Understanding Ip Routing - Cisco Catalyst 3560-X Software Configuration Manual

Understanding IP Routing

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On switches running the IP base or IP services feature set, when configuring routing parameters on the
Note
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. On switches running the LAN base feature set, IP static routing is
supported only with the default sdm template. For more information on the SDM templates, see
Chapter 1, "Configuring SDM Templates"
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 1-1
router has an interface in each VLAN.
Figure 1-1
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:
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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
1-2
Configuring BGP, page 1-45
Configuring ISO CLNS Routing, page 1-66
Configuring Multi-VRF CE, page 1-76
Configuring Protocol-Independent Features, page 1-91
Monitoring and Maintaining the IP Network, page 1-106
shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The
Routing Topology Example
VLAN 10
Switch A
A
Host
B
Host
ISL Trunks
Types of Routing, page 1-3
IP Routing and Switch Stacks, page 1-3
Chapter 1
or see the sdm prefer command in the command reference
VLAN 20
Switch B
C
Host
Configuring IP Unicast Routing
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