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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
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Chapter 1 Overview
Routing Algorithms
OSPF supports stub areas and EIGRP supports stub routers.
Routing Algorithms
Routing algorithms determine how a router gathers and reports reachability information, how it deals
with topology changes, and how it determines the optimal route to a destination. Various types of routing
algorithms exist, and each algorithm has a different impact on network and router resources. Routing
algorithms use a variety of metrics that affect calculation of optimal routes. You can classify routing
algorithms by type, such as static or dynamic, and interior or exterior.
This section includes the following topics:
• Static Routes and Dynamic Routing Protocols, page 1-8
• Interior and Exterior Gateway Protocols, page 1-8
• Distance Vector Protocols, page 1-9
• Link-State Protocols, page 1-9
Static Routes and Dynamic Routing Protocols
Static routes are route table entries that you manually configure. These static routes do not change unless
you reconfigure them. Static routes are simple to design and work well in environments where network
traffic is relatively predictable and where network design is relatively simple.
Because static routing systems cannot react to network changes, you should not uses them for today’s
large, constantly changing networks. Most routing protocols today use dynamic routing algorithms,
which adjust to changing network circumstances by analyzing incoming routing update messages. If the
message indicates that a network change has occurred, the routing software recalculates routes and sends
out new routing update messages. These messages permeate the network, triggering routers to rerun their
algorithms and change their routing tables accordingly.
You can supplement dynamic routing algorithms with static routes where appropriate. For example, you
should configure each subnetwork with a static route to the IP default gateway or router of last resort (a
router to which all unrouteable packets are sent).
Interior and Exterior Gateway Protocols
You can separate networks into unique routing domains or autonomous systems. An autonomous system
is a portion of an internetwork under common administrative authority that is regulated by a particular
set of administrative guidelines. Routing protocols that route between autonomous systems are called
exterior gateway protocols or interdomain protocols. BGP is an example of an exterior gateway protocol.
Routing protocols used within an autonomous system are called interior gateway protocols or
intradomain protocols. EIGRP and OSPF are examples of interior gateway protocols.