © 1992–2007 Cisco Systems, Inc.
All rights reserved. This document is Cisco Public
Information. Page 1 of 11
Learning Objectives
Perform basic switch
configurations.
Configure the Ethernet interfaces on the host PCs.
Configure VLANs.
Configure spanning tree.
Optimizing STP.
Introduction
In this activity, you
will perform basic switch
configurations, configure addressing on PCs, configure
VLANs, examine the Spanning Tree Protocol and learn how to optimize it.
Task 1: Perform
Basic Switch Configurations
Configure the S1, S2, and S3 switches
according to the following guidelines and save all your configurations:
Configure the switch
hostname as indicated on the topology.
Disable DNS lookup.
Configure an encrypted privileged EXEC mode password of class.
Configure a password of cisco for
console connections.
Configure a password of cisco for vty connections.
Switch>enable
Switch#configure terminal
Enter configuration commands,
one per line. End with CNTL/Z.
Switch(config)#hostname S1
S1(config)#enable
secret class S1(config)#no ip domain-lookup S1(config)#line
console 0
S1(config-line)#password
cisco
S1(config-line)#login
S1(config-line)#line vty
0 15
S1(config-line)#password
cisco
S1(config-line)#login
S1(config-line)#end
%SYS-5-CONFIG_I: Configured
from console by console
S1#copy running-config
startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
Your completion results should
be 18 percent. If not, check for missing configuration statements.
Task 2: Configure
the Ethernet Interfaces on the Host PCs
From the Desktop tab,
use the IP Configuration window
to configure the Ethernet interfaces of PC1, PC2, and PC3 with the IP address,
subnet mask, and gateway indicated in the addressing table.
Your completion results should
be 26 percent. If
not, check for missing configuration steps.
Task 3: Configure VLANs
Step 1. Enable the user ports on
S2 in access mode.
Refer to the topology
diagram to determine which switch
ports on S2 are activated for end-user device access. These three ports will
be configured for access mode
and enabled with the no shutdown
command.
S2(config)#interface fa0/6
S2(config-if)#switchport mode access
S2(config-if)#no shutdown
S2(config-if)#interface fa0/11
S2(config-if)#switchport mode access
S2(config-if)#no shutdown
S2(config-if)#interface fa0/18
S2(config-if)#switchport mode access
S2(config-if)#no shutdown
Step 2. Configure VTP.
Configure VTP on the three switches
using the following table. Remember that VTP
domain names and passwords are case-sensitive. The default
operating mode is server.
Switch Name
|
VTP Operating Mode
|
VTP Domain
|
VTP
Password
|
S1
|
Server
|
Lab5
|
cisco
|
S2
|
Client
|
Lab5
|
cisco
|
S3
|
Client
|
Lab5
|
cisco
|
S1(config)#vtp mode server Device mode
already VTP SERVER. S1(config)#vtp domain Lab5
Changing VTP domain name from NULL to Lab5
S1(config)#vtp password cisco
Setting device VLAN database password to
cisco
S1(config)#end
S2(config)#vtp mode client
Setting device to VTP CLIENT mode
S2(config)#vtp domain Lab5
Changing VTP domain name from NULL to Lab5
S2(config)#vtp password cisco
Setting device VLAN database password to
cisco
S2(config)#end
S3(config)#vtp mode client
Setting device to VTP CLIENT mode
S3(config)#vtp domain Lab5
Changing VTP domain name from NULL to Lab5
S3(config)#vtp password cisco
Setting device VLAN database password to
cisco
S3(config)#end
Step 3. Configure
Trunk Links and Native VLAN.
Configure
trunking ports and native VLAN. For each switch, configure ports Fa0/1
through Fa0/5 as trunking
ports. Designate VLAN 99 as the native VLAN for
these trunks. When this activity was started, these ports were disabled and must
be re-enabled now using the no shutdown command.
Only the commands for the FastEthernet0/1
interface on each switch are shown,
but the commands should be applied up to the
FastEthernet0/5 interface.
S1(config)#interface fa0/1
S1(config-if)#switchport mode trunk
S1(config-if)#switchport trunk native vlan 99
S1(config-if)#no shutdown
S1(config)#end
S2(config)#interface fa0/1
S2(config-if)#switchport mode trunk
S2(config-if)#switchport trunk native vlan 99
S2(config-if)#no shutdown
S2(config-if)#end
S3(config)#interface fa0/1
S3(config-if#switchport mode trunk
S3(config-if)#switchport trunk native vlan 99
S3(config-if)#no shutdown
S3(config-if-#end
Step 4. Configure
the VTP server with VLANs.
VTP allows you
to configure VLANs on the VTP server and
have those VLANs populated to the VTP
clients in the domain. This ensures consistency in
the VLAN configuration across the
network.
Configure the following VLANS on the VTP
server:
VLAN
|
VLAN Name
|
VLAN 99
|
management
|
VLAN 10
|
faculty-staff
|
VLAN 20
|
students
|
VLAN 30
|
guest
|
S1(config)#vlan 99
S1(config-vlan)#name management
S1(config)#vlan 10
S1(config-vlan)#name faculty-staff
S1(config)#vlan 20
S1(config-vlan)#name students
S1(config)#vlan 30
S1(config-vlan)#name guest
S1(config-vlan)#end
Step 5. Verify the VLANs.
Use the show vlan brief command on S2 and S3 to verify that all four VLANs have been distributed to the client switches.
S2#show vlan brief
VLAN Name Status Ports
---- -------------------------------- ---------
-----------------------------
1
default
active Fa0/1, Fa0/2, Fa0/4,
Fa0/5
Fa0/6, Fa0/7, Fa0/8, Fa0/9
Fa0/10, Fa0/11, Fa0/12,Fa0/13
Fa0/14, Fa0/15, Fa0/16,Fa0/17
Fa0/18, Fa0/19, Fa0/20,Fa0/21
Fa0/22, Fa0/23, Fa0/24, Gi0/1
Gi0/2
10
faculty/staff
active
20
students
active
30 guest active
99
management
active
S3#show vlan brief
VLAN Name Status Ports
---- -------------------------------- ---------
-----------------------------
1
default active Fa0/1, Fa0/2, Fa0/4, Fa0/5
Fa0/6, Fa0/7, Fa0/8, Fa0/9
Fa0/10, Fa0/11, Fa0/12,Fa0/13
Fa0/14, Fa0/15, Fa0/16,Fa0/17
Fa0/18, Fa0/19, Fa0/20,Fa0/21
Fa0/22, Fa0/23, Fa0/24, Gi0/1
Gi0/2
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
Step 6. Configure
the management interface address
on all three switches.
S1(config)#interface vlan99
S1(config-if)#ip address 172.17.99.11
|
255.255.255.0
|
S2(config)#interface vlan99
S2(config-if)#ip address 172.17.99.12
|
255.255.255.0
|
S3(config)#interface vlan99
S3(config-if)#ip address 172.17.99.13
|
255.255.255.0
|
Verify that the switches are correctly configured by pinging between them. From S1, ping the management interface on S2 and S3. From S2, ping the management interface on S3.
Were
the pings successful?
Yes, the pings are succesful.
If not, troubleshoot the switch configurations and try again.
Step 7. Assign switch ports
to the VLANs.
Port assignments
are listed in the table at the beginning
of the activity. However, since
Packet Tracer
4.11 does
not support the interface
range command, only assign
the first port from each range.
S2(config)#interface fa0/6
S2(config-if)#switchport access vlan 30
S2(config-if)#interface fa0/11
S2(config-if)#switchport access vlan 10
S2(config-if)#interface fa0/18
S2(config-if)#switchport access vlan 20
S2(config-if)#end
S2#copy running-config startup-config
Destination filename
[startup-config]? [enter] Building configuration...
[OK] S2#
Your completion results should
be 99 percent. If not, check for missing configuration
statements.
Task 4: Configure
Spanning Tree
Step 1. Examine the
default configuration of 802.1D Spanning Tree Protocol (STP).
On each switch, display the spanning tree table with the show spanning-tree command. The output is shown for S1 only. Root selection
varies depending on the default BID of each switch. In this activity S3 is currently the root.
S1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID
Priority 32769
Address
0030.F20D.D6B1
Hello Time 2
sec Max Age 20 sec Forward Delay 15 sec
Bridge
ID Priority 32769
(priority 32768 sys-id-ext 1) Address 0050.0F68.146E
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- --------------------------------
Fa0/1 Root FWD 19 128.3
Shr Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Desg FWD
19 128.3
Shr Fa0/4 Desg FWD
19 128.3 Shr
VLAN0010
Spanning tree enabled protocol ieee
Root ID
Priority 32778
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge
ID Priority 32778 (priority 32768 sys-id-ext 10) Address 0050.0F68.146E
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
-------------------------------- Fa0/1 Root FWD 19 128.3
Shr
Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Desg FWD
19 128.3 Shr Fa0/4 Desg FWD 19 128.3
Shr
VLAN0020
Spanning tree enabled protocol ieee
Root ID
Priority 32788
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID
Priority 32788 (priority 32768 sys-id-ext 20)
Address 0050.0F68.146E Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
-------------------------------- Fa0/1 Root FWD 19 128.3
Shr
Fa0/2 Altn BLK 19 128.3
Shr
Fa0/3 Desg FWD 19 128.3
Shr
Fa0/4 Desg FWD 19 128.3
Shr
VLAN0030
Spanning tree enabled protocol ieee
Root ID
Priority 32798
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID
Priority 32798 (priority 32768 sys-id-ext 30)
Address
0050.0F68.146E Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- --------------------------------
Fa0/1 Root FWD 19 128.3
Shr Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Desg FWD
19 128.3 Shr Fa0/4
Desg FWD 19 128.3
Shr
VLAN0099
Spanning tree enabled protocol ieee
Root ID
Priority 32867
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge
ID Priority 32867
(priority 32768 sys-id-ext 99) Address 0050.0F68.146E
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
-------------------------------- Fa0/1 Root FWD 19 128.3
Shr
Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Desg FWD
19 128.3 Shr Fa0/4 Desg FWD 19 128.3
Shr
Note that there are
five instances of STP on each switch. Examine the VLAN 99
spanning tree for all three switches: S1#show spanning-tree vlan 99
VLAN0099
Spanning tree enabled protocol ieee
Root ID
Priority 32867
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge
ID Priority 32867
(priority 32966 sys-id-ext 99) Address 0050.0F68.146E
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
-------------------------------- Fa0/1 Root FWD 19 128.3
Shr
Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Desg FWD
19 128.3 Shr Fa0/4 Desg FWD 19 128.3
Shr
S2#show spanning-tree
vlan 99
VLAN0099
Spanning tree enabled protocol
ieee
Root ID
Priority 32867
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32867
(priority 32966 sys-id-ext 99) Address 00E0.F7AE.7258
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- -------------------------------- Fa0/1 Root FWD 19 128.3
Shr
Fa0/2 Altn BLK 19 128.3
Shr Fa0/3 Altn BLK 19 128.3
Shr Fa0/4 Altn BLK
19 128.3 Shr
S3#show spanning-tree
vlan 99
VLAN0099
Spanning tree
enabled protocol ieee
Root ID
Priority 32867
Address
0030.F20D.D6B1
This bridge is the root
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge
ID Priority 32867
(priority 32966 sys-id-ext 99) Address 0030.F20D.D6B1
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
-------------------------------- Fa0/1 Desg FWD 19 128.3
Shr
Fa0/2 Desg FWD 19 128.3
Shr
Fa0/3 Desg FWD 19 128.3
Shr
Fa0/4 Desg FWD 19 128.3
Shr
Step 2. Examine the
output.
Answer the following questions based on the output.
What is the priority
for switches S1, S2, and S3 on VLAN
99?
S1=32867,
S2=32867, S3=32867
What is the priority for S1 on VLANs 10, 20,
30, and 99?
VLAN10 =
32867, VLAN20 = 32867, VLAN30 = 32867, VLAN99 = 32867
Which
ports are blocking
VLAN 99 on the root switch?
none
Which
ports are blocking
VLAN 99 on the non-root switches?
varies, but using
default settings, one port is blocking
on one non-root switch, and three
ports are blocking on the other
non-root switch
How
does STP select the root?
Lowest bridge ID
Since the bridge priorities are
all the same, what else does the switch use to determine the root?
Switch MAC address
Task 5: Optimizing STP
Because
there is a separate instance of the spanning tree
for every active VLAN, a separate root election is conducted for each instance. If the default switch
priorities are used
in root selection, the same root is elected for every spanning tree, as we have seen. This
could lead to an inferior design.
Some reasons to control the
selection of the root switch
include:
The root
switch is responsible
for generating BPDUs in STP 802.1D and is the focal point for
spanning tree control
traffic. The root switch must be capable of handling this additional
load.
The placement of the root defines the active switched paths in the network. Random placement is likely to lead to suboptimal paths. Ideally the root is in the
distribution layer.
Consider the
topology used
in this activity. Of the
six trunks configured, only two are carrying traffic. While this prevents loops, it is a waste of resources.
Because the root can be
defined on the basis of the VLAN, you can have some ports
blocking for one VLAN and forwarding
for another. This is demonstrated
below.
In this example, it has been determined that the root selection using
default values has led to under-
utilization of the available switch
trunks. Therefore, it is necessary
to force another switch to become
the root switch for VLAN 99 to impose
some load-sharing on the trunks.
In the example output below, the
default root switch for all VLANs is S3.
Selection of the root switch is accomplished by changing
the spanning-tree priority for the
VLAN. The default priority, as you have observed, is 32768
plus the VLAN ID. The lower number
indicates a higher priority for root selection. Set the priority for VLAN 99 on S1 to 4096.
S1(config)#spanning-tree vlan 99 priority
4096
S1(config)#exit
Give the switches a little time to recalculate the
spanning tree and then check the
tree for VLAN 99 on switch S3 (the
original VLAN 99 root) and switch S1 (the non-root
switch selected to become the
new VLAN 99 root).
S3#show spanning-tree vlan 99
VLAN0099
Spanning tree enabled protocol ieee
Root ID
Priority 4195
Address 0050.0F68.146E
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID
Priority 32867 (priority 32966 sys-id-ext 99)
Address
0030.F20D.D6B1
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- --------------------------------
Fa0/4 Desg FWD 19 128.3
Shr Fa0/1 Root FWD 19 128.3
Shr Fa0/2 Altn BLK
19 128.3 Shr Fa0/3 Desg FWD 19 128.3
Shr
S1#show spanning-tree
vlan 99
VLAN0099
Spanning tree enabled protocol ieee
Root ID
Priority 4195
Address
0050.0F68.146E This bridge is the root
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 4195
(priority 4294 sys-id-ext 99) Address 0050.0F68.146E
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- -------------------------------- Fa0/4 Desg FWD 19 128.3
Shr
Fa0/3 Desg FWD 19 128.3
Shr Fa0/2 Desg FWD 19 128.3
Shr Fa0/1 Desg FWD
19 128.3 Shr
Which switch is the root for VLAN 99?
S1
Which
ports are blocking VLAN
99 traffic on the new root?
None
Which
ports are now blocking VLAN 99 traffic on the old root?
fa0/2
Compare the S1 VLAN 99 spanning tree above
with the S1 VLAN 10 spanning tree.
S1#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID
Priority 32778
Address
0030.F20D.D6B1
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778
(priority 32788 sys-id-ext 10) Address 0050.0F68.146E
Aging Time 300
Interface
Role Sts Cost Prio.Nbr Type
---------------- ---- ---
--------- -------- -------------------------------- Fa0/4 Desg FWD 19 128.3
Shr
Fa0/3 Desg FWD 19 128.3
Shr Fa0/2 Altn BLK
19 128.3 Shr Fa0/1 Root FWD 19 128.3
Shr
Note that S1 can now use all four ports for VLAN 99 traffic as long as they are not blocked at the
other
end of the trunk. However, the original spanning tree topology, with one of four S1
ports in blocking mode,
is still in place for the four other
active VLANs. By configuring groups of VLANs to use different
trunks as their primary forwarding
path, we retain the redundancy of failover trunks, without having to leaves trunks totally
unused.
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