Source Independence

The next hop destination does not depend upon the source of the packet.
This phenomena is called source independence
The packet only have destination information for the next hop, this means even if the network topology changes without notifying the network, it would still work.
Hence, increasing the dynamic robustness.

Hierarchical Addressing and Routing

The process of forwarding packets, is called routing.
The information about destinations is kept in routing tables.

Figure b represents the routing table of switch 2.

Routing in a WAN

There are 2 types of switch according to attached computers.

Interior Switch

This switch has no computers attached to it.

Exterior Switch

This switch has computers attached to it.

Both of these switches need routing tables which need 2 things.

Universal Routing

The table should have next hop for each possible destination.

Optimal Routes

The table should have next hop for shortest path possible.

Modeling a WAN

We can model a WAN by representing switches as nodes and direct connections as edges.

Route Computation with a Graph

Redundant and Routing Information

Notice how the above figure shows same next hop for node 1. We can collapse the table as such: