Cisco lab, guide to configure interface stp attributes

1. Configuring Interface STP Attributes
Real World Application & Core Knowledge
Now that you are familiar with the basic operation of PVST, Rapid-PVST and MST its
time to take another step down the sidewalk of complex avenue. There are several
interface configuration commands specific to spanning tree on the Cisco Catalyst
Series switches. This lab will discuss and demonstrate the following features; STP
bpdufilter, STP interface cost, STP interface link-type and STP interface port-priority.
Let’s first started with Spanning-Tree BPDUFilter. This feature is quite useful in some
scenarios where you do not want send or process received BPDU’s out a particular
port on a Cisco switch. The use of this command can however be very dangerous as a
wrongful configuration can result in a layer two switching loop until spanning-tree
re-converges based on the new information.
An example scenario for using BPDUFilter could be that your corporate policy
mandates that host ports on the network should never receive BPDU’s from the
access switches. To abide by this policy BPDUFilter must be enabled. This prevents
the sending and processing received BPDU’s. Take not that in a policy requirement
like this you want to ensure that no rouge switches are connected to the network
which is done by BPDU Guard as previously discussed in Lab 4-17 – Configuring
Switchport BPDU Guard.
The spanning-tree cost interface configuration will statically set the cost of the
interface so that it modifies the spanning-tree root port election process. On a switch
with multiple equal cost links to the root bridge, setting the cost statically on a
switchport can be the tie breaker to determine which port becomes the root port.
The spanning-tree link-type interface configuration statically configures the link-type
on a per interface basis. The Rapid in Rapid Spanning Tree Protocol makes this
concept quite simplistic. By default the link type is derived from a ports duplex. Full
duplex is considered a point to point link whereas half duplex is considered a “shared”
medium. So why is Rapid, rapid on rapid-spanning-tree? The old 802.1d standard
took up to 45 seconds to set the port in forwarding mode, whereas 802.1s now
transitions the port to designated (forwarding) very quickly. If a link-type is set to p2p
then the RSTP does its think and quickly transitions the port into forwarding however
if the link-type is half-duplex or configured as “shared” in interface configuration
mode by using the spanning-tree link-type shared then the switch does not
transition the port quickly. It goes through the entire process of determining rather
or not a port poses a potential layer two network loop.
Configuring spanning-tree port-priority in interface configuration mode statically
configures the port-priority used as a tie breaker for switches with multiple
redundant links to a particular network segment where the root bridge can be
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2. reached.. So the question is, is if you have multiple links to a root bridge from a single
switch and each link has the exact same cost, how does the switch know which link
to use as the “root” port? The tie breaker is done by port-priority. If you do a show
spanning-tree vlan # you’ll notice that Prio.Nbr is between cost and type. The
priority is by default set to 128 on all Catalyst series switches and the Nbr is the port
number. For example SW1 and SW2 are connected via Fa0/10, Fa0/11 and Fa0/12. All
ports have the same cost so the port-priority will determine which port becomes the
root, which in this case Port Fa0/10 will be the root port, Fa0/11 and Fa0/12 will be
alternate ports (blocking).
Now that you have a basic understanding of some of the STP interface specific
configurations you need to familiarize yourself with the following commands below;
Command Description
This command is executed in interface configuration mode and enables
spanning-tree BPDUFilter which disables sending and processing received BPDU
bpdufilter frames on the interface.
This command is executed in interface configuration mode and statically
sets the interface cost used for manipulating the root path in a given
spanning-tree cost # spanning-tree topology.
This command is executed in interface configuration mode and statically
spanning-tree sets the interface link type. This command manipulates the the ports
link-type p2p | shared rapid transition processing.
This command is executed in interface configuration mode and statically
sets the interface port-priority in spanning-tree to manipulate the
spanning-tree election of the root port when multiple equal cost links in a given
port-priority network segment exist.
This command is executed in privileged mode to view the current
show spanning-tree spanning-tree properties on a per vlan basis. Used to view root port,
vlan # alternate port(s), cost, port-priority and port-type.
Lab Prerequisites
If you are using GNS3 than load the Free CCNA Workbook GNS3 topology
than start devices; SW1, SW2 and SW3.
Establish a console session with devices SW1, SW2 and SW3 than configure
the devices respected hostname(s).
Configure all switches to run Rapid-PVST.
Configure interfaces Fa0/10 and Fa0/11 on both SW1 and SW2 to trunk.
Configure interfaces Fa0/13 and Fa0/14 on SW1 to trunk then configure
interfaces Fa0/10 and Fa0/11 on SW3 to trunk.
Configure interfaces Fa0/13 and Fa0/14 on both SW2 and SW3 to trunk.
For this lab you’ll only need to use VLAN 1, so remove all other VLANs and
configure SW1 as the root bridge for VLAN 1.
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3. Lab Objectives
Configure BPDUFilter on SW2 interface Fa0/10 then verify it by using BPDU
Guard on SW1 Fa0/10. Once completed remove the BPDUFilter and
BPDUGuard before proceeding.
Configure SW3 to use Fa0/14 as the root port in the spanning-tree by using
manipulating the interface cost; use a cost lower then the default
FastEthernet interface cost. Afterward, configure SW2 to use interface Fa0/11
as the root port to SW1. Verify your configuration; once verified remove the
interface costs before proceeding.
SW3 Interface Fa0/10 is connected to a 10/100Mbps hub then the hub is
connected to SW1. Configure the link type accordingly and verify your
configuration.
Assuming SW2′ s interface Fa0/10 and Fa0/11 are configured with their
defaults for spanning tree, the root port will become Fa0/10. Influence
spanning-tree to use interface Fa0/11 and do not use the cost command.
Lab Instruction
Step1. – Configure BPDUFilter on SW2 interface Fa0/10 then verify it by using BPDU
Guard on SW1 Fa0/10. Once completed remove the BPDUFilter and BPDUGuard
before proceeding.
To configure bpdufilter you’ll use the spanning-tree bpdufilter enable command in
interface configuration mode as shown below;
SW2 con0 is now available
Press RETURN to get started.
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4. SW2>enable
SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface fa0/10
SW2(config-if)#spanning-tree bpdufilter enable
To verify that BPDUFilter is operating properly you can enable BPDUGuard on SW1
interface Fa0/10 then bounce the interface. As previously stated in the lab
BPDUFilter prevents transmitting and processing received BPDU’s on a particular port.
Since no BPDU’s will be sent out SW2 interface Fa0/10 then SW1 Fa0/10 will not shut
down as BPDU’s wont be detected as shown below;
SW1 con0 is now available
Press RETURN to get started.
SW1>enable
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5. SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface fa0/10
SW1(config-if)#spanning-tree bpduguard enable
SW1(config-if)#shutdown
SW1(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/10, changed state to
administratively down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/10,
changed
state to down
SW1(config-if)#no shut
SW1(config-if)#
%LINK-3-UPDOWN: Interface FastEthernet0/10, changed state to up
SW1(config-if)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/10,
changed
state to up
SW1(config-if)#exit
SW1(config)#exit
As shown above you can see that the interface Fa0/10 did not go into err-disabled
state as no BPDU’s were received since Fa0/10 on SW2 is configured to filter BPDU’s
(Not send them).
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6. But however if you check over on SW3, you’ll notice that all interfaces have been
shutdown into Err-Disabled state as SW3 has detected a loop in the network.
Ethernet by default sends a loopback keepalive out each interface every ten seconds.
If this keepalive is received back on the same interface the the interface goes into
Err-Disabled mode as a physical topology loop has been detected.
This occurs due to spanning-tree not blocking that interface and forwarding all traffic
out the interface. Since this happens frames loop back around in the network and
SW3 detects it.
You’ll need to remove BPDUGuard off SW1 Fa0/10 and BPDUFilter off SW2 Fa0/10,
then bounce interfaces fa0/10, fa0/11, fa0/13 and fa0/14 on SW3 as shown below
before proceeding as they are in Err-Disabled state;
SW1(config)#interface fa0/10
SW1(config-if)#no span bpduguard enable
SW1(config-if)#end
SW1#
SW2(config)#interface fa0/10
SW2(config-if)#no span bpdufilter
SW2config-if)#end
SW2#
SW3 con0 is now available
Press RETURN to get started.
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7. SW3>enable
SW3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW3(config)#interface range fa0/10 , fa0/11 , fa0/13, fa0/14
SW3(config-if-range)#shutdown
SW3(config-if-range)#
%LINK-5-CHANGED: Interface FastEthernet0/10, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/11, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/13, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/14, changed state to
administratively down
SW3(config-if-range)#no shutdown
SW3(config-if-range)#
%LINK-3-UPDOWN: Interface FastEthernet0/10, changed state to up
%LINK-3-UPDOWN: Interface FastEthernet0/11, changed state to up
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8. %LINK-3-UPDOWN: Interface FastEthernet0/13, changed state to up
%LINK-3-UPDOWN: Interface FastEthernet0/14, changed state to up
SW3(config-if-range)#end
SW3#
Step3. – SW3 Interface Fa0/10 is connected to a 10/100Mbps hub then the hub is
connected to SW1. Configure the link type accordingly and verify your configuration.
As discussed in the Lab core knowledge section; link-type on Rapid Spanning Tree
Protocol determines rather or not the interface is rapidly transitioned into forwarding
state or not. To configure link type on a particular interface use the spanning-tree
link-type command followed by the link type point-to-point or shared as shown
below;
SW3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW3(config)#int fa0/10
SW3(config-if)#spanning-tree link-type shared
SW3(config-if)#end
SW3#
%SYS-5-CONFIG_I: Configured from console by console
SW3#
To verify the link-type of a particular interface use the show spanning-tree vlan
# command as shown below;
SW3#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
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9. Root ID Priority 24577
Address 0014.f2d2.4180
Cost 19
Port 10 (FastEthernet0/10)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0014.a964.2e00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Fa0/10 Root FWD 19 128.10 Shr
Fa0/11 Altn BLK 19 128.11 P2p
Fa0/13 Desg FWD 19 128.13 P2p
Fa0/14 Desg FWD 19 128.14 P2p
SW3#
Step4. – Assuming SW2’s interface Fa0/10 and Fa0/11 are configured with their
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10. defaults for spanning tree; the root port will become Fa0/10. Influence spanning-tree
to use interface Fa0/11 and do not use the cost command.
To complete this objective you’ll need to change the port priority number as cost
cannot be changed. The lowest priority number port wins the root port election if all
costs on the redundant links to the root bridge are equal as shown below;
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface fa0/11
SW1(config-if)#spanning-tree port-priority 64
SW1(config-if)#end
SW1#
%SYS-5-CONFIG_I: Configured from console by console
SW1#
To verify that the priority does manipulate the root port selection on SW2 to Fa0/11
instead of Fa0/10 use the show spanning-tree vlan #command as shown below;
SW2#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 24577
Address 0014.f2d2.4180
Cost 19
Port 13 (FastEthernet0/11)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
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11. Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 001c.57d8.9000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Fa0/10 Altn BLK 19 128.12 P2p
Fa0/11 Root FWD 19 128.13 P2p
SW2#
Keep in mind that the port-priority is propagated from switch to switch via BPDU’s.
So when you want influence a traffic transit path you must configure the port-priority
on the advertising switch. In this case, SW1 is advertising two traffic paths to the root
bridge, but interface Fa0/11 is now advertising a better port priority and therefore
will be selected as the root port on SW2.
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