BGP zombies and excessive path hunting

Cloudflare explains that BGP zombies are undesirable routes that linger in the Default-Free Zone, causing packets to be misrouted or arrive via longer paths. The piece outlines path hunting as the process routers use to seek the best path after a more-specific prefix is withdrawn, with convergence time influenced by the Minimum Route Advertisement Interval (MRAI), which RFC4271 recommends as 30 seconds for eBGP updates.

It notes that zombies can form when a withdrawn more-specific prefix is not yet forgotten by all routers, with examples showing path hunting delays extending traffic disruption; observed impacts include zombie lifetimes of around six to eleven minutes in some cases, and even about 20 seconds in others, depending on the network.

The article also discusses BYOIP on-demand advertisement and how withdrawal cycles can increase zombie activity, and it presents a proposed multi-step draining process to avoid route loops and blackholes when removing traffic from Cloudflare’s edge. Finally, it mentions refinements under consideration, such as more graceful traffic draining and RFC9687-like measures to detect stalled BGP messaging, with the aim of reducing the impact of zombies on customers relying on on-demand BGP functionality. 31 October 2025