Uplink fast

                            Uplinkfast

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Topology :         + SW 1-2-3 chay STP

+ R1 la root

+ F1/2 SW3 block status

Từ SW3 có duy nhất 1 tuyến đường tới Root SW1 .giả sử nếu SW1-SW3 down , thì STP phải mất ít nhất 30s để chuyển trạng thái block f1/2 sang trạng thái forwading . Configuring a port with Portfast is one way to get around that, but again, you can only use it when a single host device is found off the port. What if the device connected to a port is another switch?

The delay is there to prevent switching loops, and wecan't use Portfast to shorten the delay since these are switches, not host devices. What we can use is Uplinkfast.

The ports that SW3 could potentially use to reach the root switch arecollectively referred to as an uplink group. The uplink group includes the ports in forwarding and blocking mode. If the forwarding port in the uplink group sees that the link has gone down,another port in the uplink group will be transitioned from blocking toforwarding immediately.

Cisco recommends that Uplinkfast not be usedon switches in the distribution and core layers

Some additional details regarding Uplinkfast:

1..The actual transition from blocking to forwarding isn't really "immediate" - it actually takes 1 - 3 seconds. Next to a 50-second delay, that certainly seems immediate!

2.. Uplinkfast cannot be configured on a root switch.

3..When Uplinkfast is enabled, it's enabled globally and for all VLANs residing on the switch. You can't run Uplinkfast on some ports or on a per-VLAN basis - it's all or nothing.

4.The CAM table is flooded (using dummy sourced multicasts) out of this new root port to expedite the MAC learning process of its upstream neighbors

(The last component of UplinkFast feature is quick MAC-address table re-learning. Since the primary uplink has failed, the core switches might lose the MAC addresses associated with the access switch. This will result in connectivity disruption, till the time the addresses are re-learned via the secondary path. Thus, in addition to bringing the secondary uplink up quickly, the switch will also flood it with dummy multicast frames, sourced from all the MAC addresses known to the switch. This will allow the upstream switches (the destination is multicast) to quickly re-learn the MAC addresses via the new path. Of course, the penalty is excessive network flooding with multicast frame)

5. Apply caution if the bridge has more than two uplinks, as it might become transit.

6. As a built-in safeguard when enabled, the bridge priority value and all ports cost values are incremented.

=== > save 30s

Luu y: khi f1/3 up thi no se o trang thai block.

Command :     SW3(config)#spanning-tree uplinkfast ?

                 max-update-rate Rate at which station address updates are sent

            

Before [ this switch is connected to the root switch]

After:

After applying this command the switch’s priority will shoot up to 49,152 + sys-id-ext and will add 3000 to the existing STP port costs:

The rational behind this is that the high priority value will make it unlikely that the switch will become a root.  The high STP port costs will make it unlikely that the switch will become a transit switch (a switch that forwards frames between other switches).  That way if one of the uplinks fails the switch can quickly switch to another uplink with little chance of causing a loop.

Where To Apply Uplinkfast

Uplinkfast is a wiring-closet switch feature - it's not recommended for core and distribution-layer switches. Uplinkfast should be configured only on access-layerswitches. It's a safe betthat the root switches are going to be found in the core layer, and the switches that are farthest away from the root switches will be the access switches. The accessswitches will be the ones closest to the end users

Log message:

R3#

*Jul 10 14:30:37.911: STP: VLAN0001 new root port Et0/2, cost 3200

*Jul 10 14:30:37.911: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 Ethernet0/2 moved to Forwarding (UplinkFast).

*Jul 10 14:30:37.911: STP[1]: Generating TC trap for port Ethernet0/2

*Jul 10 14:30:37.911: STP[1]: Generating TC trap for port Ethernet0/3

*Jul 10 14:30:37.911: STP: VLAN0010 new root port Et0/2, cost 3200

*Jul 10 14:30:37.911: STP[10]: Generating TC trap for port Ethernet0/2

*Jul 10 14:30:37.911: STP[10]: Generating TC trap for port Ethernet0/3

*Jul 10 14:30:37.911: STP: VLAN0020 new root port Et0/2, cost 3200

*Jul 10 14:30:37.911: STP[20]: Generating TC trap for port Ethernet0/2

*Jul 10 14:30:37.911: STP[20]: Generating TC trap for port Ethernet0/3

*Jul 10 14:30:37.911: STP: VLAN0030 new root port Et0/2, cost 3200

*Jul 10 14:30:37.911: STP[30]: Generating TC trap for port Ethernet0/2

*Jul 10 14:30:37.911: STP[30]: Generating TC trap for port Ethernet0/3

*Jul 10 14:30:39.918: STP: VLAN0001 sent Topology Change Notice on Et0/2

*Jul 10 14:30:39.918: STP: VLAN0010 sent Topology Change Notice on Et0/2

*Jul 10 14:30:39.918: STP: VLAN0020 sent Topology Change Notice on Et0/2

*Jul 10 14:30:39.918: STP: VLAN0030 sent Topology Change Notice on Et0/2