Routing Table Principles

These principles are from Alex Zinin's book, Cisco IP Routing.

1. Every router makes its decision alone, based on the information it has in its own routing table. 
2. The fact that one router has certain information in its routing table does not mean that other routers have the same information. 
3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

Asymmetric Routing

Because routers do not necessarily have the same information in their routing tables, packets can traverse the network in one direction, using one path, and return via another path. This is called asymmetric routing. Asymmetric routing is more common in the Internet, which uses the BGP routing protocol than it is in most internal networks.