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Cisco ONS 15454 Reference Manual, R7.0
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Chapter 11 Circuits and Tunnels
11.16 11.16 Constraint-Based Circuit Routing
Although virtual path protection segments are possible in VT tunnels, VT tunnels are still considered
unprotected. If you need to protect VT circuits, use two independent VT tunnels that are diversely routed
or use a VT tunnel that is routed over 1+1, BLSR, or a mixture of 1+1 and BLSR links.
11.16 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 composed entirely of BLSR or 1+1 links, or the path between source and destination can
be entirely protected using 1+1 or BLSR links, no path-protected mesh network (PPMN), or virtual path
protection is used.
If PPMN protection is needed to protect the path, set the level of node diversity for the PPMN portions
of the complete path in the Circuit Routing Preferences area of the Circuit Creation dialog box:
• Nodal Diversity Required—Ensures that the primary and alternate paths of each PPMN 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 PPMN domain in the complete path.
• Link Diversity Only—Creates only a link-diverse path for each PPMN 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 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 have most of their bandwidth available. By selecting a required node and/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 that 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
PPMN domains/segments. The alternate path is computed normally; CTC uses excluded nodes/links
when finding all primary and alternate paths on PPMNs.
At Source Nodes Only
VT tunnel endpoint —1——
At Destination Nodes Only
VT tunnel endpoint 1 ———
Table 11-15 Bidirectional VT Tunnels (continued)
Connection Type
Number of
Inbound Links
Number of
Outbound Links
Number of
Sources
Number of
Destinations