3.3 Fundamental routing concepts

3.3 Fundamental routing concepts

There are 3 steps to routing:

1. find outgoing interface
2. move the packet/switch between interfaces
3. rebuild the packet’s layer 2 headers

In step 1, the router finds the longest match. Then the router does a recursive look up to find the outgoing interface.
note: A route cannot be installed in the RIB if the recursive lookup fails

In step 2, this is where CEF, or process switching happens. This is also where load balancing happens.

In step 3 we build the layer 2 header. Depending on the egress interface encapsulation we would obviously choose different headers (eth vs ppp).
Also, depending on multipoin (eth) vs point to point (ppp) we might have to do layer 3 to layer 2 lookups