Overview of Unidirectional Links

Most of the traffic in a broadcast video cable network is directed downstream to the user. Conventional bidirectional links do not optimize bandwidth allocation to match the bandwidth requirements of this mostly one-way traffic flow. In addition, the bidirectional nature of ports requires a port to receive data from the same port that it transmits data to. This behavior quickly consumes port resources without using them effectively.

You can conserve port resources and address the bandwidth requirements by implementing unidirectional links in the network.

Physical interfaces operate in bidirectional mode by default, both transmitting and receiving traffic. When you configure unidirectional link mode on the interface, two new physical interfaces are automatically created. One interface, designated by -tx in the interface name, can only transmit traffic. The other interface, designated by -rx in the interface name, can only receive traffic. The parent physical interface is still present, but you effectively see a port with two unidirectional links. Figure 9 illustrates the unidirectional nature of the new interfaces.

You can configure unidirectional link mode on a per-port basis on the 10-Gigabit Ethernet interfaces of the following hardware only:

• 4-port 10-Gigabit Ethernet DPC on the MX960 Ethernet Services Router
• IQ2 PIC and IQ2E PIC on the T Series Core Routers

You can configure both unidirectional and bidirectional ports on a single DPC or PIC.

Configurable Options

The transmit-only and receive-only interfaces created on a port act independently. On the parent interface, you configure only the physical interface attributes common to both links. These attributes include clocking, framing, gigabit Ethernet options, and SONET options. On each of the unidirectional interfaces, you independently configure encapsulation (Ethernet only), MAC address, MTU size, address family (inet or inet6), and logical interfaces. VLAN tagging (untagged, single, stacked, or flexible) and VLAN IDs are also independently configurable on the receive-only and transmit-only interfaces. The full range of numbers for logical interfaces and VLAN IDs is available to be shared between both unidirectional interfaces. That is, the number configured on receive-only interfaces plus the total configured on the transmit-only interfaces cannot exceed the available range.

To forward packets, you can configure only static ARP entries and static routes separately on each of the unidirectional interfaces. This configuration enables the transmit-only and receive-only interfaces to link to different ports on different routers. No other method of packet forwarding is currently supported.

The transmit-only and receive-only interfaces are removed when you delete unidirectional link mode from the parent interface. The parent interface resumes operation as a normal, bidirectional interface.

Logical Interfaces

You cannot configure logical interfaces on the parent interface after you have configured unidirectional link mode. However, you can configure logical interfaces on both the transmit-only interface and the receive-only interface.

Alarm Reporting

Alarms and defects are not reported for the transmit-only interface. Only local alarms and defects are reported for the receive-only interface. This behavior enables the use of SONET in a WAN-PHY configuration. SONET alarms, defects, and performance monitoring require bidirectional communication between sender and receiver. By accepting only local defects and alarms, the receiver interfaces in such a configuration are decoupled from the senders.

Operational State

The transmit-only link on a unidirectional port is always operationally up. Operational state is not influenced by the state of the receive-only link on that port.

Operational state of the receive-only link on a unidirectional port is independent of the state of the transmit-only link on that port. Link state for a receive-only link is determined only by the status of locally detected faults on the that link. Change in the state of the receive-only link can trigger traps, flap messages, and alarms.

Statistics

Statistics are reported differently for each of the three interfaces.

• Parent physical interface: No logical interfaces can be configured on the parent interface when it is in unidirectional link mode. Therefore all traffic statistics for this interface are reported as zero. All port-level statistics are reported on the parent physical interface rather than the rx or tx physical interfaces.
• Transmit-only physical interface: All transmit traffic statistics are reported for this interface. All receive (input) statistics are reported as zero. Also shown here are statistics for any logical interfaces configured on the transmit-only interface.
• Receive-only physical interface: All receive traffic statistics are reported for this interface. All transmit (output) statistics are reported as zero. Also shown here are statistics for any logical interfaces configured on the receive-only interface.