What is uplinkfast?

 

Spanning tree UplinkFast is a Cisco-specific enhancement to the Spanning Tree Protocol (STP). It's designed to improve the convergence time in the event of a direct link failure, especially in access-layer switches with redundant links to the distribution layer.

Here's how it works:

Quick Recovery: When the primary uplink fails, UplinkFast allows the switch to quickly transition the blocked port to forwarding, bypassing the usual listening and learning states.

Preventing Loops: It modifies the Bridge Protocol Data Units (BPDUs) to prevent loops by increasing the path cost and setting the local bridge priority.

Immediate Restoration: This feature ensures that connectivity is restored almost immediately, reducing downtime significantly.

UplinkFast is typically used in access-layer switches and should be deployed carefully to avoid potential issues with MAC address tables. 

Key points:-

• legacy/cisco proprietary feature
• uplinkfast is for speeding convergence when a direct link to an upstream switch fails. 
• when uplinkfast is enabled, it is enabled for the entire switch and all VLANs.

• this feature is not allowed on the root bridge switch. 
• when uplinkfast is configured, the bridge priority is changed to 49,152 so that this switch will not be selected as root.

We have three switches. Switch-1 is our Root, and two others are non-root switches. Switch-1's ports are Desg FWD because it's a root. Switch-2's gig0/0 port is a Root FWD, and gig 0/1 is a Desg FWD. Switch-3's gig0/0 is Root FWD, and gig1/0 is an Altn BLK state port. so that's what we have, now switch-3 uses gig0/0 to reach the root bridge, what if the port shuts down? how long does it take for switch-3 to make a transition? let's see. 

switch-1#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     0c58.a85e.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Desg FWD 4         128.1    Shr
Gi0/1               Desg FWD 4         128.2    Shr

switch-2#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0c2e.adf7.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    Shr
Gi0/1               Desg FWD 4         128.2    Shr

switch-3#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0c33.c353.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    Shr
Gi0/1               Altn BLK 4         128.2    Shr

switch-3#debug spanning-tree events

Spanning Tree event debugging is on

switch-3(config)#interface gigabitEthernet 0/0

switch-3(config-if)#shutdown

*Jan  5 08:15:36.684: STP: VLAN0001 new root port Gi0/1, cost 8

*Jan  5 08:15:36.684: STP: VLAN0001 Gi0/1 -> listening

*Jan  5 08:15:36.684: STP[1]: Generating TC trap for port GigabitEthernet0/0

*Jan  5 08:15:36.685: STP: VLAN0100 we are the spanning tree root

*Jan  5 08:15:38.685: STP: VLAN0001 sent Topology Change Notice on Gi0/1

*Jan  5 08:15:51.686: STP: VLAN0001 Gi0/1 -> learning

*Jan  5 08:16:06.686: STP[1]: Generating TC trap for port GigabitEthernet0/1

*Jan  5 08:16:06.686: STP: VLAN0001 Gi0/1 -> forwarding

switch-3#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             Cost        8
             Port        2 (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     0c33.c353.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Root FWD 4         128.2    Shr

after a long time of listening and learning forwarding. our previous  Gig0/1 Altn BLK port is now Root FWD. This process takes time. now we have configure the Cisco Uplinkfast feature.  

switch-3(config)#interface gigabitEthernet 0/0

switch-3(config-if)#no shutdown

switch-3(config-if)#exit

switch-3(config)#do show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0c33.c353.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    Shr
Gi0/1               Altn BLK 4         128.2    Shr

switch-3(config)#spanning-tree uplinkfast

switch-3(config)#end

switch-3#debug spanning-tree events

Spanning Tree event debugging is on

*Jan  5 08:22:15.071: STP: VLAN0001 new root port Gi0/1, cost 3008

*Jan  5 08:22:15.071: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 GigabitEthernet0/1 moved to Forwarding (UplinkFast).

wooo no listening no learning direct forwarding) this is Uplinkfast....

switch-3#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0c58.a85e.0000
             Cost        3008
             Port        2 (GigabitEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    49153  (priority 49152 sys-id-ext 1)
             Address     0c33.c353.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
  Uplinkfast enabled
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Root FWD 3004      128.2    Shr

you see after we configure the Uplinkfast feature this switch means switch-3 bridge ID priority is changed. now this switch will never be a root bridge. unless we disable uplink fast. so never configure Uplink fast on the root switch.