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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 22 Configuring IP Unicast Routing
Understanding Routing
Understanding Routing
Network devices in different VLANs cannot communicate with one another without a Layer 3 device
(router) to route traffic between the VLANs. Routers can perform routing 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 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 Catalyst 3550 switch are Routing Information Protocol
(RIP), which uses a single distance metric (cost) to determine the best path, and Interior Gateway
Routing Protocol (IGRP), which uses a series of metrics. 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 IGRP to improve efficiency.
In some network environments, VLANs are associated with individual networks or subnetworks. In an
IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the
size of the broadcast domain and keeps local traffic local. However, when an end station in one VLAN
needs to communicate with an end station in another VLAN, inter-VLAN communication is required.
This communication is supported by inter-VLAN routing. You configure one or more routers to route
traffic to the appropriate destination VLAN.
Figure 22-1
shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The
router has an interface in each VLAN.
Figure 22-1 Routing Topology Example
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A
B
C
Host
Host
Host
Switch A
Switch B
VLAN 10
VLAN 20
ISL Trunks
