Cisco Catalyst 3750-E Series, Software Configuration Manual

Page: 980 / 1414

40-2
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-08
Chapter 40 Configuring IP Unicast Routing
Understanding IP Routing
•
Monitoring and Maintaining the IP Network, page 40-105
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 40-1 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 40-1 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.
This section contains information on these routing topics:
• Types of Routing, page 40-2
•
IP Routing and Switch Stacks, page 40-3
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
18071
A
B
C
Host
Host
Host
Switch A Switch B
VLAN 10 VLAN 20
ISL Trunks