How to: use IS-IS overload bit

Overload bit is special bit in the IS-IS LSP used to inform the network that the advertising router is not yet ready to forward transit traffic.  The overload bit was first intended for signaling overload or resource shortage on specific router for the rest of the network.

You can use the command set-overload-bit intentionally on specific router to signal other routers not to use it as a transit hop in their SPF calculations. Typically this is done for a temporary situation like an overloaded router due to memory or processing shortage and released when the router recovers from the problematic situation.

Overload bit is a simple and handy technique that can be used whenever you want to isolate a specific router in your network before a maintenance, attack or to avoid problematic path.

we usually use overload bit in the following situations:

  • To verify operation of new installed routers before allowing them to forward transit traffic.
  • Preventing control plane routers like (Route Reflectors) from being used accidentally in the forwarding path.
  • On routers' start-up to avoid traffic black-holes until routing protocols are fully converged.
  • To isolate a specific router before decommissioning or a maintenance operation.

I advice you to try it in the lab and to review its options  if you had not yet and fell the beauty of the simple networking tricks.

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