PTP over Ethernet on ACX Series Routers Overview

Precision Time Protocol (PTP) is supported over IEEE 802.3 or Ethernet links on ACX Series routers. This functionality is supported in compliance with the IEEE 1588-2008 specification. PTP over Ethernet enables effective implementation of packet-based technology that enables the operator to deliver synchronization services on packet- based mobile backhaul networks that are configured in Ethernet rings. Deployment of PTP at every hop in an Ethernet ring by using the Ethernet encapsulation method enables robust, redundant, and high-performance topologies to be created that enables a highly precise time and phase synchronization to be obtained.

The ACX Series routers can be directly connected to different types of base stations (for example, base transceiver station (BTS) in 2G, NodeB in 3G, and eNodeB in 4G networks) and different types of routers that hand off time- division multiplexing (TDM), ATM, and Ethernet traffic to the base station controller. ACX Series routers must extract the network clock from these sources and pass on synchronization information to the base stations to help the routers synchronize with the base station controller.

Most of the network deployments that use Ethernet contain a minimum of two Ethernet rings, while some of the network topologies might also contain up to three Ethernet rings. Consider a scenario in which the first ring contains aggregation routers (MX Series routers) and the second ring contains access routers (ACX Series routers). In such a network, about 10 or 12 nodes of MX Series routers and ACX Series routers are present in the aggregation and access Ethernet rings.

Some of the 4G base stations that are connected to ACX Series routers need to receive the timing and synchronization information in a packet-based form. Such base station vendors support only packet interfaces that use Ethernet encapsulation for PTP packets for time and phase synchronization. Therefore, any node (an ACX Series router) that is directly connected to a 4G base station must be able to use the Ethernet encapsulation method for PTP on a primary port to support a packet-based timing capability.

PTP over Ethernet encapsulation also facilitates an easier, optimal network deployment model than PTP over IPv4. Using IPv4, the nodes (primary and client devices) participate in unicast negotiation in which the client node is provisioned with the IP address of the primary node and requests unicast messages to be sent to it from the primary node. A primary node is the router that functions as the PTP server where the primary clock is located and a client node is the router that functions as the PTP client where the client clock is located. Because PTP over Ethernet uses multicast addresses, the client node automatically learns about the primary nodes in the network. Also, the client node is able to immediately receive the multicast messages from the primary node and can begin sending messages to the primary node without the need for any provisioning configuration.

An interface on which the primary clock is configured is called a primary interface and an interface on which the client clock is configured is called a client interface. A primary interface functions as the primary port and a client interface functions as the client port. For PTP over Ethernet, apart from configuring a port or a logical interface to operate as a primary clock or a client clock, you can also configure a port or a logical interface to function as both a primary clock and a client clock. This type of port is called a dynamic port, stateful port, or a bidirectional port. Such a stateful port enables the network to more efficiently adapt to the introduction and failure of timing sources by forming the shortest synchronization trees from a particular source. This behavior is implemented as defined by the best primary clock algorithm (BMCA) in the ITU-T G.8265.1 Precision time protocol telecom profile for frequency synchronization specification.

On both MX Series and ACX Series routers, you can achieve the highest quality performance if you configure every node in a synchronization chain as a PTP boundary clock. In Ethernet ring-based topologies, you can configure a port or a logical interface to function either as a primary port or as a client port to enable redundancy when a node or link failure occurs. This dynamic port or dual-port functionality is in accordance with the IEEE 1588-2008 standard and enables the implementation of PTP in data center or financial applications.

Apart from enabling every node to be available for configuration as a PTP boundary clock, it is also necessary to enable a logical interface to be configured either as a primary port or a client port. When you configure a logical interface or even a shared IP address to be a primary port or a client port, a PTP protocol stack can represent dynamic ports and the PTP application selects the correct state (primary or client) for any specific port in the system based on the output of the default PTP BMCA and the states of other ports in the system.

While an ACX Series router supports the PTP over Ethernet functionality, a Brilliant Grand Primary such as an MX Series router or a TCA Series Timing Client does not support PTP over Ethernet. In such a scenario, the ACX Series router functions as a boundary clock with a PTP client port using IPv4 as the encapsulation mode and primary ports using Ethernet as the encapsulation mode for PTP traffic. For example, consider an ACX Series router named ACX1 to have two potential client interfaces, one that is fixed as a client-only port using IPv4 on the link toward an MX Series router named MX1, and a dynamic port that functions as a client port using PTP over Ethernet on the link toward another ACX Series router named ACX2. In addition, ACX1 also contains a port that is a primary-only port using PTP over Ethernet and connects to the base station.

Because PTP over Ethernet uses multicast addresses, a client port can automatically start receiving the multicast announce messages transmitted by the primary ports on a network and can also start communication with the primary node with minimal or no configuration. Unlike PTP over IPv4 where IP addresses are used to identify the primary and client ports, with PTP over Ethernet, multicast MAC addresses are used in the forwarding of PTP traffic. The IEEE 1588 standard defines two types of multicast MAC addresses 01-80-C2-00-00-0E (link local multicast) and 01-1B-19-00-00-00 (standard Ethernet multicast) for PTP over Ethernet operations.