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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing
Understanding Routing
Understanding Routing
Network devices in different VLANs cannot communicate with one a noth er w it hout a L ay er 3 devi ce
(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 sing le 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 destina tions. As networks grow,
static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamical ly c alcul at e t he be st r oute for fo rw arding
traffic. There are two types of dynamic routing protocols:
•Routers using distance-vector protocols maintain routing tables with distan ce val ues of net work ed
resources, and periodically pass these tables to their nei ghbors. D istan ce -v ect or pr otoc ols use on e
or a series of metrics for calculating the best routes. These p rotocol s ar e e as y to co nfig ure a nd use.
•Routers using link-state protocols maintain a complex database of netw ork t opol ogy, 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 Infor mat ion Pr otocol
(RIP), which uses a single distance metric (cost) to determine the best path, and Interior Gatewa y
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 netw or ks or sub ne two rks. In a n
IP network, each subnetwork is mapped to an individual VLAN. Configu ring 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. SwitchA is in VLAN 10, and Switch B is in VLAN 20. The
router has an interface in each VLAN.
Figure22-1 Routing Topology Example
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A
BC
Host
Host
Host
Switch A Switch B
VLAN 10 VLAN 20
ISL Trunks