Datasheet

12-18
Cisco ONS 15454 SDH Reference Manual, R7.0
October 2008
Chapter 12 SDH Topologies and Upgrades
12.4 Dual Ring Interconnect
12.4 Dual Ring Interconnect
Dual ring interconnect (DRI) topology provides an extra level of path protection for circuits on
interconnected rings. DRI allows users to interconnect MS-SPRings, SNCPs, or an SNCP with an
MS-SPRing, with additional protection provided at the transition nodes. In a DRI topology, ring
interconnections occur at two or four nodes.
The drop-and-continue DRI method is used for all ONS 15454 SDH DRIs. In drop-and-continue DRI, a
primary node drops the traffic to the connected ring and routes traffic to a secondary node within the
same ring. The secondary node also routes the traffic to the connected ring; that is, the traffic is dropped
at two different interconnection nodes to eliminate single points of failure. To route circuits on DRI, you
must choose the Dual Ring Interconnect option during circuit provisioning. Dual transmit is not
supported.
Two DRI topologies can be implemented on the ONS 15454 SDH:
• A traditional DRI requires two pairs of nodes to interconnect two networks. Each pair of
user-defined primary and secondary nodes drops traffic over a pair of interconnection links to the
other network.
• An integrated DRI requires one pair of nodes to interconnect two networks. The two interconnected
nodes replace the interconnection ring.
For DRI topologies, a hold-off timer sets the amount of time before a selector switch occurs. It reduces
the likelihood of multiple switches, such as:
• Both a service selector and a path selector
• Both a line switch and a path switch of a service selector
For example, if an SNCP DRI service selector switch does not restore traffic, then the path selector
switches after the hold-off time. The SNCP DRI hold-off timer default is 100 ms. You can change this
setting in the SNCP Selectors tab of the Edit Circuits window. For MS-SPRing DRI, if line switching
does not restore traffic, then the service selector switches. The hold-off time delays the recovery
provided by the service selector. The MS-SPRing DRI default hold-off time is 100 ms, but it can be
changed.
12.4.1 MS-SPRing DRI
Unlike MS-SPRing automatic protection switching (APS) protocol, MS-SPRing DRI is a path-level
protection protocol at the circuit level. Drop-and-continue MS-SPRing DRI requires a service selector
in the primary node for each circuit routing to the other ring. Service selectors monitor signal conditions
from dual feed sources and select the one that has the best signal quality. Same-side routing drops the
traffic at primary nodes set up on the same side of the connected rings, and opposite-side routing drops
the traffic at primary nodes set up on the opposite sides of the connected rings. For MS-SPRing DRI,
primary and secondary nodes cannot be the circuit source or destination.
Note A DRI circuit cannot be created if an intermediate node exists on the interconnecting link. However, an
intermediate node can be added on the interconnecting link after the DRI circuit is created.
DRI protection circuits act as protection channel access (PCA) circuits. In CTC, you set up DRI
protection circuits by selecting the PCA option when setting up primary and secondary nodes during DRI
circuit creation.