| Sorry about the delay. The following text is adapted from the
DECrepeater 90FS (and 900FP) documentation. The first few sections
define terminology; the supported configurations follow.
Redundant Fiber-Optic Link Overview
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Redundant ports are usually configured between two or more repeaters to provide
a standby link in case the active link fails. If the active link fails, the
standby link is automatically activated to handle traffic between the repeaters.
The basic redundant configuration consists of a pair of master ports and two
remote ports. The master ports control which of the two lines is active and
which is standby, and, therefore, the master ports are referred to as a master
pair.
The master ports can be connected to two remote ports on the same repeater
or on different repeaters, but the remote ports must be on the same LAN (or
extended LAN).
NOTE:
This redundant configuration does not result in a repeater loop, because only
one of the links is active.
The remote ports can be one of two types: a responder port or a non-responder
port. A responder port provides full fault detection because it signals the
master if it detects a failure. A non-responder port provides only partial
fault detection because it does not signal the master if a failure occurs.
NOTE:
The DECrepeater 90FS and the DECrepeater 900FP are the only current
DECrepeaters that can be configured as redundant port repeaters.
Redundant Fiber-Optic Link Components
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The following components make up redundant fiber-optic link configurations:
o Redundant-Link Pair
The RLP is a pair of fiber-optic links that provides backup communication in
the event of transmitter, receiver, or cable failures.
o Master
The master is the DECrepeater that contains the master control algorithm (MCA)
for the ports in the RLP. This algorithm controls the Link Enable signal that
goes to each of two ports in the RLP. The master uses this signal to enable
one port at a time.
o Responder Port
The RLP can contain two responder ports (referred to as responders) on the
remote end of the links from the master ports. The responders are
independent of each other, but reside on the same LAN (or extended LAN) as
each other. A responder can signal the master when it detects a failure on
its receive portion of the link. The responder does this by disabling the
Link Idle signal on its transmit port until the receive portion of the
link is operational again.
o Non-Responder Port
Any standard 10BaseFL device ports can be non-responder ports (referred to
as non-responders). Unlike a responder, a non-responder cannot signal the
master when there are receive link failures on its end of the link. If you
define an RLP with non-responders, only partial fault detection is
provided on the RLP.
o Primary Link
Defined at initialization time by the MCA, the primary link is initially
the active link providing Ethernet communications. Activity defaults to
the primary link after a reset condition.
o Secondary Link
The secondary link is the other link of the RLP to which the master fails
over if there is a failure in the primary link.
o Active Link
The active link is the link that is currently enabled by the master to carry
Ethernet traffic.
o Standby Link
The standby link is the link that is currently disabled by the master and
cannot carry Ethernet traffic. This port is held in readiness as a backup
link in case the active link fails. On a functional link, the responder
can still place traffic on the link, but there is no traffic from the
master on that link.
o Master Control Algorithm
The Master Control Algorithm (MCA) enables only one link of the RLP at any
time and controls the RLP as follows:
1. The MCA enables the primary link at initialization time (assuming that
there are no failures on that link). This causes the primary link to
become active.
2. The MCA monitors the primary link status for failures.
3. When the MCA detects a failure of the active link, the MCA:
a. Disables the active link.
b. Enables the standby link.
The active link always remains enabled (active) until a failure is detected
on that link.
Redundant Fiber-Optic Link Operation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When you initially configure the DECrepeater for redundant operation,
the primary port is the active port and the secondary port is the standby
port. In operation, the active port communicates with its remote responder
port until the master detects a link failure or until the port is
internally partitioned. If the standby port is operational, the primary
port is disabled, and the secondary port is enabled as the active port.
The failover typically occurs within 4 to 8 ms (15 ms maximum).
Once the primary link is repaired, it becomes the standby link. If the
repeater detects a failure in the (now active) secondary link, or if the
port is internally partitioned, failover occurs back to the primary port.
A link failure can be caused by any of the following occurrences:
o Loss of carrier (light in a fiber-optic device)
o Auto-partitioning of the port
Simple Redundant Link Configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The DECrepeater firmware allows the configuration of a redundant
fiber-optic Ethernet link from multiple ports. In its simplest form, a
redundant link consists of a pair of master ports that reside on one
module, each connected to independent ports on a remote module. These
remote ports may be responder ports or non-responder ports.
Once these ports are properly configured using HUBwatch and connected
together, one connection is the active link and the other is the standby
link.
Complex Redundant Link Configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The DECrepeater also supports a more complex redundant link topology.
In general, the redundant link consists of a pair of master ports on one
module, each connected to enabled remote ports. The remote ports must
reside on the same LAN (or extended LAN), but can be on separate modules.
The remote ports can be responders, non-responders, or a combination of
the two. If your configuration includes a combination of a responder and a
non-responder, configure the responder as the primary link, and the
non-responder as the secondary link.
Partial Fault Detection
~~~~~~~~~~~~~~~~~~~~~~~
Partial fault detection causes the active master port to fail over to the
standby port if the active port detects any of the following
circumstances:
o A link failure in in the receive portion of the active port
o An internally detected failure or partitioning of the active port
- An excessive number of collisions
- An excessively long collision
- Received jabbering
- Internally detected MAU failure
When the DECrepeater fiber-optic ports are connected to a
non-responder repeater, such as the DECrepeater 90FL, 90FA, or any IEEE
802.3 10BaseFL repeater port, partial fault detection is available.
Partial fault detection adequately detects most failure conditions, but
cannot detect a failure in which the remote port cannot receive
transmissions. That level of failure detection requires full fault
detection.
Full Fault Detection
~~~~~~~~~~~~~~~~~~~~
The DECrepeater supports full fault detection. This feature enables the
master module to react to failures at either end of the receive or transmit
media.
If a failure occurs in the responder's receive port, the responder signals
its partner master port. This allows the master to switch over to the standby
port.
When the DECrepeater fiber-optic ports are connected to another
redundant fiber-optic repeater, such as the DECrepeater 900FP or another
DECrepeater full fault detection is available.
Table 3 shows the difference in failure detection between partial fault
detection and full fault detection.
Table 3: Partial Fault Detection Versus Full Fault Detection
---------------------------------------------------------------------------
Link Failure from the | |
Master Port's Perception| |
| |
TX RX | Partial Fault Detection | Full Fault Detection
---------------------------------------------------------------------------
Failure | not detected | detected
| |
Failure | detected | detected
| |
Failure Failure | detected | detected
---------------------------------------------------------------------------
Recommendations for Redundant-Link Configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following recommendations when configuring the DECrepeater
for redundancy to increase fault detection and improve network reliability:
o Use responder ports (rather than non-responder ports) whenever possible.
o If you use a combination of responder ports and non-responder ports,
assign the responder as the primary link, and the non-responder as the
secondary link.
o Assign the remote ports to separate modules. This ensures that one of
the two remote ports is functional in the event of a remote module
failure.
o Whenever possible, configure primary and secondary link cables into
separate pathways, to avoid a single point of failure.
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