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Finally, check that the forwarding policy for the egress scheduler is set to "strict priority forwarding"
(at least for the PTP traffic class, but also recommended for the audio traffic class).
Note that on larger / corporate networks, especially if stretching across WAN connections, DSCP tags
may not be respected by edge routers (they may be configured to not trust the DSCP markings
originating from the local subnets and may even delete them). This will break the tight priority
forwarding requirements and may lead to increased packet delay variations, resulting in longer
latencies and degraded clock accuracy. Furthermore, after traversing any WAN link employing this
"DSCP no-trust" policy, the DiffServ priority mechanism may be irreparably broken for any
subsequent local network segments, eventually resulting in AES67 ceasing to work at all after
traversing a WAN link. You may have to consult with your network administrator to discuss options
to remove or bypass this constraint, if it exists.
2.1.5
PTP
Planning for PTP deployment is a topic on its own which may exhibit many complex facets, especially
if your network is larger and stretches several subnets. Larger networks in most cases require PTP-
aware switches (Boundary or Transparent Clocks) in strategic positions in the network. Due to the
complexity which may be involved in configuration of such networks, we limit the discussion of PTP
planning to a single LAN segment without PTP-aware switches.
2.1.5.1
PTP parameters
In most cases, PTP-aware switches are not required in LAN segments up to a medium size (several
tenths of end nodes). With standard COTS switches, proper QoS configuration should result in a
decent PTP performance. However, there are a few parameters of choice:
•
Domain number: unless required for certain reasons, leave the domain number to the default
value (0).
•
SYNC message interval: all AES67 devices are required to operate with the PTP Default
profile which has a default sync message interval of 1 second (2^0). Other choices under the
Default profile are 2^1 and 2^-1 - we recommend setting the SYNC message rate to 2^-1
for faster settlement and better stability.
AES67 also defines its own PTP profile, the Media profile. If all AES67 devices on the network
support this profile (this is not a requirement), you can reduce the SYNC message interval
down to 2^-4 – we recommend that you keep the SYNC message interval at the Media
profile default value of 2^-3.
•
ANNOUNCE message interval: ANNOUNCE messages are required to establish the best
master clock currently available on the network. We suggest that you keep the ANNOUNCE
message interval at the default value of 2^1 (applies both for the PTP Default and Media
profiles) and the ANNOUNCE message timeout interval at 3. Note: it is very important that
ALL devices have the same setting, otherwise the BMCA may not work as expected and
devices may not synchronize properly.
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AES67 Practical Guide
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