Holddown Timer
a.k.a. Hold-down timer
Key Points
- Suppresses rapid route churn after failure
- Used in distance-vector routing protocols
- Helps prevent transient loops and route flapping
- Delays acceptance of potentially unstable updates
- Allows network time to settle after disruption before accepting new routing information
Definition
Holddown Timer is a routing control mechanism that temporarily suppresses acceptance of route changes after a failure to reduce routing instability by holding a route in a suppressed state for a defined period.
Concept
Holddown Timer is a networking mechanism used in routing protocols to delay acceptance of route updates after a failure or change. It reduces instability from rapid route flapping or misleading alternate updates by giving the network time to settle before accepting new routing information. It is commonly used in distance-vector routing protocols and other designs where transient changes can cause loops or oscillation. The timer helps stabilize the routing process after network disruption.
Explainer
Holddown Timer temporarily suppresses acceptance of route changes after a failure by holding a route in a suppressed state for a defined period. This prevents the network from immediately accepting unstable updates that could create loops or service disruption. The mechanism is used in distance-vector routing and related stability control designs.
Key tradeoffs include stronger routing stability versus slower convergence time, reduction of transient loops versus potential delays in accepting legitimate route recovery, and simpler control mechanisms versus reduced network responsiveness. Failure modes include prolonged inaccessibility of valid routes if the timer duration is too long, route instability if the timer is too short for the environment, and delayed network recovery.
Holddown Timer matters operationally because unstable routing updates accepted too quickly can create loops and service disruption. It is critical in telecommunications and IP network stability across multiple network topologies and routing dynamics.