If the internet were to remain in one configuration, the routing tables of BGP systems could eventually reach a steady state. However, the internet is constantly changing, causing the BGP routing changes can happen when a variety of events occur. These events include:
- A BGP session closes due to equipment failure
- A new BGP session is opened when a system comes online
- Changes occur in the policies used to choose routes
- Routing or topology changes within an AS
When BGP routing changes occur, many update messages are sent between BGP systems before they converge to stable routing table configurations. While the tables are converging, packets in transit may get sent in loops as paths to their destinations are updated. This wastes bandwidth and causes congestion and delays in the network. The update messages also waste bandwidth, as well as consuming processing time of the routers.
The majority of instability in BGP routers is caused by a small fraction of destinations in the internet. These destinations tend to be unpopular and receive very little traffic. In fact, the most popular destinations, which receive the majority of internet traffic, are involved in very few events that cause routing changes. Therefore, while BGP instability remains a cause for concern, it can be assumed that the routes taken by most internet traffic are relatively stable. [number on reference of stability paper]