ALLIED Telesis Resilient Networking with EPSR User Manual
Solutions Guide
Resilient Networking with EPSR
Introduction
IP over Ethernet is now a well-proven technology in the
delivery of converged services. Ethernet-based Triple-Play
services have become an established commercial reality
world-wide, with service providers offering advanced
voice, video and data packages to their customers. This
requires a highly available network infrastructure for
service providers to meet service level agreements,
and meet the expectations of customers for a seamless
multimedia experience.
Now, the convergence of services and applications in the
enterprise has led to increasing demand for high availability
in the Local Area Network (LAN). High bandwidth is also
required for the multiple applications simultaneously using
the network. For many businesses, real-time applications
like surveillance, automated control, video streaming and
voice over IP (VoIP) are used right alongside data and
Internet access.
The key to providing maximum network uptime is
extremely rapid failover in the event of link failure.
Allied Telesis’s carrier-grade resiliency feature, Ethernet
Protection Switching Ring (EPSR), ensures mission critical
services are not interrupted in the event of link or node
outages. EPSR provides failover times as low as 50ms, and
can be coupled with today’s maximum Ethernet standard
of 10Gbps, to provide high bandwidth in multiples of
10GbE.
Equally at home in the enterprise network, or demanding
service provider metro networks, EPSR provides a
solution that meets the modern network requirements of
high bandwidth and high availability. This advanced selfhealing network technology provides ‘always-on’ access to
online resources and applications.
Allied Telesis supports this technology on a wide range
of sophisticated switching platforms, as well as advanced
telecommunication chassis.
Ethernet Protection Switching Ring
EPSR is Allied Telesis’ premier solution for providing
extremely fast failover between nodes in a resilient ring. EPSR
enables rings to recover within as little as 50ms, preventing
a node or link failure from affecting customer experience,
even with demanding applications such as IP telephony and
streaming video.
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The Technology
Putting a ring of Ethernet switches and/or iMAP (integrated
Media Access Platform) chassis at the core of a network is a
simple way to increase the network’s resilience - such a network
is no longer susceptible to a single point of failure. However, the
ring must be protected from layer-2 trafc loops. Traditionally,
Spanning Tree (STP)-based technologies were used to protect
rings, but they are relatively slow to recover from link failure.
This can create problems for applications that have strict loss
requirements, such as voice and video trafc, where the speed
of recovery is highly signicant.
EPSR enables rings to recover rapidly from link or node
failures within as little as 50ms, depending on por t type and
conguration. This is much faster than STP at up to 30 seconds,
or even Rapid STP (RSTP) at 1 to 3 seconds. EPSR, much
like STP, provides a polling mechanism to detect ring-based
faults and failover accordingly. But unlike STP, EPSR uses a fault
detection scheme to alert the ring that a break has occurred.
The ring then takes immediate action, instead of going through
an STP-like reconvergence.
Extremely low-latency signalling between the switches in the
ring enables very rapid detection of lost connectivity. The simple
topology enables immediate remedial action by the master
switch, with no requirement to spend any time exchanging
further signalling to conrm the network status. This almostinstant decision making makes EPSR a powerful solution, with
failover under fault conditions unnoticed by network and
application users.
Allied Telesis EPSR solution is extremely robust; its patented
technology providing the ability to handle unlikely complex fault
situations, like multiple failures.
The key proof of technology is customer experience. The
strong uptake of EPSR in demanding applications is testament it
has provided a superior solution to service providers and their
end users.
Diagram 1 shows a ring of switches that could be employed
as a network core or distribution solution for an enterprise
business, or service provider network. EPSR maintains ‘alwayson’ network availability by monitoring the health of the ring, and
utilizing a reverse path for trafc almost instantaneously in the
event of a link or node failure.
Transit
Node
Master
Transit
Node
10Gbps Ethernet
Transit
Node
Diagram 1: EPSR in a ring of switches
Resilient Net working with EPSR | 3
In some scenarios it is useful to implement an EPSR multi-ring
topology. For example, a service provider may have an access
ring connecting customers, and a distribution ring connecting
to services. A multi-ring topology is an excellent way to provide
a broadly distributed network that is still high performing,
and may require different bandwidth in different parts of the
network.
The multiple EPSR rings are likely to share a set of protected
VLANs. If these rings share a common segment, as shown in
diagram 2 below, there is the possibility of an undesirable loop
Master
forming out of both EPSR rings if the common segment was to
fail – this is known as a SuperLoop. The resultant SuperLoop
would leave a network storm state, with trafc circling the
SuperLoop indenitely causing performance issues and outages.
To prevent any possibility of a super-loop being formed, Allied
Telesis EPSR solution provides Super Loop Prevention (SLP).
EPSR-SLP ensures that multi-ring topologies are managed, and in
the event of any common segment failure no network loop can
be formed. The network gracefully handles any fault condition,
and ensures access to online services is always maintained.
Transit
Node
Common Segment
Transit
Node
Master
Both EPSR instances carry the same Data VLANs, therefore the
shared link is called a Common Segment, and this is a SuperLoop
topology which requires SuperLoop Prevention.
Diagram 2: Multi-ring topology using EPSR-SLP
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