Networking Concepts Review - CCIE Routing and Switching Exam Certification Guide - page 37

networks prohibits the manual configuration of every router. This is why there are dynamic
protocols that use algorithms that can react to network changes.
The main benefit of static routing is that the router generates no routing protocol overhead.
Static routing is recommended for hub-and-spoke topologies with low bandwidth links.
Dynamic routing protocols can determine the best routes to a destination automatically. If the
network topology changes, the routing protocol can adjust the routes without user intervention.
Dynamic routing protocols use metrics to determine the best path. Some use one metric and
other protocols use a combination of metrics. Routing metrics are discussed later in this section.
Interior Versus Exterior Routing Protocols
Routing protocols can be labeled as Interior Gateway Protocols (IGPs) or exterior gateway
protocols (EGPs). IGPs are meant for routing within a company's administrative domain. EGPs
are routing protocols that communicate with exterior domains. One of the first EGPs was called
exactly that, exterior gateway protocol. Today, Border Gateway Protocol (BGP) is the defacto
EGP. All other IP routing protocols are IGPs: RIP, OSPF, IS-IS, IGRP, and EIGRP.
Link-State Versus Distance Vector Routing Protocols
The first routing protocols were distance vector routing protocols, which are occasionally
referred to as Bellman-Ford algorithms. In distance vector routing protocols, routes are
advertised as vectors of distance and direction. The distance metric is usually the router hop
count. The direction is the next-hop router to which the packet is forwarded. For RIP, the
maximum number of hops is 15, which is a serious limitation especially in nonhierarchical
networks.
Distance vector algorithms call for each router to send all or some portion of its routing table
only to its neighbors. The table is sent periodically (every 30 or 60 seconds). The router builds
a new table and sends it to its neighbors, and so on. In today's networks, waiting half a minute
for a new routing table with new routes is too long. Some distance vector protocols send
triggered updates (a full routing table update sent before the update timer has expired), but a
router can receive a routing table with 500 routes with only one route change. This creates
serious overhead on the network, which is another drawback. Distance vector protocols are
discussed in Chapter 7.
Another protocol, Enhanced Interior Gateway Routing Protocol (EIGRP), is considered a
hybrid routing protocol. EIGRP is a distance vector protocol that implements some link-state
routing protocol characteristics. Although using similar metrics as its predecessor, IGRP and
EIGRP sends partial updates and maintains neighbor state information, similar to link-state
protocols. EIGRP does not send periodic updates. The important thing to remember for the test
is that EIGRP can be presented as a hybrid protocol. EIGRP is discussed in Chapter 7.