|
|
 |
| Spantasmic |
| STP, RSTP, MST (802.1 d
/ w / s) |
|
Spantasmic
is a flexible embedded implementation of the
Layer 2 spanning tree protocol (STP) and enhancements for
rapid recovery of connectivity (RSTP – IEEE 802.1W) and
VLAN-sensitivity (MST – IEEE 802.1S). It enables interoperable network redundancy on networking and
industrial equipment such as bridges and switches, or in
embedded switching frameworks within specialized embedded
applications. It also includes a bridging framework with
built-in forwarding and an address resolution logic-based
filtering database which can optionally stand-in for a
switch fabric chipset implementing similar functionality.
Seamless integration with native OS driver models
and standard MIB capabilities for any SNMP agent make
Spantasmic an excellent fit for managed embedded
environments.
|
|
|
|
|
|
|
|
|
|
|
|
 |
| Redundancy and Loops |
| Redundant links
are an important part of highly available networks,
serving 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 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. |
|
|
|
Spanning Tree Protocol |
|
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 |
|
 |
Customization
Flexibility |
 |
 |
|
|
|
 |
Configurable relative priority of each bridge
and each port. |
|
|
Configurable path cost associated with each
port. |
|
|
Management APIs are provided to control and
monitor spanning tree parameters. |
|
|
Available in full-source format |
|
|
Customization hooks and callouts. |
|
|
Unwanted components can be scaled out. |
|
|
Designed to support hardware
switch implementations |
|
|
 |
 |
 |
 |
|
|
|
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.
Spanning Tree Protocol is defined in the IEEE 802.1D standard as
an essential element of enterprise and carrier infrastructure.
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. |
|
|
|
 |
|
|
|
RSTP |
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 compatibility with equipment based on STP
on a per-port basis. These improvements are due to 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. |
|
|
|
MSTP |
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 includes 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. MSTP is
interoperable with STP and RSTP bridges in the same bridged LAN,
ensuring cross-protocol compatibility. |
|
|
|
Built-in Bridge Framework |
|
For implementations that do not include a
switch fabric or external bridge implementation, 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 internal 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. |
|
|
|
 |
|
|
| |
|
|
|
