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10-22
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter 10 Circuits and Tunnels
10.13 Constraint-Based Circuit Routing
10.13 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 CTC calculates for the circuit. If the
network is composed entirely of MS-SPRing or 1+1 links, or the path between source and destination
can be entirely protected using 1+1 or MS-SPRing links, no path-protected mesh network (Extended
SNCP) or virtual SNCP protection is used.
If Extended SNCP protection is needed to protect the path, set the level of node diversity for the
Extended SNCP portions of the complete path in the Circuit Creation dialog box:
• Nodal Diversity Required—Ensures that the primary and alternate paths of each Extended SNCP
domain 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 domain in the complete path.
• Link Diversity Only—Creates only a link-diverse path for each Extended SNCP domain.
When you choose automatic circuit routing during circuit creation, you have the option to require or
exclude nodes and links in the calculated route. You can use this option to:
• 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 have most of their bandwidth available. By selecting a required node or a link,
you force the 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 only used during the primary path computation and only for
Extended SNCP domains/segments. The alternate path is computed normally; CTC uses excluded
nodes/links when finding all primary and alternate paths on Extended SNCPs.
10.14 Virtual Concatenated Circuits
Virtual concatenated (VCAT) circuits, also called VCAT groups (VCGs), transport traffic using
noncontiguous time division multiplexing (TDM) time slots, avoiding the bandwidth fragmentation
problem that exists with contiguous concatenated circuits. The cards that support VCAT circuits are the
FC_MR-4 (both enhanced and line rate mode), ML-100T, and ML-1000 cards.
In a VCAT circuit, circuit bandwidth is divided into smaller circuits called VCAT members. The
individual members act as independent TDM circuits. All VCAT members should be the same size and
must originate/terminate at the same end points. For two-fiber MS-SPRing configurations, some
members can be routed on protected time slots and others on PCA time slots.
10.14.1 VCAT Member Routing
The automatic and manual routing selection applies to the entire VCAT circuit, that is, all members are
manually or automatically routed. Bidirectional VCAT circuits are symmetric, which means that the
same number of members travel in each direction. With automatic routing, you can specify the