Understanding Ip Routing; Types Of Routing - Cisco Catalyst 3120 Software Manual

Understanding IP Routing

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
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 38-1
router has an interface in each VLAN.
Figure 38-1
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:
•
•

Types of Routing

Routers and Layer 3 switches can route packets in three different ways:
•
•
•
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
38-2
Chapter 8, "Configuring SDM Templates"
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 38-2
IP Routing and Switch Stacks, page 38-3
By using default routing
By using preprogrammed static routes for the traffic
By dynamically calculating routes by using a routing protocol
Chapter 38
or see the sdm prefer command in
VLAN 20
Switch B
C
Host
Configuring IP Unicast Routing
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