Page 117 - Handout Computer Network.

BGP is the most pervasive example of an EGP.

https://www.youtube.com/watch?v=KjNYEzEBRD8&t=10s
5.6 Routing Protocols

Routing protocols can be classified in many different ways, such as IGPs or EGPs.
Another classification that describes routing protocols is distance-vector or link-state. Whereas IGP
and EGP describe the physical relationships of routers, the distance-vector and link-state categories
describe how routers interact with each other in terms of routing updates.

5.6.1 Distance-Vector Protocols

The distance-vector routing approach determines the direction (vector) and distance (hop
count) to any link in the internetwork. Distance-vector algorithms periodically (such as every 30
seconds) send all or some portion of their routing table to their adjacent neighbors.

Routers running a distance-vector routing protocol send periodic updates even if there are no
changes in the network. By receiving a neighbor’s routing table, a router can verify all the known
routes and make changes to the local routing table based on updated information received from the
neighboring router.

This process is called “routing by rumor” because the understanding that a router has of the
network is based on the neighbor’s perspective of the network topology.
Distance-vector protocols use the Bellman-Ford Algorithm to calculate the best paths.

Examples of distance-vector protocols include the following:

• Routing Information Protocol (RIP)—The most common IGP in the Internet, RIP uses hop
count as its routing metric.
• Interior Gateway Routing Protocol (IGRP)—Cisco developed this IGP to address the issues
associated with routing in large, heterogeneous networks.

5.6.2 Link-State Protocols

Link-state routing protocols were designed to overcome the limitations of distance vector
routing protocols. Link-state routing protocols respond quickly to network changes, send trigger
updates only when a network change has occurred, and send periodic updates (called link-state
refreshes) at long time intervals, such as every 30 minutes.

When a link changes state, the device that detected the change creates a link-state
advertisement (LSA) concerning that link (route), and that LSA is propagated to all neighboring
devices. Each routing device takes a copy of the LSA, updates its link state (topological) database, and
forwards the LSA to all neighboring devices.

This flooding of the LSA is required to ensure that all routing devices update their databases
before creating an updated routing table that reflects the new topology, as shown in Figure 5-8.