Datasheet

11-19
Cisco ONS 15454 SDH Reference Manual, R7.0
October 2008
Chapter 11 Circuits and Tunnels
11.12 Automatic Circuit Routing
11.12 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 domains. For unprotected segments of circuits
provisioned as fully protected, CTC finds an alternate route to protect the segment, creating a virtual
SNCP. Each segment of a circuit path is a separate protection domain. Each protection domain is
protected in a specific protection scheme including card protection (1+1, 1:1, etc.) or SDH topology
(SNCP, MS-SPRing, etc.).
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. Low-order tunnels are preferable for low-order circuits because low-order tunnels are
considered shortcuts when CTC calculates a circuit path in path-protected mesh networks.
• 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 links and/or
segments can have some protection. CTC does not look at link protection while computing a path
for unprotected circuits.
• Circuit routing does not use links that are down. If you want all links to be considered for routing,
do not create circuits when a link is down.
• Circuit routing computes the shortest path when you add a new drop to an existing circuit. It tries to
find the shortest path from the new drop to any nodes on the existing circuit.
• If the network has a mixture of low-order-capable nodes and low-order-incapable nodes, CTC might
automatically create a low-order tunnel. Otherwise, CTC asks you whether or not a low-order tunnel
is needed.
11.12.1 Bandwidth Allocation and Routing
Within a given network, CTC routes circuits on the shortest possible path between source and destination
based on the circuit attributes, such as protection and type. CTC considers 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.
• The link has the required time slots to enforce the same time slot restrictions for MS-SPRing.
If CTC cannot find a link that meets these requirements, an error appears.
The same logic applies to low-order circuits on low-order tunnels. Circuit routing typically favors
low-order tunnels because low-order tunnels are shortcuts between a given source and destination. If the
low-order tunnel in the route is full (no more bandwidth), CTC asks whether you want to create an
additional low-order tunnel.
11.12.2 Secondary Sources and Destinations
CTC supports secondary sources and destinations (drops). Secondary sources and destinations typically
interconnect two “foreign” networks (Figure 11-6). Traffic is protected while it goes through a network
of ONS 15454 SDH nodes.