When you design the synchronization plan for a network of Cisco WAN
switches, the fundamental design goals include:
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
It is the responsibility of the network administrator to identify and
define the available network clock sources during the creation of the network
Network versus Node Clock Sources
It is critical to note that the network administrator does not
explicitly identify which clock source is to be used by each node in an IGX or
BPX network. Instead, the network, as a part of switch software, automatically
selects the best available clock source and path for each node.
For MGX nodes, the network can do this as well if the Network Clock
Distribution Protocol (NCDP) features are enabled. Otherwise, the administrator
is required to manually select the clock sources per node.
Available clock sources can be any number of these items:
Any trunk between nodes that is explicitly clocked (typically by
the carrier that provides the facility). The definition of one or both
endpoints of such a "clocked" trunk as clock source(s) allows the network to
synchronize to the high-quality clock that the carrier provides.
Note: This includes subrate trunks with X.21, V.35, or RS-449
interfaces on NTM cards in IGX nodes.
Any line that connects a piece of customer-premise equipment to the
network. This includes any T1, E1, T3, E3, OC3, and OC12 line (provided the
line provides a clock, which may not be the case in all networks). Note that
this does not include any T1 or E1 Frame relay ports, or any RS232, V.35, X.21,
or RS449 data or Frame Relay ports in the network.
Any external clock source that is connected to the External Clock
This table details the required clock signal formats:
In BPX/IGX networks, each clock source is defined as a primary,
secondary, or tertiary clock source. The designation of a clock source as
primary, secondary, or tertiary is strictly at the discretion of the network
administrator. The best available clock sources are typically defined as
primary with other clock sources defined as secondary or tertiary. In MGX
nodes, the selections are Primary or Secondary. Under NCDP, the stratum level
of a clock can be specified and the protocol selects from the available clock
sources considering the stratum level.
Automatic Node Clock Selection Algorithm – BPX/IGX
Once the available clock sources are defined for the network, the
system software of the network automatically determines the specific clock
source to be used by each node in the network.
In order to ensure resiliency, the node clock source selection
algorithm is re-executed as a result of these items:
The addition or deletion of a network clock source by the network
The failure of any defined network clock source.
The failure or repair of any node in the network.
The failure, repair, or clock configuration of any trunk in the
The algorithm used for node clock selection is very straightforward.
Each node uses the nearest, highest-priority (primary, secondary, tertiary, or
internal) clock source available to it.
Therefore, if there is only one primary clock source defined in the
network, then all nodes synchronize to it, if possible. If there is more than
one primary clock source defined in the network, then each node synchronizes to
the nearest (measured by hop count) primary source. (See item 6 for a
discussion of the implications when you have nodes synchronize to multiple
If there are no primary clock sources defined (or all are failed),
then each node synchronizes to the nearest secondary clock source.
If there are no primary or secondary clock sources defined (or all
are failed), then each node synchronizes to the nearest tertiary clock
If there are no primary, secondary, or tertiary clock sources
defined (or all are failed), then each node synchronizes to the internal
(stratum 3) clock source of the BPX node with the highest internal node number.
If there are no primary, secondary, or tertiary clock sources defined
(or all are failed), and there is no BPX node available, then each node
synchronizes to the internal (stratum 4) clock source of the IGX node with the
highest internal node number.
Pass Synchronization on a Trunk: Yes or No? What Does it
In the algorithm in item 2, and for NCDP, a node must be able to
indirectly synchronize to a remote clock source. This is accomplished with the
identification of a clock path between the remote clock source and the node.
Each element (node or trunk) in the path is synchronized to the previous
element "upstream" in the path. Thus, a node is frequency-locked to the
upstream trunk, which is then frequency-locked to the upstream node, which is
then frequency-locked to the next upstream trunk, and so on. This continues
until the defined clock source is reached.
The key to the success of such a scheme is the ability of a node to
frequency-lock to its neighbor over the trunk that joins them. This requires
that the trunk between the nodes be "unclocked," or able to pass
synchronization between nodes.
A trunk can have a setting of "Pass Sync : Yes" or "Pass Sync: No".
Use the cnftrk command to change the parameter.
Configure the trunk to not pass synchronization:
If the trunk is clocked by the carrier.
Unfortunately, there is no way for the nodes to determine
automatically whether a particular trunk is clocked or not. Similarly, there is
no test procedure that can be performed by the network administrator to
determine whether the trunk is clocked or not. This information must be
provided by the service provider.
If, for any reason, the network administrator wishes to prevent the
trunk from being included in the clock path between any nodes in the network.
This is sometimes done for trunks that are prone to frequent
Note: Subrate trunks by definition cannot pass clocks and are therefore
blocked from being configured as Pass Sync. Virtual trunks are physically
unable to pass clocking information but are not restricted from being
configured as "Pass Sync: Yes." Ensure that you do not configure the network to
pass clocking information across virtual trunks.
A trunk configured as "Pass Sync: Yes" cannot be configured as a
network clock source.
A trunk configured as "Pass Sync: No" is not used in the clock path
for any node.
Note: An IGX node cannot be included anywhere in the clock path of a BPX
node. The reason for this is that the clock recovery circuitry and internal
oscillator of the IGX is stratum 4 whereas the internal oscillator on the BPX
is stratum 3.
How Can I Tell if a Trunk is Clocked or Unclocked?
The simple answer is that only the service provider who provides the
trunk can determine this. The reason is that a particular trunk can be either
clocked or unclocked based on what equipment the trunk traverses inside the
infrastructure of the service provider.
Some reasonable rules of thumb are:
A cable is unclocked.
A fractional T1 trunk is usually clocked because it goes through
the Digital Access and Crossconnect System (DACS) of a carrier
A full T1 is usually not clocked unless it is provided by Sprint.
However, some short-haul trunks provided by other carriers may be
A T3 trunk is rarely clocked because broadband framing structures
are specifically designed to support a large number of DS3 data streams. Each
is clocked independently with the performance of dynamic bit
Loop Clock on a Trunk or a Line: Yes or No? What Does it
In the configuration command for each trunk and each line (the
cnftrk and cnfln
commands, respectively), there is a parameter that allows the network
administrator to specify "Loop Clock: Yes" or "Loop Clock: No." This parameter
specifies the source of the transmit clock (used to send bits from the node out
onto the trunk or line).
If "Loop Clock: No" (the default) is chosen, then the transmit clock
on the trunk or line is derived from the master clock of the node. (This is not
necessarily the internal oscillator of the node. If the node is
frequency-locked to a remote clock source or the internal oscillator on a
remote node, then the master clock of the node is not its internal oscillator.)
If "Loop Clock: Yes" is chosen, then the transmit clock on the trunk
or line is frequency-locked to the receive clock (derived from the incoming bit
stream) on the trunk or line. This is commonly done on:
A time-division multiplexing (TDM)-based line (such as one that
connects to a PBX) when the device at the other end of the line cannot be
synchronized to the node. This allows the device to transmit and receive bits
at its own frequency (which can be different than the frequency of the node).
This prevents the data loss associated with uncontrolled frame slips. In such a
case, the line and the attached CPE have no problem using a frequency that is
independent of the master clock of the node.
A trunk that is clocked by the carrier and the carrier's clock is
not used as the clock source for the node. This
configuration prevents uncontrolled frame slips (and the corresponding data
loss) in the carrier's facilities.
Is it OK to Have Multiple Clock Sources in Use in the
In some cases, it is unavoidable for some nodes and trunks in a
network to synchronize to one clock source and to synchronize other nodes and
trunks in the network to another clock source. This is especially common in
international networks or in networks in which trunks are obtained from a
variety of service providers. Such a network is said to be synchronized in a
If two pieces of equipment that are synchronized to different clock
sources are joined by an unclocked trunk, input buffers on the interfaces at
each node periodically overflow (at one end) or underflow (at the other end).
This overflow or underflow condition is commonly known as a frame slip because
an overflow condition usually causes one (or more) frame of data to be
In a TDM-based network, almost every frame slip causes data to be
lost, since there is likely to be data contained in at least one timeslot of
On a trunk in a FastPacket or ATM network, many idle packets or cells
transmit every second. All IGX and BPX trunk cards discard idle cells from the
network before they are buffered and processed. This prevents the error
condition of input buffer overflow from occurring.
Because of the fundamental characteristic of cell-based networking, a
network with a plesiochronous synchronization plan can usually operate free of
What Errors Do Clock Problems Cause?
Clocking problems typically cause frame slips on circuit-line
interfaces, especially circuit lines to TDM devices such as a PBX. Frame slips
can occur on either or both ends of the line. Both the PBX and the switch can
record frame slips. In order to help resolve frame slips, configure the
external equipment to receive clock from the network. If the external equipment
cannot accept network clock, configure the circuit line interface for loop
clock. If the configuration of one end of the circuit line for loop clock does
not eliminate frame slips, evaluate the clocking architecture of the network
and external equipment.
Clocking problems typically cause Packet Errors, HEC errors, PLCP
errors, or frame-sync errors. The error depends upon the type of trunk
interface used. Errors result from a difference in frequency between adjacent
nodes or Telco clocking of trunks. Clock errors on trunks typically occur on
one end. This is because the BPX or IGX trunk-card suppresses the input buffer
overflow by deleting idle cells. The errors indicate underflow slips. Configure
the end of the trunk that does not experience errors for loop clock to minimize
A trunk clocked by the carrier can exhibit errors on both ends.
Configure either or both ends of the trunk for loop clock to minimize the error