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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing Steps for Configuring Routing
When HostA in VLAN 10 needs to communicate with HostB in VLAN 10, it sends a packet addressed
to that host. SwitchA forwards the packet directly to Host B, without sending it to the router.
When Host A sends a packet to HostC in VLAN 20, SwitchA forwa rds the packet to the router, which
receives the traffic on the VLAN 10 interface. The router checks the routing table, determines the correct
outgoing interface, and forwards the packet on the VLAN20 interface to Switch B. Switch B receives
the packet and forwards it to HostC.
Steps for Configuring Routing
By default, IP routing is disabled on the Catalyst 3550 switch, and you must enable it before routin g can
take place. For detailed IP routing configuration information, refer to the Cisco IOS IP and IP Routing
Configuration Guide for Release 12.1.
In the following procedures, the specified interface must be one of these Layer 3 interfaces:
•A routed port: a physical port configured as a Layer 3 port by using the no switchport interface
configuration command.
•A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id
global configuration command and by default a Layer 3 interface.
•An EtherChannel port channel in Layer 3 mode: a port-cha nn el lo gical in terf ac e c rea ted by using
the interface port-channel port-channel-number global configuration command and binding the
Ethernet interface into the channel group. For more information, see the “C onf iguri ng L a yer 3
EtherChannels” section on page21-11.
Note A Layer 3 switch can have an IP address assigned to each routed port and SVI. The num ber of ro uted
ports and SVIs that you can configure is not limited by software. However, the interrelationship
between this number and the number and volume of features being implemented might have an
impact on CPU utilization because of hardware limitations. For more information about feature
combinations, see the “Optimizing System Resources for User-Selected Features” section on
page 6-57.
All Layer 3 interfaces must have IP addresses assigned to them. See the “Assigning IP Addresses to
Network Interfaces” section on page 22-5.
Configuring routing consists of several main procedures:
•To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN
membership to Layer 2 interfaces. For more information, se e Chapter9, “Creating and Maintaining
VLANs.”
•Configure Layer 3 interfaces.
•Enable IP routing on the switch.
•Assign IP addresses to the Layer 3 interfaces.
•Enable selected routing protocols on the switch.
•Configure routing protocol parameters (optional).