The destination network entries in the routing table can be added in several ways:
- Local Route interfaces – These are added when an interface is configured and active. This entry is only displayed in IOS 15 or newer for IPv4 routes, and all IOS releases for IPv6 routes.
- Directly connected interfaces – These are added to the routing table when an interface is configured and active.
- Static routes – These are added when a route is manually configured and the exit interface is active.
- Dynamic routing protocol – This is added when routing protocols that dynamically learn about the network, such as EIGRP or OSPF, are implemented and networks are identified.
Dynamic routing protocols exchange network reachability information between routers and dynamically adapt to network changes. Each routing protocol uses routing algorithms to determine the best paths between different segments in the network, and updates routing tables with these paths.
Dynamic routing protocols have been used in networks since the late 1980s. One of the first routing protocols was RIP. RIPv1 was released in 1988. As networks evolved and became more complex, new routing protocols emerged. The RIP protocol was updated to RIPv2 to accommodate growth in the network environment. However, RIPv2 still does not scale to the larger network implementations of today. To address the needs of larger networks, two advanced routing protocols were developed: Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). Cisco developed the Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.
Additionally, there was the need to connect different internetworks and provide routing between them. The Border Gateway Protocol (BGP) is now used between Internet Service Providers (ISPs). BGP is also used between ISPs and their larger private clients to exchange routing information.
The table classifies the protocols. Routers configured with these protocols will periodically send messages to other routers. As a cybersecurity analyst, you will see these messages in various logs and packet captures.
| Protocol | Interior Gateway Protocols | Exterior Gateway Protocols | |||
|---|---|---|---|---|---|
| Distance Vector | Link State | Path Vector | |||
| IPv4 | RIPv2 | EIGRP | OSPFv2 | IS-IS | BGP-4 |
| IPv6 | RIPng | EIGRP for IPv6 | OSPFv3 | IS-IS for IPv6 | BGP-MP |