CCNA Advance- STP (Spanning Tree Protocol) : Part 2
STP- Enhancements
PortFast:
As for now we are well equipped with STP, now we turn to STP
optimization techniques. STP optimization is necessary for fast convergence times
in the network, as the standard 4 step convergence of STP can cause a lot of
havoc in the real time network. In this article we would like to discuss STP:
Port fast, Uplink fast, and Backbone fast.
When a switch powers up or when some device is connected to
a switch, STP immediately comes into action. In the initial phase the port enters
into a spanning tree listening state. Listening state is just like a network
topology exploration, this state lasts for a certain time, and then the port
transitioned into a learning state. After an STP forward timer threshold the
port state changes into either blocking mode or forwarding mode. In a real time
network, most of the time we can’t afford the switch port to transition through
all these 4 stages. We want the port to immediately shift into a forwarding
state, once a network is alive, to avoid un-necessary packet delays in the
network. For this purpose we use an STP
PortFast feature. Once PortFast is enabled on a switch/trunk port, the port
skips the listening and learning phases and immediately shifts into the
forwarding state. So one important point we need to remember is:
Only enable PortFast on End Stations, because it can create network
loops if used carelessly (as it turns the port into a forwarding state
immediately)!
So it was easy? Yes it was, now we move towards a new
strange concept UpLink Fast.
UpLinkFast:
By using UpLink fast on a port, fast convergence is achieved
via creating UpLink Groups. Once a topology change occurs, convergence is
achieved using these UpLink groups, which activate the redundant links (ports)
instantly. This redundancy is achieved without hassle of passing the redundant
link through all STP transition phases (i.e. listening, learning), within 1-5 seconds:
redundant link (port) is in forwarding
state. An UpLink group consists of the root port and set of blocked ports. This
UpLink group consists of alternate path if the active root port fails. Some of
the worth remembering points regarding UpLink fast are:
- It cannot be configured on a root switch.
- When UF
is enabled, it’s enabled globally and for all VLANs residing on the switch.
- The
designated port (root port) will retain its status once it detects that
the failed link has been restored and fully operational.
- The
wait interval for the port to become root port again is determined by: (2
x FwdDelay) + 5 seconds.
- UF will
take immediate action to prevent the switch (on which UF is enabled) from becoming
the root switch by: changing the switch priority to 49,152, making it the
last option in a network topology for becoming a root switch. The STP port
cost is increased up to 3000 making it least feasible part for any switch
to use it to reach the root switch.
I think that much theory is
enough, now we will do some configurations to solidify our concepts. For our
example scenario we are using GNS3 and emulating C2961 router as a switch because
packet tracer is not giving us much options to implement advance STP.
For complete article, please download it from below mentioned link and Enjoy:
http://www.mediafire.com/download/r69bzogi3g3lid8/STP-Enhancements_Part_1.pdf
Categories:
CCNA,
CCNP,
CCNP STP,
IEEE 802.1d,
Spanning tree advance,
STP advance
planning to learn them :-)
ReplyDeleteGreat, STP is one of the best loop prevention protocol.
ReplyDelete