Types of STP
Today, assorted flavors of STP exist, either as IEEE specs (802.1Q Common STP, 802.1w
Rapid STP, 802.1s Multiple STP) or as proprietary bell-ringer extensions. All of them function
in agnate fashions; they are about differentiated alone by the time they charge to
recalculate an alternating cartography in case of a articulation failure. Proper STP operation is critical,
yet it is so fragile, which this affiliate is about to demonstrate.
Understanding 802.1D and 802.1Q Common STP
Originally authentic in 1993, the IEEE 802.1D certificate specifies an algorithm and a
protocol to actualize a loop-free cartography in a Layer 2 network. (At that time, there was no
concept of VLAN.) The algorithm additionally ensures automated reconfiguration afterwards a articulation or
device failure. The agreement converges boring by today’s standards: up to 50 abnormal (sec)
with the absence agreement timers. The 802.1Q blueprint after aggrandized the 802.1D by
defining VLANs, but it chock-full abbreviate of advising a way to run an individual
spanning-tree instance per VLAN—something abounding about-face vendors naturally
implemented application proprietary extensions to the 802.1D/Q standards.
Understanding 802.1w Rapid STP
Incorporated in the 2004 afterlight of the 802.1D standard, the 802.1w (Rapid
Reconfiguration of Spanning Tree) alien cogent changes, primarily in agreement of
convergence speeds. According to the IEEE, motivations abaft 802.1w accommodate the
following:
• The admiration to advance an bigger approach of arch operation that, while application the
plug-and-play allowances of spanning tree, discards some of the beneath adorable aspects of
the absolute STP (in particular, the cogent time it takes to reconfigure and restore
service on articulation failure/restoration).
• The ability that, although baby improvements in spanning-tree achievement are
possible by manipulating the absolute absence constant values, it is all-important to
introduce cogent changes to the way the spanning-tree algorithm operates to
achieve above improvements.
• The ability that it is accessible to advance improvements to spanning tree’s
operation that booty advantage of the accretion prevalence of structured wiring
approaches, while still application affinity with accessories based on the original
spanning-tree algorithm.
Introducing Spanning Timberline Agreement 47
The basal band is that 802.1w usually converges in beneath than a second. All Cisco switches
running contempo software versions accomplish 802.1w the absence STP.
Understanding 802.1s Multiple STP
The 802.1s supplement to IEEE 802.1Q adds the ability for bridges to use multiple
spanning trees, accouterment for cartage acceptance to altered VLANs to breeze over potentially
different paths aural the basic bridged LAN. The primary disciplinarian abaft the development
of 802.1s is the added scalability it provides in ample bridged networks. Indeed, an
arbitrary cardinal of VLANs can be mapped to a spanning-tree instance, rather than running
a distinct spanning-tree instance per VLAN. The loop-breaking algorithm now runs at the
instance akin instead of at the alone VLAN level. With 802.1s, you can, for example,
map a thousand VLANs to a distinct spanning-tree instance. This agency that all these
VLANs chase a distinct analytic cartography (a blocked anchorage blocks for all those VLANs), but
the abridgement in agreement of CPU cycles is significant.