IS IS Overload Bit


Overload bit is used as a mechanism to route around an IS IS router. This could be used in a maintenance or during convergence time.

Overload Bit

  • Originally the overload bit was there as a flow control mechanism to notify other ISIS routers not route to a particular router experiencing load issues.
  • Setting the overload bit can be performed:
    • On Startup - this option can set a manual timer to wait or wait for BGP to converge based on receipt o keepalive messages.
    • Manually - set the overload bit manually without a timer to expire.
  • Checking the overload bit can be done using the command show isis database
  • When this feature is enabled, directly connected interfaces are reachable.
  • Additional feature of Overload Bit are to suppress only certain prefixes either external or interlevel.
    • External are any redistributed routes that will have the OL set.
    • Interlevel are any routes advertised between ISIS Levels.

Relevant Commands


Manually sets the over load bit. Router is not be used to forward traffic.

R1(config-router)#        set-overload-bit

set-overload-bit suppress

Depending on how the overload bit is set, as long as it is set just certain prefixes can be suppressed.

R1(config-router)#        set-overload-bit [..] suppress (interlevel | external)

set-overload-bit on-startup

After a reboot set the overload bit and wait defined timer.

R1(config-router)#        set-overload-bit on-startup (seconds)

set-overload-bit on-startup wait-for-bgp

After a reboot set the overload bit and wait for bgp to finish converging. If BGP keepalives are not received from all the BGP neighbors, and wait-for-bgp is configured, IS-IS will disable the overload bit after 10 minutes.

R1(config-router)#        set-overload-bit on-startup wait-for-bgp

show isis database

A way to see if a node has the overload bit set.

show isis database

Additional Resources

Uses of the overload Bit with IS-IS - Cisco
RFC3277 - Intermediate System to Intermediate System (IS-IS) Transient Blackhole Avoidance - IETF


rating: 0+x
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License