Table of Contents

Configuring Packet over SONET

This chapter describes basic configuration tasks for Packet over SONET (POS) connections, and includes the following sections:

About Packet over SONET

Synchronous Optical Network (SONET) is an octet-synchronous multiplex scheme that defines a family of standard rates and formats. Optical specifications are defined for single-mode fiber and multimode fiber. The transmission rates are integral multiples of 51.840 Mbps. For example, the POS OC-12c uplink interface provides 622.080 Mbps over single-mode optical fiber.

POS provides for the serial transmission of data over SONET frames using either High-Level Data Link Control (HDLC) protocol, the default, or Point-to-Point Protocol (PPP) encapsulation. On serial interfaces, Cisco's implementation provides the error detection and synchronous framing functions of traditional HDLC without the windowing or retransmission functions that are found in traditional HDLC.

Because SONET/SDH (Synchronous Digital Hierarchy) is by definition a point-to-point circuit, PPP is well suited for use over SONET links. The octet stream is mapped into the SONET/SDH synchronous payload envelope (SPE) in accordance with RFC 2615, "PPP over SONET/SDH," and RFC 2615, "PPP in HDLC-like Framing." Octet boundaries are aligned with the SPE octet boundaries, and the PPP frames are located by row within the SPE payload. Because frames are variable in length, the frames can cross SPE boundaries. Using this scheme, multiprotocol data can be encapsulated and transported directly into SONET frames without relying on ATM to provide Layer 2 capability (for example, in IP over ATM over SONET).

About the POS OC-12c Uplink Interface

POS technology is ideally suited for networks that are built for providing Internet or IP data. It provides superior bandwidth utilization and efficiency over other transport methods. For expensive WAN links, POS can provide as much as 25 to 30 percent higher throughput than ATM-based networks. Transporting frames directly into the SONET/SDH payload eliminates the overhead required in the ATM cell header, IP over ATM encapsulation, and segmentation and reassembly (SAR) functionality.

Figure 6-1 shows a typical application of the POS OC-12c uplink interface module in an enterprise setting. Here, POS links among Catalyst 8540 campus switch routers in each building comprise the enterprise backbone.

Figure 6-2 shows an example of a service provider application of the POS OC-12c uplink interface module. Here, traffic aggregates from Catalyst 8500 CSRs over OC-12c POS interfaces to Cisco 12000 GSRs. POS OC-48 interfaces on the Cisco 12000 gigabit switch routers then provide the uplinks to the Internet backbone.

Configuring the POS OC-12c Uplink Interface

This section describes the default configuration of the POS OC-12c uplink interface and initial properties you must configure for a newly installed interface. It also describes optional configurations you can use to customize the interfaces for the requirements of your network.

Note   The POS OC-12c uplink interface module consists of one OC-12c port and one enhanced Gigabit Ethernet port. For instructions on configuring the Gigabit Ethernet interface, see the "About the Enhanced Gigabit Ethernet Interfaces (Catalyst 8540)" section.

Default Configuration

Table 6-1 shows the default configuration of an enabled POS OC-12c uplink interface. To change any of these values, see the instructions in the "Initially Configuring the POS Interface" section and the "Customizing the Configuration" section.

Initially Configuring the POS Interface

Configure the following properties for a newly installed POS OC-12c uplink interface:

Configure the following properties to match those of the end interface:

To initially configure the POS OC-12c uplink interface, perform the following steps, beginning in global configuration mode:

Example

The following example shows how to initially configure a POS OC-12c uplink interface:

Automatic Reverting of Clock Source

If your system clock source is set to line clock, it uses the recovered received clock to transmit. Some conditions cause severe degradation of the signal quality and the received clock becomes unreliable. Because your system software monitors SF (signal failure), it knows when there is severe degradation in the signal quality and resorts to using the internal clock temporarily. When the conditions that caused the signal quality to deteriorate clear, your system reverts to the line clock.

When two POS interface modules are connected and configured with the default line clock, the signal quality can degrade over time, and both POS interfaces will revert to the internal clock. As soon as the signal quality improves, both POS interfaces revert to using the line clock. This cycle repeats itself causing the line protocol on both interfaces to toggle. You can prevent this situation by configuring one end of the connection with the default line clock and the other with the internal clock.

In addition, degradation in the signal quality causes the POS interfaces to revert back to the clock source under the following conditions:

Additional Configurations

To configure additional properties to match those of the far end interface, perform the following steps, beginning in global configuration mode:

Note   These steps apply to the POS OC-12c uplink interface on the switch router and to the far end interface.

Customizing the Configuration

This section describes how to customize the configuration of the POS OC-12c uplink interface to match your network environment.

Setting the MTU Size

To set the maximum transmission unit (MTU), perform the following steps, beginning in global configuration mode:

Note   The POS OC-12c uplink interface supports IP unicast and IP multicast fragmentation. For IP unicast fragmentation, the packet must ingress on a POS interface and egress on any interface. For IP multicast fragmentation, IP multicast data packets greater than 1500 bytes are fragmented to 1500 bytes on the ingress POS interface before being switched to other members in the multicast group. All the members in the multicast group must have an MTU equal to or greater than 1500 bytes.

Configuring Framing

The default framing mode for the POS OC-12c uplink interface is SONET STS-12c. You can also configure the interface for SDH STM-4, which is more widely used in Europe. To configure the framing mode on the POS OC-12c uplink interface, perform the following steps, beginning in global configuration mode:

Configuring SONET Overhead

You can set the SONET overhead bytes in the frame header to meet a specific standards requirement or to ensure interoperability of the POS OC-12c uplink interface with another vendor's equipment. To configure the SONET overhead, perform the following steps, beginning in global configuration mode.

The value of the c2 byte is determined as follows:

The value of the s1s0 bits is determined as follows:

Configuring POS SPE Scrambling

SONET payload scrambling applies a self-synchronous scrambler of polynomial X**43+1 to the synchronous payload envelope (SPE) of the interface to ensure sufficient bit transition density. Both ends of the connection must use the same scrambling algorithm.

To configure POS SPE scrambling, perform the following steps, beginning in global configuration mode:

Configuring SONET Alarms

The OC-12c POS uplink interface supports SONET alarm monitoring. To configure alarm monitoring, perform the following steps, beginning in global configuration mode:

To determine which alarms are reported on the POS interface, and to display the BER thresholds, use the show controllers pos command. For a detailed description of the pos report and pos threshold commands, refer to the Cisco IOS Interface Command Reference publication.

Configuring SONET Delay Triggers

A trigger is an alarm, which, when activated, causes the line protocol to go down.

This section describes how to configure line and section triggers, and path level triggers.

Line and Section Triggers

Table 6-2 lists the line and section alarms that are default triggers:

When one or more of the alarms in Table 6-2 are activated, the interface line protocol goes down without a delay. To delay triggering the line protocol of the interface from going down, you can issue a pos delay triggers line command. You can set the delay from 50 to 10000 ms. If you do not specify a time interval, the default delay is set to 100 ms.

Path Level Triggers

Table 6-3 lists path alarms that are not triggers by default. You can configure these path alarms as triggers and also specify a delay.

To configure the path alarms listed in Table 6-3 as triggers, you can issue the pos delay triggers path command. These triggers will bring down the interface line protocol. When you configure the path alarms as triggers, you can simultaneously specify a delay for the triggers. You can set the delay from 50 to 10000 ms. If you do not specify a time interval, the default delay is set to 100 ms.

The pos delay triggers path configuration can also bring the line protocol of the interface down when the higher of the B2 and B3 error rates is compared with the SF (signal failure) threshold. If the SF threshold is crossed, then the interface line protocol goes down.

To configure a delay in triggering the interface line protocol from going down, perform the following steps, beginning in global configuration mode:

Verifying the POS Configuration

To verify the configuration of the POS OC-12c uplink interface, use the following commands:

Examples

The following example shows output for the show interfaces pos command:

The following example shows output for the show protocols pos command:

The following example shows output for the show controllers pos command:

The following example shows output for the show controllers pos command with the detail option:

About APS

Automatic protection switching (APS) allows a switchover of packet-over-SONET circuits if a circuit fails and is required when connecting SONET equipment to telco equipment. APS uses a protection interface in the SONET network as the backup for the primary, or working, interface. When the working interface fails, the protection interface quickly assumes its traffic load.

The OC-12c POS uplink interface supports 1+1 architecture as described in the Bellcore publication TR-TSY-000253, "SONET Transport Systems: Common Generic Criteria," Section 5.3.

You can configure APS in one of two ways on the OC-12c POS uplink interface:

Configuring APS Mode

For APS operation, the APS mode must be specified for each interface associated with a redundant pair of interfaces. You must configure the working interface before configuring the protection interface. Otherwise, the protection interface becomes the active interface and disables the working interface.

To configure the working interface, perform the following steps, beginning in global configuration mode:

To configure the protection interface, perform the following steps, beginning in global configuration mode:

Example

The following example shows how to configure APS for POS interface 1/0/0:

Switch# configure terminal
Switch (config)# interface pos1/0/0
Switch (config-if)# aps working 2
Switch (config-if)# aps group 1
Switch (config-if)# end
Switch (config)# interface pos3/0/0
Switch (config-if)# aps protect 1 172.31.235.200
Switch (config-if)# aps group 1
Switch (config-if)# end
Switch#

In the above example, interface 1/0/0 is the working interface and 3/0/0 is the protection interface.

Changing APS Settings

APS priority level commands manually control the relationship between two APS interfaces. The priority levels are defined in the Telcordia publication "GR-253-CORE, SONET Transport Systems: Common Criteria," Table 5.3.

To set the APS levels, use any of the following commands in interface configuration mode:

Note   The lockout and force priority levels post a request to switch away from the protection or working interface specified. For example, if you issue the aps force command while in interface configuration mode on the protection interface, it causes the system to switch to the working interface, away from the protection interface. The lockout and force commands do not persist over a system restart.

Example

The following example shows how to force the system to use the protection channel associated with POS interface 1/0/0:

Switch# configure terminal
Switch(config)# interface pos2/0/0
Switch(config-if)# aps force 1
Switch(config-if)#

Set APS Signal Thresholds (Catalyst 8540)

You can configure the APS signal bit error rate (BER) thresholds to initiate a signal degrade or signal failure. To configure the thresholds, use the following interface configuration mode commands:

Note   The value variable represents the exponent of the BER threshold. For instance, a value of 5 would set the threshold to 10-5. The default value is 10-3, which is adequate for most APS installations.

Example (Catalyst 8540)

The following example shows how to set the APS signal degrade and signal failure thresholds for ATM interface 1/0/0:

Switch(config)# interface atm 1/0/0
Switch (config-if)# aps signal degrade BER threshold 7
Switch (config-if)# aps signal-fail BER threshold 5

To display the current BER threshold settings for an interface, use the show running interface atm card/subcard/port command.

Configuring Other APS Options

To configure other APS options, perform any of the following tasks in interface configuration mode. The tasks listed in this section are optional.

Example

The following example shows how to configure aps authentication:

Switch# configure terminal
Switch(config)# interface pos2/0/0
Switch(config-if)# aps authentication 1
Switch(config-if)# end

Verifying the APS Configuration

To verify the APS configuration, use the following command:

The following example shows output for the show aps command: