CLNS communication over GRE is not supported on SRX. Currently, SRX only supports IPV4 communication over GRE.

When configuring IS-IS over GRE, and the CLNS communication uses GRE for the communication, packet drops are seen on SRX for the CLNS packet. 

Connectionless Network Service (CLNS) is a Layer 3 protocol similar to IP version 4 (IPv4) for linking hosts (end systems) with routers (intermediate systems) in an Open Systems Interconnection (OSI) network.​ You can configure IS-IS to exchange CLNS routes within a CLNS island. For more details, refer to the technical document, CLNS Configuration Overview

CLNS communication over GRE in a hub-and-spoke environment is not supported on SRX devices operating in flow mode, as only IPv4 over GRE is supported in flow mode.
However, it is supported when the SRX devices are operating in packet mode, as shown below.

root@SRX345> show security flow status 
  Flow forwarding mode:
    Inet forwarding mode: packet based
    ISO forwarding mode: packet based

2025-03-28: Content in the solution field was modified for clarification based on PR1866363.