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Catalyst 3560 Switch Software Configuration Guide
OL-8553-06
Chapter 36 Configuring IP Unicast Routing
Understanding IP Routing
Configuring Protocol-Independent Features, page 36-86
Monitoring and Maintaining the IP Network, page 36-100
Note When configuring routing parameters on the switch and to allocate system resources to maximize the
number of unicast routes allowed, you can use the sdm prefer routing global configuration command
to set the Switch Database Management (sdm) feature to the routing template. For more information on
the SDM templates, see Chapter 8, “Configuring SDM Templates” or see the sdm prefer command in
the command reference for this release.
Understanding IP Routing
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, network devices in different VLANs
cannot communicate with one another without a Layer 3 device (router) to route traffic between the
VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the
appropriate destination VLAN.
Figure 36-1 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 36-1 Routing Topology Example
When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed
to that host. Switch A forwards the packet directly to Host B, without sending it to the router.
When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which
receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct
outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives
the packet and forwards it to Host C.

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.
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A
B
C
Host
Host
Host
Switch A Switch B
VLAN 10 VLAN 20
ISL Trunks