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Redundancy and Loops | Spanning
Tree Protocol | RSTP | MST | Learning
Bridge Framework Spantasmic is a lean, robust implementation
of the Layer 2 spanning tree protocol (STP) designed to run on bridges and switches,
that provides path redundancy while preventing undesirable loops in a network.
The STP algorithm enables a bridge to use the most efficient path when faced with
multiple paths, and when the best path fails, to recalculate the network and find
the next best path. Spantasmic includes support for the IEEE 802.1D STP specification,
as well as enhancements for rapid recovery of connectivity when a bridged port
fails (RSTP – IEEE 802.1W) and VLAN-sensitivity (MST – IEEE 802.1S).
It also includes a bridging framework with built-in forwarding and an address
resolution logic-based filtering database. With its extensive support of standards
and underlying reliability and flexibility, Spantasmic is an ideal fit for LAN
and MAN applications, where multiple bridged links without loops are required.
Redundant links are as important as backups in case of failure in a network. For
bridged (or switched) networks, it is common to add a second bridge between two
segments as a backup in case the primary bridge fails. Redundancy eliminates a
single point of hardware failure in a network, but it is not without its problems.
For a network to function properly, only one active path can exist between two
stations. The redundant bridge in the network causes multiple active paths between
stations which can cause loops in the network topology, and the potential exists
for duplication of messages. When loops occur, some bridged see stations appear
on both sides of the bridge. This condition confuses the basic forwarding algorithm
and allows duplicate frames to be forwarded, which can lead to an explosion in
traffic and can adversely impact the performance of the network.
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Spantasmic’s Spanning Tree Protocol implementation is used to selectively
block bridge ports in a bridge loop situation, to keep exactly one active path
open, thus allowing for redundant bridges to be used without loops. STP specifies
an algorithm that bridges can use to create a loop-free logical topology, and
creates a tree structure of loop-free leaves and branches that spans the entire
Layer 2 network. Whenever a loop is created, the root bridge will reconfigure
the network and take the redundant bridged path creating the loop out of service.
By disabling these inter-bridge links STP ensures that no L2 frames are forwarded
in such a way as to "loop" the frames back to the original LAN that
originated them. Further, the single path left active is the one determined to
be the most efficient one. The redundancy feature is not lost - if a link in forwarding
state becomes unavailable, STP reconfigures the network and reroutes data paths
by activating the appropriate standby path.
As an essential element of enterprise and carrier infrastructure,
Spanning Tree Protocol is defined in the IEEE 802.1D standard. Spantasmic includes
support for this specification, and is interoperable with other 802.1D compliant
bridges and switches. Further, Spantasmic operation is transparent to end-stations,
which are unaware of whether they are connected to a single LAN segment or a bridged/switched
LAN of multiple segments.
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Although the basic STP avoids data loops in a bridged network, it takes 30 to
60 seconds to converge, depending upon the complexity of the bridged network topology.
Moreover, reconfiguration in the event of bridge failure or link failure is also
slow when compared to other Layer 3 protocols that support path reconfiguration.
This can be critical in networks carrying delay-sensitive traffic such as voice
and video. To avoid this limitation, Spantasmic also implements Rapid STP (RSTP),
based on the IEEE 802.1W specification, an enhancement to the original IEEE 802.1D
specification. RSTP has a much faster convergence and reconfiguration time in
the event of a change in network topology, which is usually less than a second,
while retaining comptability with equipment based on STP on a per-port basis.
Those improvements come from the ability of the protocol to distinguish point-to-point
vs. shared links.
Spantasmic’s RSTP implementation is a distributed algorithm
that selects a single bridge to act as the spanning tree's root. The algorithm
assigns port roles to individual ports on each bridge. Port roles determine whether
the port is to be part of the active topology connecting the bridge or switch
to the root bridge (a root port), or connecting a LAN through the bridge to the
root bridge (a designated port). Regardless of their roles, ports can serve as
alternate or back-up ports that provide connectivity in the case of failure. State
machines associated with port roles maintain and change the port states that control
the processing and forwarding of frames. A new root port can rapidly transition
to the forwarding port state. Explicit acknowledgements between bridges and switches
in the LAN allow designated ports to rapidly transition to the forwarding port
state. Thus, Spantasmic ensures rapid recovery of connectivity following the failure
of a bridge, bridge port or LAN.
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Another limitation of the basic STP is visible when 802.1Q-capable bridges are
involved (and most bridges/switches today are 802.1Q-capable), in scenarios where
asymmetrical connectivity between VLANs exists. With the advent of Metro Ethernet
networks, different VLANs commonly have different connectivity and redundancy
requirements, and can have different underlying physical links. This asymmetrical
connectivity creates a requirement for the infrastructure to be able to execute
separate STP instances for different VLANs, which result in the ability to have
a given physical port perform forwarding for one VLAN while doing blocking for
another. To address this, Spantasmic adds the facility for VLAN bridges to use
multiple spanning trees (MST), providing for traffic belonging to different VLANs
to flow over potentially different paths within the virtual bridged LAN.
Spantasmic sports an implementation of the IEEE 802.1S multiple
spanning trees specification which enables VLANs to be grouped into a spanning-tree
instance, provides for multiple forwarding paths for data traffic, and also enables
load balancing. Each spanning tree instance is independent of other instances,
thereby increasing the fault tolerance of the network since failure in one instance
does not affect other instances. MST is interoperable with STP and RSTP bridges
in the same bridged LAN, ensuring cross-protocol compatibility.
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Spantasmic also includes a bridging component to forward unicast and broadcast
frames received on a given port to the appropriate port(s), which reduces the
overall traffic in a bridged LAN and provides greater effective bandwidth with
its filtering abilities. The filtering engine drops frames that arrive on a port
and are destined to hosts in the same LAN, thus reducing the overhead of forwarding.
The port that a frame is forwarded to, is decided based on an address resolution
logic (ARL) table that maps MAC addresses to individual LANs connected to the
bridge. Entries in the ARL table are filled automatically by the bridge through
its learning capability or may be added statically. Spantasmic’s ARL table
also provides the unique capability to age entries, wherein entries that exceed
a customizable maximum age get deleted from the table automatically. Other features
of the bridge include maintaining the port state of individual ports, and bridge
management to control and monitor the parameters associated with the bridge. Spantasmic
also provides customized APIs to attach and configure any spanning tree protocol
to on a bridge port. The bridging component may be used in its software-only form,
or may be interfaced to switch fabric hardware to enable fast data-paths, while
the control logic resides in software.
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