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Hierarchy Level


When you enable IS-IS to exchange Connectionless Network Service (CLNS) routes, Junos OS sets the reserved (R) bit in the default metric field inside type, length, and value (TLV) type-3 (ES-Neighbor) as a marker for routing loop prevention. Junos OS uses the up/down bit for marking prefixes on the Level 2-to-Level 1 boundary as being propagated Down, such that any router in that area never propagates it Up on a Level 1-to-Level 2 boundary. For detailed information about how this works in IP routing environments, see RFC 2966, Domain-wide Prefix Distribution with Two-Level IS-IS.

Some other vendors’ platforms might not support up/down bit setting in CLNS route TLVs. If one of these vendors’ platforms receives this TLV with the R bit set, the platform discards the information.

When you use the clns-updown-compatibility statement in the IS-IS configuration, the R bit is set to 0, and the issue is resolved. The clns-updown-compatibility statement causes Junos OS to use the Internal/External metric-type bit in the TLV header instead of using the R bit as the up/down bit marker. This has the advantage that older end system (ES) equipment does not receive TLV headers with the R bit set.


Not using the R bit can lead to potential routing loops. You can use the site-of-origin (SoO) extended community to prevent a looped BGP update from being injected back to IS-IS when received from a remote provider edge (PE) device. The receiving PE device can check against the SoO community, and if the value matches its own, the NLRI is not accepted.

Required Privilege Level

routing—To view this statement in the configuration.

routing-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 11.4R7.