There are two basic methods of building a routing table:
(i) Adaptive (Dynamic) Routing
(ii) Non Adaptive (Static) Routing
A static ro table is created, maintained and updated by a network administrator, manually. A static route to every network must be configured on every router for full connectivity. This provides a granular level of control over routing, but quickly becomes impractical on large networks. Routers will not share static routes with each other, thus reducing CPU/RAM overhead and saving bandwidth. However, static routing is not fault-tolerant, as any change to the routing infrastructure (such as a link going down, or a new network added) requires manual intervention. Routers operating in a purely static environment cannot seamlessly choose a better route if a link becomes unavailable. Static routes have an Administrative Distance (AD) of 1 and thus are always preferred over dynamic routes, unless the default AD is changed. A static route with an adjusted AD is called a floating static route and is covered in greater detail in another guide.
A dynamic routing table is created, maintained and updated by a routing protocol running on the router. Examples of routing protocols include RIP (Routing Information Protocol), EIGRP (Enhanced Interior Gateway Routing Protocol) and OSPF (Open Shortest Path First). Specific dynamic routing protocols are covered in great detail in other guides. Routers do share dynamic routing information with each other, which increases CPU, RAM and bandwidth usage. However, routing protocols are capable of dynamically choosing a different (or better) path when there is a change to the routing infrastructure.
Do not confuse routing protocols with routed protocols.
(i) A routed protocol is a layer 3 protocol that applies logical addresses data between networks (such as IP).
(ii) A routing protocol dynamically builds the network, topology and next hop routing tables (such as RIP, EIGRP, etc.).
The following briefly outlines the advantages and disadvantages of static routing:
(i) Minimal CPU/Memory overhead.
(ii) No bandwidth overhead (updates are not shared between routers).
(iii) Granular control on how traffic is routed.
(i) Infrastructure changes must be manually adjusted.
(ii) No "dynamic" fault tolerance if a link goes down.
(iii) Impractical on large network..
The following briefly outlines the advantages and disadvantages of dynamic routing:
(1) Simpler to configure on larger networks.
(ii) It will dynamically choose a different (or better) route if a link goes down.
(iii) Ability to load balance between multiple links.
(iv) Updates are shared between routers, thus consuming bandwidth.
(i) Routing protocols put additional load on router CPU/RAM.
(ii) The choice of the "best route" is in the hands of the routing protocol and not the network administrator.