Introduction
Uplink fast is a feature of STP that allows very fast converge in case of uplink failure.
Configuration example – no UplinkFast enabled
A simple topology, S1 is a root bridge, S3 has two uplinks eth0/0 and eth0/1.
Let’s ping 192.168.1.2 and shutdown eth0/0 on S3 and see how many pings we lose:
S3(config)#int ethernet 0/1 S3(config-if)#shut S3(config-if)#shutdown S3(config-if)# *Jan 19 15:24:29.003: STP: VLAN0001 new root port Et0/0, cost 200 *Jan 19 15:24:29.003: STP: VLAN0001 Et0/0 -> listening *Jan 19 15:24:29.003: STP[1]: Generating TC trap for port Ethernet0/1 S3(config-if)# *Jan 19 15:24:31.008: %LINK-5-CHANGED: Interface Ethernet0/1, changed state to administratively down S3(config-if)# *Jan 19 15:24:31.008: STP: VLAN0001 sent Topology Change Notice on Et0/0 *Jan 19 15:24:32.015: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to down S3(config-if)# *Jan 19 15:24:44.003: STP: VLAN0001 Et0/0 -> learning S3(config-if)# *Jan 19 15:24:59.004: STP[1]: Generating TC trap for port Ethernet0/0 *Jan 19 15:24:59.004: STP: VLAN0001 sent Topology Change Notice on Et0/0 *Jan 19 15:24:59.004: STP: VLAN0001 Et0/0 -> forwarding
I’ve lost about 20 pings with a single STP reconverge! Port had to go through all STP states: listening, learning and forwarding. We’d have to double it if the link flaps (goes down and comes back up).
UplinkFast
Let’s enable UplinkFast feature and see how it helps with convergence times.
S3(config)#spanning-tree uplinkfast
Once enabled, UplinkFast feature does following things:
- sets ports costs to 3000 (so switch doesnt become a transit switch)
- increases the STP priority (so this switch is unlikely to become a root bridge) to 49152
S03#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address aabb.cc00.1000
Cost 3100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)
Address aabb.cc00.2000
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
Et0/1 Root FWD 3100 128.1 Shr
Et0/0 Desg FWD 3100 128.2 Shr
Et0/2 Altn BLK 3100 128.3 Shr
Et0/3 Desg FWD 3100 128.4 Shr
Let’s verify blocked ports:
S3#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address aabb.cc00.2000
Cost 3100
Port 2 (Ethernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)
Address aabb.cc00.3000
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
Et0/0 Altn BLK 3100 128.1 Shr
Et0/1 Root FWD 3100 128.2 Shr
Et0/2 Desg FWD 3100 128.3 Shr
Et0/3 Desg FWD 3100 128.4 Shr
And we can verify uplink fast ports by entering command below:
S3#sh 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) : 0 Name Interface List VLAN0001 Et0/1(fwd), Et0/0
OK, we can see that eth0/1 is the current active forwarding port and eth0/0 is a backup port.
Let’s shutdown eth0/1 again and see how many pings we lose:
S3(config)#int et S3(config)#int ethernet 0/1 S3(config-if)#shu S3(config-if)#shutdown S3(config-if)# *Jan 19 15:36:47.096: STP: VLAN0001 new root port Et0/0, cost 3200 *Jan 19 15:36:47.096: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 Ethernet0/0 moved to Forwarding (UplinkFast). S3(config-if)# *Jan 19 15:36:47.096: STP[1]: Generating TC trap for port Ethernet0/0 *Jan 19 15:36:47.096: STP[1]: Generating TC trap for port Ethernet0/1 S3(config-if)# *Jan 19 15:36:49.095: %LINK-5-CHANGED: Interface Ethernet0/1, changed state to administratively down S3(config-if)# *Jan 19 15:36:49.103: STP: VLAN0001 sent Topology Change Notice on Et0/0 S3(config-if)# *Jan 19 15:36:50.099: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to down
In this test, I didn’t lose a single ping, UPLINK fast is working, great! Let’s bring back up the eth0/1 and see if that causes any issues:
S3(config)#int ethernet 0/1 S3(config-if)# S3(config-if)#no sh S3(config-if)# *Jan 19 15:39:42.437: set portid: VLAN0001 Et0/1: new port id 8002 *Jan 19 15:39:42.437: STP: VLAN0001 Et0/1 -> listening S3(config-if)# *Jan 19 15:39:43.553: STP: VLAN0001 Et0/1: root port delay timer active *Jan 19 15:39:43.553: STP: VLAN0001 Et0/1 -> blocking *Jan 19 15:39:44.439: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up S3(config-if)# *Jan 19 15:39:45.442: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up S3(config-if)#
As we can see above, et0/1 went straight into blocking mode. After about 30seconds eth0/1 got moved into forwarding state again by UplinkFast:
*Jan 19 15:40:18.555: STP: VLAN0001 Et0/0 -> blocking (uplinkfast)
*Jan 19 15:40:18.555: STP[1]: Generating TC trap for port Ethernet0/0
*Jan 19 15:40:18.555: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 Ethernet0/1 moved to Forwarding (UplinkFast).
S3(config-if)#
*Jan 19 15:40:18.555: STP[1]: Generating TC trap for port Ethernet0/1
*Jan 19 15:40:18.555: STP: VLAN0001 sent Topology Change Notice on Et0/1
