Cisco ONS 15600 SDH Reference Manual, Release 1.4
Chapter 6, Circuits and Tunnels

Table Of Contents

Circuits and Tunnels

Circuit Properties

Circuit Status

Circuit Protection Types

Viewing Circuit Information on the Edit Circuit Window

Circuit Filter

DCC Tunnels

Multiple Drops for Unidirectional Circuits

SNCP Circuits

Path Trace

Automatic Circuit Routing

Bandwidth Allocation and Routing

Secondary Sources and Drops

Manual Circuit Routing

Constraint-Based Circuit Routing

Bridge and Roll

Roll States

Roll Window

Single and Dual Rolls

Circuit Bridge and Roll Restrictions

Protected Circuits

Circuits and Tunnels

This chapter explains CiscoONS15600SDH VC4 high-order circuits and data communications channel (DCC) tunnels. To provision circuits and tunnels, refer to the CiscoONS15600SDH Procedure Guide.

Chapter topics include:

Circuit Properties

DCC Tunnels

Multiple Drops for Unidirectional Circuits

SNCP Circuits

Path Trace

Automatic Circuit Routing

Manual Circuit Routing

Constraint-Based Circuit Routing

Bridge and Roll

Note In this chapter, "cross-connect" and "circuit" have the following meanings: cross-connect refers to the connections that occur within a single ONS15600SDH to allow a circuit to enter and exit an ONS15600SDH. Circuit refers to the series of connections from a traffic source (where traffic enters the ONS15600SDH network) to the destination (where traffic exits an ONS15600SDH network).

6.1  Circuit Properties

The ONS15600SDH supports unidirectional and bidirectional circuits. Subnetwork connection protection (SNCP) or multiplex section-shared protection ring (MS-SPRing) circuits can be revertive or nonrevertive. Circuits will route automatically or you can manually route them. The autorange feature eliminates the need to build circuits of the same type individually; Cisco Transport Controller (CTC) can create up to five sequential circuits. You must specify the number of circuits that you need and build the first circuit.

You can provision circuits at either of the following points:

Before cards are installed. The ONS15600SDH allows you to provision slot and circuits before installing the traffic cards. However, circuits will not carry traffic until the cards are installed, the circuit status is In Service (IS), and the port status is IS or Out of Service-Maintenance (OOS-MT).

Cards are installed and their ports are in service. Circuits will carry traffic as soon as the signal is received.

The ONS15600SDH Circuits window ( Figure6-1), which is available from network, node, and card view, is where you can view information about circuits, including:

Name—The name of the circuit (user-assigned or automatically generated).

Type—For the ONS15600SDH, the circuit type is VC4 (VC4 circuit).

Size—VC4 circuit sizes can be VC4, VC4-2c, VC4-3c, VC4-4c, VC4-8c, VC4-16c, or VC4-64c.

Protection—The protection type; see the "Circuit Protection Types" section .

Direction—The circuit direction, either two-way or one-way.

Status—The circuit status; for details, see the "Circuit Status" section .

Source—The circuit source in the format node/slot/port/VC4.

Destination—The circuit destination in the format node/slot/port/VC4.

# of VLANS—The number of VLANs used by an Ethernet circuit (future use for the ONS15600SDH).

# of Spans—The number of internode links that compose the circuit.

State—The circuit state. The ONS15600SDH Release 1.4 does not support a full state model. As a result, you cannot change the circuit state; it is always IS.

Note You cannot set up low-order (VC3 and VC12) circuits to terminate on an ONS15600SDH node. However, you can create both VC4 and low-order circuits that have an ONS 15454 SDH source and destination with an ONS15600SDH as a pass-through node. For information on low-order circuit creation and tunneling, refer to the circuit chapters in the CiscoONS15454SDH Reference Manual. Note that you cannot mix protection schemes, for example, 1+1 to SNCP. Acceptable schemes are unprotected to unprotected, 1+1 to 1+1, MS-SPRing to MS-SPRing, and SNCP to SNCP.

Figure 6-1 ONS 15600 SDH Circuit Window in Network View

6.1.1  Circuit Status

The circuit statuses that appear in the Circuit window Status column are generated by CTC based on an assessment of conditions along the circuit path. Table6-1 lists the statuses that can appear in the Status column.

Table 6-1 ONS 15600 SDH Circuit Status 



CTC is creating a circuit.


CTC created a circuit. All components are in place and a complete path exists from circuit source to destination.


CTC is deleting a circuit.


A CTC-created circuit is missing a connection or circuit span (network link), a complete path from source to destination(s) does not exist, or a MAC address change occurred on one of the circuit nodes and the circuit is in need of repair. (In the ONS 15454 SDH, the MAC address resides on the alarm interface panel [AIP]; in the ONS15600SDH, the MAC address resides on the backplane EEPROM.)

In CTC, circuits are represented using cross-connects and network spans. If a network span is missing from a circuit, the circuit status is INCOMPLETE. However, an INCOMPLETE status does not necessarily mean a circuit traffic failure has occurred because traffic might be on a protect path.

Network spans are in one of two states: up or down. On CTC circuit and network maps, up spans appear as green lines ( Figure6-1) and down spans appear as gray lines. If a failure occurs on a network span during a CTC session, the span remains on the network map but its color changes to gray to indicate the span is down. If you restart your CTC session while the failure is active, the new CTC session cannot discover the span and its span line will not appear on the network map.

Subsequently, circuits routed on a network span that goes down will appear as ACTIVE during the current CTC session, but they will appear as INCOMPLETE to users who log in after the span failure.


A TL1-created circuit or a TL1-like CTC-created circuit is complete and has upgradable cross-connects. A complete path from source to destination(s) exists. The circuit can be upgraded.


A TL1-created circuit or a TL1-like CTC-created circuit with upgradable cross-connects is missing a cross-connect or circuit span (network link), and a complete path from source to destination(s) does not exist. The circuit cannot be upgraded until missing components are in place.


A TL1-created circuit or a TL1-like CTC-created circuit is complete but has at least one nonupgradable cross-connect. SNCP_HEAD, SNCP_EN, SNCP_DC, and SNCP_DROP connections are not upgradable so all unidirectional SNCP circuits created with TL1 are not upgradable.


A TL1-created circuit or a TL1-like CTC-created circuit with one or more nonupgradable cross-connects is missing a cross-connect or circuit span (network link); a complete path from source to destination(s) does not exist.


Roll is awaiting completion or cancellation. When a roll is in the ROLL PENDING state, you can complete a manual roll and cancel an automatic or manual roll.

6.1.2  Circuit Protection Types

The Protection column on the Circuit window shows the card (MS) and SDH topology (AU4) protection used for the entire circuit path. Table6-2 lists the protection type indicators that you will see in this column.

Table 6-2 Circuit Protection Types 

Protection Type

Circuit protection is not applicable.

2F MS-SPRing

The circuit is protected by a 2-fiber MS-SPRing.


The circuit is protected by an SNCP.


The circuit is protected by a 1+1 protection group.


The circuit is protected by diverse SDH topologies, for example a MS-SPRing and an SNCP, or an SNCP and 1+1.

Unprot (black)

The circuit is not protected.

Unprot (red)

A circuit created as a fully protected circuit is no longer protected due to a system change, such as a traffic switch.


Circuit protection types appear in the Protection column only when all circuit components are known, that is, when the circuit status is ACTIVE or UPGRADABLE. If the circuit is in some other status, the protection type appears as "unknown."

6.1.3  Viewing Circuit Information on the Edit Circuit Window

When Show Detailed Map is checked on the Edit Circuit window, you can view information about ONS15600SDH circuits. Routing information includes:

Circuit direction (unidirectional or bidirectional)

The nodes and VC4s that the circuit traverses, including slots and port numbers

The circuit source and destination points

Open Shortest Path First (OSPF) area IDs

Link protection (SNCP, unprotected, MS-SPRing, 1+1) and bandwidth (STM-N)

For MS-SPRings, the detailed map shows the number of MS-SPRing fibers and the MS-SPRing ring ID. For SNCPs, the map shows the active and standby paths from circuit source to destination, and it also shows the working and protect paths.

Alarms and states can also be viewed on the circuit map, including:

Alarm states of nodes on the circuit route

Number of alarms on each node organized by severity

Port service states on the circuit route

Alarm state/color of most severe alarm on port


Path trace states

Path selector states

Figure6-2 shows a bidirectional VC4 circuit routed on an SNCP.

Figure 6-2 SNCP Circuit on the Edit Circuits Window

By default, the working path is indicated by a green, bidirectional arrow, and the protect path is indicated by a purple, bidirectional arrow. Source and destination ports are shown as circles with an S and D, respectively. Port status is indicated by colors, shown in Table6-3.

Table 6-3 Port State Color Indicators

Port Color



Light blue


Figure6-3 shows a popup for an SNCP span. The detailed circuit map also provides popup span information for MS-SPRings.

Figure 6-3 Detailed Circuit Map Showing Span Information

In addition to providing circuit information, the detailed circuit map allows you to add a drop and initiate a path trace:

To add a drop to a circuit, right-click a unidirectional circuit destination node and choose from the menu.

To initiate a path trace, right-click a port containing a path trace capable card and choose from the menu.

6.1.4  Circuit Filter

The ONS15600SDH will support up to 2048 VC4 circuits. The Circuit Filter feature allows you to reduce the number of circuits that appear on the Circuits window ( Figure6-4). You can specify certain filter criteria, such as name, direction, and state; only the circuits that match the criteria will appear on the Circuits window.

Figure 6-4 Filtering Circuits

6.2  DCC Tunnels

SDH provides four DCCs for network element Operation, Administration, Maintenance, and Provisioning (OAM&P): one on the SDH regenerator section (RS) layer (DCC1) and three on the SDH multiplex section (MS) layer (DCC2, DCC3, DCC4). The ONS15600SDH uses the RS DCC for ONS15600SDH management and provisioning.

You can use the three MS DCCs and the RS DCC (when not used for ONS15600SDH DCC terminations) to tunnel third-party SDH equipment across ONS15600SDH networks. A DCC tunnel endpoint is defined by slot, port, and DCC, where DCC can be either the RS DCC or one of the MSDCCs. You can link MS DCCs to MS DCCs and link RS DCCs to RS DCCs. You can also link a RSDCC to a MS DCC and a MS DCC to a RS DCC. To create a DCC tunnel, connect the tunnel endpoints from one ONS15600SDH optical port to another.

Table6-4 lists the DCC tunnels that you can create.

Table 6-4 DCC Tunnels

SDH Layer
SDH Bytes



D1 to D3



D4 to D6



D7 to D9



D10 to D12

When you create DCC tunnels, keep the following guidelines in mind:

Each ONS15600SDH can have up to 64 DCC tunnel connections.

A RS DCC that is terminated cannot be used as a DCC tunnel endpoint.

A RS DCC that is used as an DCC tunnel endpoint cannot be terminated.

All DCC tunnel connections are bidirectional.

6.3  Multiple Drops for Unidirectional Circuits

Unidirectional circuits can have multiple drops for use in broadcast circuit schemes. In broadcast scenarios, one source transmits traffic to multiple destinations, but traffic is not returned to the source. The ONS15600SDH supports either of the following:

Up to 2048 1:2 nonblocking broadcast connections

Up to 682 1:N nonblocking broadcast connections (where N is less than or equal to 8)

When you create a unidirectional circuit, the card that does not have its backplane receive (Rx) input terminated with a valid input signal generates a loss of service (LOS) alarm. To mask the alarm, create an alarm profile suppressing the LOS alarm and apply the profile to the port that does not have its Rx input terminated.

6.4  SNCP Circuits

Use the Edit Circuits window to change SNCP selectors and switch protection paths ( Figure6-5). In the SNCP Selectors tab, you can:

View the SNCP circuit's working and protection paths

Edit the reversion time

Edit the signal fail/signal degrade thresholds

Figure 6-5 Editing SNCP Selectors

On the SNCP Switch Counts tab, you can view switch counts for the selectors ( Figure6-6).

Figure 6-6 Viewing SNCP Switch Counts

6.5  Path Trace

The SDH J1 Path Trace is a repeated, fixed-length string that includes 64 or 16 consecutive J1 bytes. You can use the string to monitor interruptions or changes to circuit traffic. Table6-5 lists the ONS15600SDH cards that support path trace. Cards not listed in the table do not support the J1 byte.

Table 6-5 ONS 15600 SDH Cards Supporting J1 Path Trace


OC48/STM16SR/SH 16Port1310



OC48/STM16LR/LH 16Port1550



OC192/STM64SR/SH 4Port1310



OC192/STM64LR/LH 4Port1550



The J1 path trace transmits a repeated, fixed-length string. If the string received at a circuit drop port does not match the string the port expects to receive, an alarm is raised.

The ONS15600SDH supports manual J1 path trace monitoring to detect and report the contents of the 64-byte VC4 path trace message (nonterminated) for the designated VC4 path. You can also modify the expected path trace message. The ONS15600SDH does not support path trace auto mode or allow you to modify a transmitted path trace message.

The ONS15600SDH can also monitor a 16-byte ITU pattern.

6.6  Automatic Circuit Routing

If you select automatic routing during circuit creation, CTC routes the circuit by dividing the entire circuit route into segments based on protection schemes. For unprotected segments of protected circuits, CTC finds an alternate route to protect the segment in a virtual SNCP fashion. Each path segment is a separate protection scheme, and each protection scheme is protected in a specific fashion (virtual SNCP or 1+1).

The following list provides principles and characteristics of automatic circuit routing:

Circuit routing tries to use the shortest path within the user-specified or network-specified constraints.

If you do not choose Fully Path Protected during circuit creation, circuits can still contain protected segments. Because circuit routing always selects the shortest path, one or more segments might have protection. CTC does not look at link (segment) protection while computing a path for unprotected circuits.

Circuit routing will not use links that are out of service. If you want all links to be considered for routing, do not create circuits when a link is out of service.

Circuit routing computes the shortest path when you add a new drop to an existing circuit.

6.6.1  Bandwidth Allocation and Routing

Within a given network, CTC will route circuits on the shortest possible path between source and destination based on the circuit attributes, such as protection and type. CTC will consider using a link for the circuit only if the link meets the following requirements:

The link has sufficient bandwidth to support the circuit.

The link does not change the protection characteristics of the path.

If CTC cannot find a link that meets these requirements, an error appears.

6.6.2  Secondary Sources and Drops

CTC supports secondary sources and drops. Secondary sources and drops typically interconnect two networks containing equipment from different vendors, as shown in Figure6-7. Traffic is protected while it traverses a network of ONS15600SDH nodes.

Figure 6-7 Secondary Sources and Drops

Several rules apply to secondary sources and drops:

CTC does not allow a secondary destination for unidirectional circuits because you can specify additional destinations (drops) after you create the circuit.

Primary and secondary sources should be on the same node.

Primary and secondary destinations should be on the same node.

Secondary sources and destinations are permitted only for regular VC4 connections.

For bidirectional circuits, CTC creates an SNCP connection at the source node that allows traffic to be selected from one of the two sources on the ONS15600SDH network. If you check the FullyPathProtected option during circuit creation, traffic is protected within the ONS15600SDH network. At the destination, another SNCP connection is created to bridge traffic from the ONS15600SDH network to the two destinations. A similar but opposite path exists for the reverse traffic flowing from the destinations to the sources.

For unidirectional circuits, an SNCP drop-and-continue connection is created at the source node.

6.7  Manual Circuit Routing

Routing circuits manually allows you to:

Choose a specific path, not just the shortest path chosen by automatic routing

Choose a specific VC4 on each link along the route

CTC imposes the following rules on manual routes:

All circuits in a shared packet ring should have links with a direction that flows from source to destination.

If you enabled Fully Protected Path, choose a diverse protect (alternate) path for every unprotected segment.

For a node that has an SNCP selector based on the links chosen, the input links to the SNCP selectors cannot be 1+1 protected. The same rule applies at the SNCP bridge.

If Fully Protected Path is chosen, CTC verifies that the route selection is protected at all segments. A route can have multiple protection schemes with each scheme protected by a different mechanism.

Table6-6 summarizes the available bidirectional connections. Any other combination is invalid and will generate an error.

Table 6-6 Bidirectional VC4 Circuits 

No. of Inbound Links
No. of Outbound Links
No. of Sources
No. of Drops
Connection Type





















Double SNCP



Double SNCP



Double SNCP




Table6-7 summarizes the available unidirectional connections. Any other combination is invalid and will generate an error.

Table 6-7 Unidirectional VC4 Circuits

No. of Inbound Links
No. of Outbound Links
No. of Sources
No. of Drops
Connection Type






SNCP head end



SNCP head end



SNCP drop and continue

6.8  Constraint-Based Circuit Routing

When you create circuits, you can choose Fully Protected Path to protect the circuit from source to destination. The protection mechanism used depends on the path that CTC calculates for the circuit. If the network is comprised entirely of 1+1 links, or the path between source and destination can be entirely protected using 1+1 links, no Extended SNCP Mesh Networks protection is used.

If Extended SNCP Mesh Networks protection is needed to protect the path, set the level of node diversity for the Extended SNCP Mesh Networks portions of the complete path on the Circuit Creation dialog box:

Nodal Diversity Required—Ensures that the primary and alternate paths of each Extended SNCP Mesh Networks scheme in the complete path have a diverse set of nodes.

Nodal Diversity Desired—CTC looks for a node-diverse path; if a node-diverse path is not available, CTC finds a link-diverse path for each Extended SNCP Mesh Networks scheme in the complete path.

Link Diversity Only—Creates only a link diverse path for each Extended SNCP Mesh Networks scheme.

When you choose automatic circuit routing during circuit creation, you have the option to require and/or exclude nodes and links in the calculated route. You can use this option to achieve the following results:

Simplify manual routing, especially if the network is large and selecting every span is tedious. You can select a general route from source to destination and allow CTC to fill in the route details.

Balance network traffic; by default CTC chooses the shortest path, which can load traffic on certain links while other links are either free or use less bandwidth. By selecting a required node and/or a link, you force CTC to use (or not use) an element, resulting in more efficient use of network resources.

CTC considers required nodes and links to be an ordered set of elements. CTC treats the source nodes of every required link as required nodes. When CTC calculates the path, it makes sure the computed path traverses the required set of nodes and links and does not traverse excluded nodes and links.

The required-nodes-and-links constraint is used only during the primary path computation and only for Extended SNCP Mesh Networks segments. The alternate path is computed normally; CTC uses excluded nodes/links when finding all primary and alternate paths on Extended SNCP Mesh Networks.

6.9  Bridge and Roll

The Bridge and Roll wizard reroutes live traffic without interrupting service. The bridge process takes traffic from a designated "roll from" facility and establishes a cross connect to the designated "roll to" facility. After the bridged signal at the receiving end point is verified, the roll process creates a new cross-connect to receive the new signal. After the roll completes, the original cross-connects are released. You can use the bridge and roll feature for maintenance functions such as card or facility replacement, or for load balancing.

Note To perform bridge and roll, you should be logged into an ONS15600SDH node. If you are logged into an ONS 15454 SDH node, you should log out. When you log in, clear the cache and reload CTC from an ONS15600SDH node.

6.9.1  Roll States

Table6-8 lists the roll states.

Table 6-8 Roll States



Roll is awaiting completion or cancellation.


Roll has been canceled.

Note You can only reroute circuits in the Active state. You cannot reroute circuits that are in the Roll Pending state.

6.9.2  Roll Window

The Rolls window lists information about a rolled circuit before the roll process is complete. You can access the Rolls window by clicking the Circuits > Rolls tabs in either network or node view. Figure6-8 shows the Rolls window.

Figure 6-8 Rolls Window

The Rolls window options include:

Roll From Circuit is the circuit that has connections that will no longer be used after the roll process is complete.

Roll To Circuit identifies the circuit that will carry the traffic after the roll process is complete. The Roll To Circuit will be the same as the Roll From Circuit if a single circuit is involved in a roll.

Roll State shows the values described in Table6-8.

Roll Mode indicates whether the roll is automatic or manual. CTC implements roll mode at the cross-connect level, which means it applies to connections within a single ONS15600SDH.

Automatic—When a valid signal is received on the new path, CTC completes the roll on the node automatically. You can cancel an automatic roll only when the Roll Valid Signal value is false. One-way source rolls are always automatic.

Manual—You must complete a manual roll after a valid signal is received. You can cancel a manual roll at any time. One-way destination rolls are always manual.

Roll Path indicates the fixed point of the roll object.

Roll From Path indicates the path (VC4) that is being rerouted.

Roll To Path indicates the new path where the Roll From Path is rerouted.

Use the Complete button to terminate a manual roll. You can do this when a manual roll is in a ROLL_PENDING state and you have not yet completed the roll or have not cancelled its sibling roll.

Use the Finish button to complete the circuit processing of a roll. It changes the circuit state from ROLL_PENDING to ACTIVE.

Use the Cancel button to cancel the selected roll. You can cancel a manual roll at anytime; you can cancel an automatic roll only if the Roll Valid Signal is false.

6.9.3  Single and Dual Rolls

Circuits have an additional layer of roll types, single and dual. A single roll on a circuit is a roll on one of its cross-connections. Use a single roll to:

Change either the source or destination of a selected circuit ( Figure6-9 and Figure6-10, respectively).

Roll a segment of the circuit onto another chosen circuit ( Figure6-11). This roll also results in a new destination.

In Figure6-9, you can select any available VC4 on Node 1 for a new source.

Figure 6-9 Single Source Roll

In Figure6-10, you can select any available VC4 on Node 2 for a new destination.

Figure 6-10 Single Destination Roll

Figure6-11 shows one circuit rolling onto another circuit. The new circuit has cross-connections on Node 1, Node 3, and Node 4. CTC deletes the cross-connection on Node 2 after the roll.

Figure 6-11 Single Roll from One Circuit to Another Circuit

A dual roll involves two cross-connections. It allows you to reroute intermediate segments of a circuit, but keep the original source and destination. You can perform a dual roll on a single circuit or two circuits. When rolling two cross-connections using the CTC Bridge and Roll wizard, you can choose an existing circuit or create a new circuit. The created circuit is designated with the same name as the original circuit with the suffix _ROLL**.

Several constraints exist for dual rolls:

You must complete or cancel both cross-connects rolled in a dual roll. You cannot complete one roll and cancel the other roll.

When a Roll To circuit is involved in the dual roll, the first roll must roll onto the source of the RollTo circuit and the second roll must roll onto the destination of the Roll To circuit.

Figure6-12 illustrates a dual roll on the same circuit.

Figure 6-12 Dual Roll on the Same Circuit

Figure6-13 illustrates a dual roll involving two circuits.

Figure 6-13 Dual Roll on Two Circuits

6.9.4  Circuit Bridge and Roll Restrictions

Several restrictions apply when using the bridge and roll feature to reroute traffic using two circuits:

DCC must be enabled on the circuits involved in a roll before roll creation.

A maximum of two rolls can exist between any two circuits.

If two rolls are involved between two circuits, both rolls must be on the original circuit. The second circuit should not carry live traffic. The two rolls loop from the second circuit back to the original circuit. The roll mode of the two rolls must be identical (either automatic or manual).

If a single roll exists on a circuit, you must roll the connection onto the source or the destination of the second circuit and not an intermediate node in the circuit.

6.9.5  Protected Circuits

CTC allows you to roll the working or protect path regardless of which is active. You can upgrade an unprotected circuit to a fully protected circuit or downgrade a 1+1-protected circuit to an unprotected circuit.

When using bridge and roll on SNCP circuits, you can roll the source or destination or both path selectors in a dual roll. However, you cannot roll a single path selector.

You can also perform bridge and roll on MS-SPRing circuits.