This document reviews the recommended clock source settings for packet
over SONET (POS) router interfaces connected over dark fiber, back-to-back, or
across a telephone company (Telco) network.
Select the best clock settings to ensure accurate data recovery and to
avoid SONET-layer errors.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
This table summarizes the recommended clock settings for router POS
Clock Source at Both Ends of the POS Link
Back to Back with Dark Fiber or DWDM
Telco Network with ADM or MUX
internal - internal
internal - line
line - internal
line - line
The rest of this document discusses the reasons for these recommended
Cisco recommends that you configure internal-to-internal or
line-to-internal in this configuration. Do not set both sides to derive
clocking from the line in this configuration in order to avoid frequency drifts
and line interruptions, including intermittent errors and even link failures.
In order to configure two routers for a back-to-back connection, use
the clock source internal command.
ip address 126.96.36.199 255.255.255.0
clock source internal
ip address 188.8.131.52 255.255.255.0
clock source internal
This section explains the importance of internal-to-internal for a
back-to-back or dark fiber configuration. Figure 1
illustrates a back-to-back topology.
Figure 1 – Back-to-back Topology
Figure 2 illustrates a POS connection over
Figure 2 – POS Connection Over Dark Fiber
A common misconception about clocking is that both ends of any
synchronous link must use the same clock, so one end must derive clocking from
the line. This statement is true for DCE-to-DTE connections. However, this
statement is not true for bidirectional layer-1 links, such as SONET.
Here is an example to explain why this statement does not hold good for
bidirectional layer-1 links:
Figure 3 – Bidirectional Layer-1 Link
Here, each one-way link is synchronized.
However, both one-way links do not have to be synchronized together. In
other words, the link from left to right does not need to be synchronous with
the link from right to left.
Consider that a POS interface consists of two physical strands of
fiber. Each strand provides a one-way link.
Importantly, with clock source internal, the
router performs these actions:
Therefore, you can configure internal clocking on both router ends. The
clock source command determines the source of the
transmit clock only.
A packet-based application of SONET – and any SONET-based
point-to-point configuration – supports internal-internal clock settings with
either Stratum 3 or Stratum 4 oscillators. Clocks must comply with the SONET
Minimum Clock (SMC) specification, which defines 20 parts per million (ppm)
accuracy. The original SONET networks, which supported point-to-point OC-48
links that typically carried DS-3 frames, and the pre-SONET Plesiochronous
Digital Hierarchy (PDH) networks also were timed with 20ppm clocks. These early
SONET systems are a direct analogy to POS links of today, which define a
point-to-point connection between two routers with asynchronous interfaces to
the rest of the network.
Point-to-point means that the SONET payload terminates at each POS
interface. The router then extracts IP packets from within the PPP-encapsulated
frame and forwards the packets to an output interface as though any non-POS
interface, such as a serial or Ethernet interface receives the packets. This
means that you can time each POS link independently, and you do not need to
have all the POS interfaces on a router synchronized to a common clock.
POS mapping uses HDLC-like framing, and fills the gaps between
consecutive packets with idle flags. That way, the IP payload rate is
de-coupled from the SONET frame rate. The mapping does not require an extremely
accurate clock to generate the outgoing SONET frame rate, and a 20ppm clock
accuracy is more than sufficient. The huge buffers that the receiving
interfaces use minimizes the impact of any excessive jitter.
Multi-node SONET networks can also transport payloads reliably with
internal clocking configured at every node when the clocks are at least
Stratum-3 accurate. However, Cisco does not recommend such configuration.
Stratum 4 accurate clocks can result in a high rate of pointer justifications,
which may lead to exceeding the jitter tolerance of the serviced asynchronous
In summary, consider these points when you select a clock setting for
back-to-back or dark fiber POS links:
POS defines a point-to-point technology. The SONET link terminates
completely on the line card. No SONET information is passed between ports in a
router. In contrast, a SONET add-drop multiplexer (ADM) typically passes the
synchronous payload envelope (SPE) from ingress to egress port and modifies the
pointer bytes to accommodate any timing offset between the two ports.
POS uses an asynchronous mapping. The SONET frame determines the rate
at which packets are "stuffed" into SONET frames byte-by-byte. On the transmit
side, a router POS interface sets the H1/H2 pointer bytes to a fixed value of
522. This value is chosen because the pointer value positions the SPE at the
beginning of the frame that follows the pointers. Framer designers have to pick
some arbitrary value to use, so they tend to pick "nice" values, like 522. In a
dark fiber or DWDM configuration, the path does not include any equipment that
alters or processes the pointer bytes, so the SONET frame arrives at the
receiver with the same fixed value of 522 for the H1/H2 bytes. Thus, there is
no possibility of clocking slips or SPE slips.
Alternately, you can configure one end of the link for clock source
line. Importantly, the result of this configuration is that the transmitter now
uses the clock recovered from the line by the local receiver, to time the
Configure clock source line on one end (and
only one end) of your POS link when the derived clock source is of higher
quality than the clock available on the router POS interface. Engine 3 and
Engine 4 line cards of the Cisco 12000 Series use a Stratum 3 clock source. All
Engine 0 - 2 line cards other than the 1xOC48 SRP line card (OC48/SRP-SR-SC-B)
use an SMC source. A byproduct of a line-internal configuration is that both
directions of the link use the same clock, but this does not need to be the
The disadvantage of line-internal is that a clock hit in one direction
causes the interface trying to time itself from the line to send errors out
because it is now using a "bad" signal as its source. Internal-internal
separates the two clocking domains. An error on one side does not cause an
error on the other. Clocking internal on both sides ensures that an error in
the received clock (on loop side) does not affect Tx traffic.
The discussion so far illustrates that the clock source
line configuration on both ends of a POS link is inherently
unstable. With line-to-line, both transmitters use the clock received from the
remote end, and neither end actually supplies the clock. This incorrect
configuration leads to a timing loop.
Note: A limited batch of 1xOC12 POS line cards for the GSR experienced
timing-related errors due to a problem with the onboard oscillator. The
oscillator required the incoming and outgoing clocks to be identical.
Therefore, appropriate line-internal clock setting configurations resolved most
timing-related errors. This problem does not affect any other POS line cards.
With this configuration, Cisco recommends that you configure both sides
to derive clocking from the line. Cisco router POS interfaces use line clocking
by default. Configure clock source line if you
previously changed the clock setting.
Figure 4 illustrates a POS connection over a
Figure 4 – POS Connection Over a SONET
Typically, the SONET cloud provides a more accurate or higher Stratum
level clock source than router hardware. In rare circumstances, a POS interface
increments the PSE / NSE counters and reports pointer adjustments with
line-line clocking. Such pointer adjustments indicate a problem with timing or
clock drift in the provider network. Report any such problems to the provider.