As of now “REACHABLE TIME” value for IPv6 RA Packet is not configurable. If the ODL controller is down, then the L3 forwarding fails after exactly 2 minutes. 
Created Topology:
 Created 2 networks (N1 and N2)
 Created Subnets from each network (S1 and S2)
 Created 1 Router and attached both the subnets (S1 and S2)
 Booted VMs from each  network (VM1 from N1 and VM2 from VM2)
 Verify Ping Traffic from VM1 to VM2 ==> Success (Working as expected for IPv6 L3 Routing Traffic)

 Disconnected ODL Controller from the OVS Switch.
 Verify Ping Traffic from VM1 to VM2 ==> Success (Working as expected for IPv6 L3 Routing Traffic)
 
  Awaited 2 Mins time for VMs are removing router interface MAC from the IPv6 ND Cache. (Currently i tested with Cirros VMs. Hence i am not able to see the exact IPv6 ND Cache from the command prompt)

Initiated ping traffic again from VM1 to VM2 ==> FAILED (This is also expected as per the IPv6 ND Cache Timer)

 Have verified the OVS dump flows and observed that NS packets are punted to the ODL Controller for resolving the MAC address of the Router-Interface/Subnet-GW Interface.

cookie=0x4000000, duration=4881.845s, table=45, n_packets=67, n_bytes=5762, priority=50,icmp6,metadata=0x138a000000/0xffff000000,icmp_type=135,icmp_code=0,nd_target=fe80::f816:3eff:feab:ead2 actions=CONTROLLER:65535

cookie=0x4000000, duration=4729.940s, table=45, n_packets=67, n_bytes=5762, priority=50,icmp6,metadata=0x138b000000/0xffff000000,icmp_type=135,icmp_code=0,nd_target=fe80::f816:3eff:fecd:9e81 actions=CONTROLLER:65535

 
 opendaylight-user@root>ipv6CacheShow
Interface Cache List:
     Sno | PortId                               | Mac Address       | Owner            | dpnId           | FixedIPs
    --------------------------------------------------------------------------------------------------------------------------------------------------------------
     1   | 5ce4ba73-7c1c-4fc4-a36d-0ea16a239eaa | fa:16:3e:42:df:98 | compute          | 139627079752513 | 2001:1111:0:9:f816:3eff:fe42:df98
     2   | 9c938904-3d35-4349-a1ea-581eba13588e | fa:16:3e:02:57:06 | dhcp             | N/A             | 2001:db8:0:2:f816:3eff:fe02:5706
     3   | ac96eddb-9f65-4646-8da8-57bcfd9d9460 | fa:16:3e:74:a3:b9 | dhcp             | N/A             | 2001:1111:0:9:f816:3eff:fe74:a3b9
     4   | 0e1bba3b-356f-4af0-8812-10e4196f128d | fa:16:3e:cd:9e:81 | router_interface | N/A             | 2001:1111:0:9:0:0:0:1  fe80:0:0:0:f816:3eff:fecd:9e81
     5   | 7d949eea-eac1-4203-a6d8-3941253077f7 | fa:16:3e:e3:ef:80 | compute          | 139627079752513 | 2001:db8:0:2:f816:3eff:fee3:ef80
     6   | f06170f2-b5bf-4d6f-8d68-3685871730ae | fa:16:3e:ab:ea:d2 | router_interface | N/A             | 2001:db8:0:2:0:0:0:1  fe80:0:0:0:f816:3eff:feab:ead2