This article explains the meaning of the syslog message: "rnh_comp_output(): composite rnh <NH index>: af 2 does not have a TAG, discarding" that is seen on MX and PTX Series devices and details what needs to be done to troubleshoot such messages. 

The following syslog messages are seen for a particular NH index. In this scenario, "17630" is the NH index:

Nov 16 05:04:37 router-name kernel: %KERN-3: rnh_comp_output(): composite rnh 17630: af 2 does not have a TAG, discarding..
Nov 16 06:05:19 router-name kernel: %KERN-3: rnh_comp_output(): composite rnh 17630: af 2 does not have a TAG, discarding..

The above syslog message indicates that a host generated packet reached a "Chain Composite NH," which is a TAG NH. However, when the above logs were seen, the packet had reached a chain composite NH that did not have a TAG (MPLS label) related attribute.

If these messages are not seen consistently, then the single instance can be ignored. The messages could occur during convergence when the NH is getting installed in the kernel and the Packet Forwarding Engine (PFE). 

If these messages are seen continuously, however, then collect the following NH information and contact Support or further assistance: