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Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter10 Circuits and Tunnels
10.11 Automatic Circuit Routing
If a circuit is provisioned using a terminating card, the terminating card provides the C2 byte. A
low-order path circuit is terminated at the cross-connect card and the cross-connect card generates the
C2 byte (0x02) downstream to the VC terminating cards. The cross-conn ect generates the C2 value
(0x02) to the terminating card. If an STM-N circuit is created with no terminating cards, t he test
equipment must supply the path overhead in terminating mode. If the test equipment is in pass-through
mode, the C2 values usually change rapidly between 0x00 and 0xFF. Adding a terminating card to an
STM-N circuit usually fixes a circuit having C2 byte problems.
10.11 Automatic Circuit Routing
If you select automatic routing during circuit creation, CTC routes the circuit by dividing the enti re
circuit route into segments based on protection domains. For unprotected segments of circuits
provisioned as fully protected, CTC finds an alternate route to p rotect the segment, creating a virtual
SNCP. Each segment of a circuit path is a separate protection domain. Each protect ion domain is
protected in a specific protection scheme including card protect ion (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 st ill 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.
Table10-7 STM Path Signal Label Assignments for Signals
Hex Code Content of the STM Synchronous Payload Envelope (SPE)
0x00 Unequipped
0x01 Equipped—nonspecific payload
0x02 Tributary unit group (TUG) structure
0x03 Locked tributary unit (TU-n)
0x04 Asynchronous mapping of 34,368 kbps or 44,736 kbp s into container-3 (C-3)
0x12 Asynchronous mapping of 139,264 kbps into container-4 (C- 4)
0x13 Mapping for asynchronous transfer mode (ATM)
0x14 Mapping for Distributed Queue Dual Bus (DQDB)
0x15 Asynchronous mapping for Fiber Distributed Data Inte rface (FDDI)
0xFE 0.181 Test signal (TSS1 to TSS3) mapping SDH network (see ITU-T G.707)
0xFF Virtual container-alarm indication signal (VC-AIS)