15-27
Cisco ONS 15454 DWDM Reference Manual, R8.5
78-18343-02
Chapter 15 Management Network Connectivity
15.3.4 DCN Case Study 1: Ring Topology with Two Subnets and Two DCN Connections
GRE tunnel interface configuration:
interface Tunnel0
ip address 192.168.30.2 255.255.255.0
tunnel source Ethernet1/0
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 192.168.100.0 255.255.255.0 Tunnel0 10
ip route 192.168.100.1 255.255.255.255 192.168.200.77
The host routing path 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 occur. Table 15-5 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.
15.3.4.2 DCN Case Study 1 Limitations
DCN Case Study 1 shows how a GRE tunnel can be created between two routers to create DCN
connection resiliency. While the resiliency is a benefit, when a DCN failure forces traffic to the GRE
tunnel, the path calculated by the ONS 15454 OSPF algorithm running in the OSC/DCC/GCC network
is no longer the shortest one. Subsequently, the round-trip delay time (RTT) might increase significantly
because the DCN protection in this configuration is transparent to the ONS 15454 network. The ONS
15454 continues to use the same routing table. In addition, if a DCN failure occurs, the routing path that
uses the GRE tunnel adds additional latency because of the number and length of OSC/DCC/GCC spans
that the tunnel has to travel over the ONS 15454 network.
This latency makes this DCN Case Study 1 solution difficult to scale to large networks. If this solution
is used and the network grows significantly, a larger number of DCN-connected NEs are required. For
example, the common rule in ONS 15454 DCN design is that all nodes should be within five section data
communications channel (LDCC)/regeneration section DCC (RS-DCC/OSC or eight line DCC (LDCC)
/multiplex section DCC (MS-DCC) spans from the network attached node. If Case Study 1 design is
implemented, the maximum span numbers should be cut in half. However, if the DCN Case Study 1
design is used in networks that have full IP routing, have connectivity to every NE, and require only
CTC/CTM management, the SOCKS proxy feature can be used to provide the same DCN connectivity
resilience.
Table 15-5 DCN Case Study 1 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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