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Chapter 42 Configuring IP Unicast Routing Understanding IP Routing
Types of Routing
Routers and Layer 3 switches can route packets in three different ways:
• By using default routing
• By using preprogrammed static routes for the traffic
• By dynamically calculating routes by using a routing protocol
Default routing refers to sending traffic with a destination unknown to the router to a default outlet
or destination.
Static unicast routing forwards packets from predetermined ports through a single path into and out of a
network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes
in the network, such as link failures, and therefore, might result in unreachable destinations. As networks
grow, static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding
traffic. There are two types of dynamic routing protocols:
• Routers using distance-vector protocols maintain routing tables with distance values of networked
resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one
or a series of metrics for calculating the best routes. These protocols are easy to configure and use.
• Routers using link-state protocols maintain a complex database of network topology, based on the
exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in
the network, which speeds up the convergence time or time required to respond to these changes.
Link-state protocols respond quickly to topology changes, but require greater bandwidth and more
resources than distance-vector protocols.
Distance-vector protocols supported by the switch are Routing Information Protocol (RIP), which uses
a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which
adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state
protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional
Interior Gateway Routing Protocol (IGRP) to improve efficiency.
Note On a switch or switch stack, the supported protocols are determined by the software running on the
switch or stack master. If the switch or stack master is running the IP base feature set, only default
routing, static routing and RIP are supported. All other routing protocols require the IP services feature
set.
IP Routing and Switch Stacks
A switch stack appears to the network as a single router, regardless of which switch in the stack is
connected to a routing peer. For additional information about switch stack operation, see Chapter 5,
“Managing Switch Stacks.”
The stack master performs these functions:
• It initializes and configures the routing protocols.
• It sends routing protocol messages and updates to other routers.
• It processes routing protocol messages and updates received from peer routers.
• It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database
to all stack members. The routes are programmed on all switches in the stack bases on this database.