How to Configure UplinkFast on Cisco Switch in GNS3

When we configure the standard Spanning Tree Protocol on Layer 2 Switches, the STP Convergence process takes 50 seconds as a result of a link error. We can reduce this downtime with UplinkFast (UF) up to 20 seconds.

How to Configure UplinkFast on Cisco Switch in GNS3

How to Enable Spanning Tree UplinkFast on Layer 2 Switch

As you know, Spanning Tree UplinkFast, BackboneFast and PortFast are Cisco-specific. The purpose of configuring the Spanning Tree Protocol on Layer 2 Switches is to prevent loops in the network.

The Spanning Tree UplinkFast feature must enable on a Switch with one Blocked Port. The reason we do this is to allow Blocked Port to enter Forwarding Mode immediately as a result of a link error. Therefore, this feature will reduce the downtime of 50 seconds to 20 seconds.

In our previous articles, we have examined the PortFast Configuration and Root Bridge Configuration. In this article, we will also examine the Uplink Fast configuration on Layer 2 Switches.

How to Setup UplinkFast in GNS3

First, you need to add Layer 2 Switch to GNS3 for configuration. For additional information, Packet Tracer does not support features such as UplinkFast and BackboneFast.

To enable UplinkFast, follow the steps below.

   Step 1

Open Graphical Network Simulator-3 and create a new project for UF.

Creating a New Project in GNS3

   Step 2

In GNS3, create a topology like the one below. Also, configure TCP/IP for VPCS computers.

GNS3 Network Topology

   Step 3

Check the Spanning Tree Protocol configuration of Cisco Switches on the topology and select Root Bridge Switch to enter the Port States.

Cisco Switch SW1 show spanning-tree command output appears to be Root Bridge.

show spanning-tree

   Step 4

At the Switch SW2 CLI prompt, execute the show spanning-tree command and note the Port States as shown below.

show spanning-tree

   Step 5

In the same way, note Cisco Switch SW3 Port States.

Reviewing Switch Port States

   Step 6

Start a continuous Ping from Lenovo PC to HP PC before enabling UplinkFast.

Continuous Pinging

   Step 7

Unplug the cable between SW1 and SW2 while continuously pinging from the Lenovo PC to the HP PC.

Continuous Pinging

   Step 8

STP Convergence takes 50 seconds as shown. That’s why we can reduce this time by 30 seconds by enabling STP UplinkFast on a Switch with a Blocked Port.

STP Convergence Time

   Step 9

Since SW2 has one Blocked Port, we will enable Uplink Fast on this Switch. Open the SW2 CLI command prompt and execute the following command on the Switch.

SW2# conf t
SW2(config)# spanning-tree uplinkfast
SW2(config)# end
SW2#

 
spanning-tree uplinkfast

   Step 10

After enabling the Uplink Fast feature on the switch, verify that it enabled by using the show spanning-tree summary command.

show spanning-tree summary

   Step 11

The UplinkFast process is very simple as shown. Now, start the Ping process from Lenovo PC to HP PC again, to verify how Uplink Fast works.

Then disconnect the cable between SW1 and SW2 again.

Indirect Cable Error Generation

   Step 12

The STP Convergence process completed in almost 20 seconds. If you examine the image below, you can better understand the benefit of enabling Uplink Fast on the switch.

STP Convergence Time

   Step 13

Finally, when you reconnect the cable between SW1 and SW2, you can see that Ping continues without interruption.

Continuing Ping

Show Commands


SW2#show spanning-tree summary
Switch is in pvst mode
Root bridge for: none
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is enabled
Configured Pathcost method used is short
UF is enabled
BackboneFast is disabled

Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 1 0 0 3 4
---------------------- -------- --------- -------- ---------- ----------
1 vlan 1 0 0 3 4

Station update rate set to 150 packets/sec.

UF statistics
-----------------------
Number of transitions via UF (all VLANs) : 0
Number of proxy multicast addresses transmitted (all VLANs) : 0
SW2#

 


SW2#show spanning-tree detail

VLAN0001 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 49152, sysid 1, address 002c.78f0.2f00
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32769, address 002c.785d.8700
Root port is 1 (GigabitEthernet0/0), cost of root path is 3004
Topology change flag not set, detected flag not set
Number of topology changes 3 last change occurred 00:02:02 ago
from GigabitEthernet0/1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
UF enabled

Port 1 (GigabitEthernet0/0) of VLAN0001 is root forwarding
Port path cost 3004, Port priority 128, Port Identifier 128.1.
Designated root has priority 32769, address 002c.785d.8700
Designated bridge has priority 32769, address 002c.785d.8700
Designated port id is 128.1, designated path cost 0
Timers: message age 1, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is shared by default
BPDU: sent 31, received 272

Port 2 (GigabitEthernet0/1) of VLAN0001 is alternate blocking
Port path cost 3004, Port priority 128, Port Identifier 128.2.
Designated root has priority 32769, address 002c.785d.8700
Designated bridge has priority 32769, address 002c.78f0.1500
Designated port id is 128.2, designated path cost 4
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is shared by default
BPDU: sent 2, received 308

Port 3 (GigabitEthernet0/2) of VLAN0001 is designated forwarding
Port path cost 3004, Port priority 128, Port Identifier 128.3.
Designated root has priority 32769, address 002c.785d.8700
Designated bridge has priority 49153, address 002c.78f0.2f00
Designated port id is 128.3, designated path cost 3004
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is shared by default
BPDU: sent 301, received 0

Port 4 (GigabitEthernet0/3) of VLAN0001 is designated forwarding
Port path cost 3004, Port priority 128, Port Identifier 128.4.
Designated root has priority 32769, address 002c.785d.8700
Designated bridge has priority 49153, address 002c.78f0.2f00
Designated port id is 128.4, designated path cost 3004
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is shared by default
BPDU: sent 301, received 0

SW2#

 


SW2#
*Sep 12 13:31:34.053: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 GigabitEthernet0/1 moved to Forwarding (UF).
*Sep 12 13:31:52.253: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 GigabitEthernet0/0 moved to Forwarding (UF).
SW2#

 


SW2#show spanning-tree uplinkfast
UF is enabled

Station update rate set to 150 packets/sec.

UF statistics
-----------------------
Number of transitions via UF (all VLANs) : 2
Number of proxy multicast addresses transmitted (all VLANs) : 0

Name Interface List
-------------------- ------------------------------------
VLAN0001 Gi0/0(fwd), Gi0/1
SW2#

 


SW2#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 002c.785d.8700
Cost 3004
Port 1 (GigabitEthernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)
Address 002c.78f0.2f00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
UF enabled

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Root FWD 3004 128.1 Shr
Gi0/1 Altn BLK 3004 128.2 Shr
Gi0/2 Desg FWD 3004 128.3 Shr
Gi0/3 Desg FWD 3004 128.4 Shr

SW2#

 

How to Enable Uplink Fast ⇒ Video

You can watch the video below to activate STP UplinkFast and also subscribe to our YouTube channel to support us!

   Final Word


Configuring Uplink Fast for the standard Spanning Tree protocol on L2 Switches is a very useful process. If your Switches do not support Rapid STP, you should definitely enable UplinkFast to improve performance in the Standard STP configuration. Thanks for following us!

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