Border Gateway Protocol (BGP)
Uses a path-vector concept which enables loop prevention in complex topologies
In AS-level, shortest path may not be preferred for policy, security, cost reasons.
- Different routers have different preferences (policy) => as packet goes thru network it will encounter different policies
- Same problem for link-state. Link state also has a more serious scaling problem. Aggregation needed.
- Solution: use source-based routing and specify entire path