You remember from your CCNA studies that when a port goes through the
transition from blocking to forwarding, you're looking at a 50-second
delay before that port can actually begin forwarding frames.
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?
A switch can be connected to two other switches, giving that local
switch a redundant path to the root bridge, and that's great - we
always want a backup plan! However, STP will only allow one path to be
available, but if the available path to the root switch goes down,
there will be a 50-second delay due to the STP timers MaxAge and
ForwardDelay before the currently blocked path will be available.
The delay is there to prevent switching loops, and we can'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 are
collectively 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 to forwarding
immediately. Uplinkfast is pretty much PortFast for wiring closets.
(Cisco recommends that Uplinkfast not be used on switches in the
distribution and core layers.)
Some additional details regarding Uplinkfast:
The actual transition from blocking to forwarding mode takes about three seconds.
Uplinkfast cannot be configured on a root switch.
Uplinkfast is configured globally. You can't run Uplinkfast on some ports or on a per-VLAN basis - it's all or nothing.
The original root port will become the root port again when it detects
that its link to the root switch has come back up. This does not take
place immediately. The switch uses the following formula to determine
how long to wait before transitioning back to the forwarding state:
( 2 x FwdDelay) + 5 seconds
Uplinkfast will take immediate action to ensure that the switch upon
which it is configured cannot become the root switch. First, the switch
priority will be set to 49,152, which means that if all other switches
are still at their default priority, they'd all have to go down before
this switch can possibly become the root switch. Additionally, the STP
Port Cost will be increased by 3000, making it highly unlikely that
this switch will be used to reach the root switch by any downstream
switches.
And you just know there's got to be at least one option with this command, right? Let's run IOS Help and see.
SW2(config)#spanning-tree uplinkfast ?
max-update-rate Rate at which station address updates are sent
When there is a direct link failure, dummy multicast frames are sent to
the MAC destination 0100.0ccd.cdcd. The max-update-rate value
determines how many of these frames will be sent in a 100-millisecond
time period.
Mastering the details of UplinkFast, BackboneFast, BPDU Guard, and Loop
Guard are vital to your success on the CCNP exams, and one or more of
these features are in use on almost every network in the world. Learn
these features for success in both the exam room and the real world!