IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Release

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Overload bits on routers

Updated: June 9, 2026

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Explains the function and impact of IS-IS overload bits, outlining configuration steps, implications for route advertisement, and guidelines for using overload bits to manage network state transitions.

A overload bit is a state indicator that

  • is included in a router's Link State Protocol (LSP) data,

  • signals to other routers in the area that the router is temporarily unavailable for transit traffic, and

  • enables routing protocol stability during specific operational circumstances.

The overload bit is useful in these situations:

  • During a serious but nonfatal error, such as limited memory.

  • During the startup and restart of the routing process; the overload bit can be set until the routing protocol has converged. However, it is not used during a standard NSF restart or failover, as this would cause routing flaps.

  • During the trial deployment of a new router; it can be enabled until deployment is verified, then cleared.

  • During the shutdown of a router; the overload bit can be set to remove the router from the topology before decommissioning.

Overload bit behavior in multitopology mode

Independent configuration for IPv4 and IPv6

The overload bit applies separately to IPv4 and IPv6 address families in IS-IS multitopology mode. You can configure and clear the overload bit independently for each topology.

You set or clear the overload bit from the router’s address family configuration mode. This allows precise control over routing behavior for each protocol.

  • If the IPv4 overload bit is set, all routers in the area avoid using the router for IPv4 transit traffic but can still use it for IPv6 transit traffic.

  • If the IPv6 overload bit is set, the router is avoided for IPv6 transit traffic and remains available for IPv4 transit traffic.

Understanding this separation helps you configure and verify IS-IS operation across different address families and ensures correct routing decisions in multitopology environments.