Use this procedure to enable trace for IPv6 DHCP Relay and enable IPv6 forwarding trace.
enable
trace level 66 3
trace ipv6 forwarding enable <all|debug|error|info|pkt|warn>
Enable rcip6 trace messages and enable IPv6 forwarding trace:
Switch:1>enable Switch:1#trace level 66 3 Switch:1#trace ipv6 forwarding