Why G.8275.2?
The G.8275.2 profile is based on the partial timing support from the network. Hence nodes using G.8275.2 are not required to be directly connected.
The G.8275.2 profile is used in mobile cellular systems that require accurate synchronization of time and phase. For example, the fourth generation (4G) of mobile telecommunications technology.
PTP Clocks
Two types of ordinary clocks and three types of boundary clocks are used in this profile:
Ordinary Clocks (OCs)
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Telecom Grandmaster (T-GM): A telecom grandmaster provides timing for other devices in the network, and is usually connected to a server reference time source, such as a GNSS receiver. It does not synchronize its local clock to other network elements.
Considerations for a T-GM:
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Only one PTP port can be configured as a server port.
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One T-GM server port can have multiple clients associated with it.
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The T-GM OC server port is a fixed port; that is, it always acts as a server clock and its role does not change by negotiating with its peer.
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Partial-Support Telecom Time Subordinate/Client Clocks (T-TSC-P): A client clock synchronizes its local clock to another PTP clock (GM, T-GM or T-BC), and does not provide synchronization through PTP to any other device.
Considerations for a T-TSC-P:
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An ordinary clock with single client port can be configured.
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Only one peer clock address can be configured as clock source.
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Boundary Clocks (BCs)
Boundary clocks can assume any of the following roles:
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A BC that can only be a grandmaster (T-GM)
A server-only boundary clock can have multiple server port configured. The different server ports can be in different VLANs to serve the clients that need to be served over them.
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A BC that can become a grandmaster and can also be a client to another PTP clock (T-BC-P).
Client-only port configuration is not allowed under boundary clocks. However, one of the dynamic ports (port state negotiated based on BMCA), can assume the role of client.
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A BC that can only be a subordinate/client (T-TSC-P with more than one port).
Fixed server port, dynamic ports and virtual port can be configured under a boundary clock. However, only one clock source (peer address) can be configured with a dynamic port.
Miscellaneous Notes
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Any clock that has multiple PTP ports within a PTP domain is termed a boundary clock (BC). Ordinary clocks (OC) always have only one PTP port.
In G.8275.2 (02/2016), PTP transparent clocks are not permitted.
PTP Domain
A PTP domain is a logical grouping of clocks that communicate with each other using the PTP protocol.
A single computer network can have multiple PTP domains operating separately, for example, one set of clocks synchronized to one time scale and another set of clocks synchronized to another time scale. PTP can run over either Ethernet or IP, so a domain can correspond to a local area network or it can extend across a wide area network.
The allowed domain numbers of PTP domains within a G.8275.2 network are in the range of 44 and 63 (both inclusive). The default domain number is 44.
PTP Messages and Transport
The following PTP transport parameters are defined:
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In Cisco IOS XE Everest 16.5.1, PTP over IPv4 in unicast mode must be used..
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Either one-step or two-step clock mode must be used.
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For PTP primaary clock, both one-way and two-way operation modes are supported. This means PTP primaary can grant request to a subordinate's one-way or two-way requests.
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In case of PTP subordinate clock, two-way PTP operation is required to allow phase/time-of-day delivery. The delay-request-response mechanism is used to propagate delay measurement; the peer-delay mechanism is not used.
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The G.8275.2 profile supports unicast message negotiation.
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Interfaces carrying PTP traffic can be under different VRFs.
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Sync, Delay_Req, Announce, Follow_Up, Delay_Resp, and Signaling messages are used in this profile. See the table below for rates of transmission for these messages.
|
Message |
Default Rate (packets per second) |
Minimum Rate (packets per second) |
Maximum Rate (packets per second) |
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Sync |
32 |
1 |
128 |
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Follow_up (only if sync messages are used) |
32 |
1 |
128 |
|
Delay_Req |
16 |
1 |
128 |
|
Delay_Resp |
16 |
1 |
128 |
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Announce |
1 |
1 |
8 |
|
Signaling |
Not Specified |
1 |
Not specified |
Limitations
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Pdelay_Req, Pdelay_Resp, Pdelay_Resp_Follow_Up and management messages are not used in this profile.
PTP Ports
A port can be configured to perform either fixed primaary or subordinate role or can be configured to change its role dynamically. If no role is assigned to a port, it can dynamically assume a primaary, passive, or subordinate role based on the BMCA.
In G.8275.2, PTP ports are not tied to any specific physical interfaces, but are tied to a loopback (virtual) interface. Traffic from a PTP port is routed through any physical interface based on the routing decision.
For a Boundary Clock, multiple PTP ports are supported. The maximum number of PTP ports supported on a BC node is 64.
For a dynamic port, only one clock source can be configured.
Starting with Cisco IOS XE Release 16.12, PTP 8275.2 supports multiple loopbacks (not in VRFs) for fixed Master ports.
Virtual Port Support on T-BC
 Note |
The virtual port is not supported on the Cisco ASR 900 RSP2 Module. |
In G.8275.2 implementation, virtual PTP ports are used to provide electrical frequency and phase inputs to T-BC. With virtual ports, T-BCs are fed with frequency inputs, such as, synchronous Ethernet, 10M, BITS, and phase/time inputs, such as, 1PPS and ToD. Virtual ports participate in the BMCA of the T-BCs.
If frequency source is of Category-1 (according to G.8275.2) and if 1PPS and ToD inputs are UP, virtual port status is up. Otherwise, virtual port status is down.
A virtual port participates in BMCA only when it is in administratively up state.
A virtual port always has clock class 6, clock accuracy 0x21 (within 100ns), and clock offset Scaled Log Variance of 0x4E5D.
Note |
The virtual port has the attributes set to the above values only when it is in the UP state. |
Whenever virtual port is selected as the best server clock by the BMCA, PTP clock is driven by the electrical inputs. If virtual port is administratively up but not selected by BMCA, 1PPS and ToD inputs do not affect PTP clock.
Alternate BMCA
The BMCA implementation in G.8275.2 is different from that in the default PTP profile. The G.8275.2 implementation specifies an alternate best server clock algorithm, which is used by each device to select a clock to synchronize to, and to decide the port states of its local ports.
The following consideration apply to the G.8275.2 implementation of the BMCA:
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MasterOnly: A per port attribute, MasterOnly defines the state of the port. If this attribute is true, the port is never placed in the client state.
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Priority 1: Priority 1 is always static in this profile and is set to 128. Priority 1 is not used in BMCA.
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Priority 2: Priority 2 is a configurable value and its range if from 0 to 255.
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Local Priority: Local priority is configured locally on clock ports to set the priority on nominated clocks. The default value is 128 and valid range is from 1 to 255.
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