Chapter 38 Configuring IP Unicast Routing

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 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 38-1shows 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 38-1 Routing Topology Example

 

VLAN 10

VLAN 20

A

Switch A

Switch B

 

 

Host

 

 

B

Host

ISL Trunks

C

Host

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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 38-2

IP Routing and Switch Stacks, page 38-3

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

By dynamically calculating routes by using a routing protocol

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

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Cisco Systems 3750E manual Understanding IP Routing, Types of Routing, 38-2