15-30
Cisco ONS 15454 DWDM Reference Manual, R8.5
78-18343-02
Chapter 15 Management Network Connectivity
15.3.6 DCN Case Study 3: Linear Topology with DCN Connections on Both Ends Using OSPF Routing
tunnel destination 192.168.100.1
Static routes with alternate paths at different costs:
ip classless
ip route 0.0.0.0 0.0.0.0 192.168.20.1
ip route 10.0.0.0 255.0.0.0 192.168.20.1
ip route 10.0.0.0 255.0.0.0 Tunnel0 10
ip route 192.168.100.1 255.255.255.255 192.168.100.77
Note that the host route to the Router 1 (192.168.100.1) points to the ONS 15454 network (by
192.168.200.77). This is required to set up the GRE tunnel. In this configuration, only the external route
to 10.0.0.0 (that includes the NOC network) is overloaded with the alternate path. However, overloading
the last-resort route might also occur.
Table 15-6 shows network settings on the four ONS 15454 nodes. The static routes are created so the
DCN-connected nodes advertise their capability to act as last-resort routers.
15.3.5.2 DCN Case Study 2 Limitations
The linear configuration in DCN Case Study 2 does not effectively protect the management network
communication for every fiber failure because the DCN router is not notified of the failures. Therefore,
it continues to send packets on the low-cost path. This problem does not occur in ring topologies where
the fiber failure is internally protected from the optical ring network. However, the OSPF dynamic
routing protocol can be used over the DCN network to provide a solution to this problem. An OSPF
configuration is shown in DCN Case Study 3.
15.3.6 DCN Case Study 3: Linear Topology with DCN Connections on Both Ends
Using OSPF Routing
DCN Case Study 3 is the same linear topology as DCN Case Study 2 except OSPF routing is used on the
DCN network. This requires the OSPF active on LAN option, located on the node view (single-shelf
mode) or multishelf view (multishelf mode) Provisioning > Network > OSPF tab, to be enabled at the
end ONS 15454 nodes. In addition, OSPF must be running between Router 1, Router 2, and the NOC
router.
Because the DCN connection usually passes over a public network where OSPF is not always an option,
the connection between Router 1, Router 2, and the NOC router is configured as a GRE tunnel so OSPF
can run on the tunnel itself.
Figure 15-20 shows the linear configuration with the separate OSPF areas, the tunnel connections, and
the required OSPF virtual link. (The physical connections where the tunnels are passed are not shown in
the figure because they are not directly part of the actual routing path.)
Table 15-6 DCN Case Study 2 Node IP Addresses
Node IP Address/Mask Default Gateway
Static Routes:
Destination/Mask – Next Hop
Node 1 192.168.100.80/24 192.168.100.1 0.0.0.0/0 – 192.168.100.1
Node 2 192.168.100.79/24 0.0.0.0 —
Node 3 192.168.100.78/24 0.0.0.0 —
Node 4 192.168.100.77/24 192.168.100.1 0.0.0.0/0 – 192.168.200.1
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