In an attempt by the Pakistani Government to block a Youtube video in the country, the Pakistan Telecoms has accidentally re-routed Youtube’s IP somewhere else and this has cascaded to other international providers which eventually caused temporary Youtube outage last Sunday.
A more detailed and technical explanation on how the events happened is summarized by Renesys blog.
Just before 18:48 UTC, Pakistan Telecom, in response to government order (thanks nsp-sec-d) to block access to YouTube (see news item) started advertising a route for 18.104.22.168/24 to its provider, PCCW (AS 3491). For those unfamiliar with BGP, this is a more specific route than the ones used by YouTube (22.214.171.124/22), and therefore most routers would choose to send traffic to Pakistan Telecom for this slice of YouTube’s network.
In a more layman and simple terms, the blocking ISP (Pakistan Telecom) re-directed Youtube’s IP address to a different IP address. Other ISPs automatically followed suit (because of inherent trust relationships) which brought people visiting the video sharing site nowhere.
This is not the first time for Pakistan either. Their supreme court previously ordered to block a BlogSpot account which practically banned the entire Blogger.com network in the country (have personally experienced it during my visit there in late 2006).