cisco learning

Routing Table Principles

At times in this course we will refer to three principles regarding routing tables that will help you understand, configure, and troubleshoot routing issues. 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.

This example implies that when designing and troubleshooting a network, the network administrator should check the following routing information:
Is there a path from source to destination available in both directions?
Is the path taken in both directions the same path? (Asymmetrical routing is not uncommon, but sometimes can pose additional issues)