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Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 23 Configuring Layer 3 Interfaces
Overview of Layer 3 Interfaces
Logical Layer 3 VLAN Interfaces
The logical Layer 3 VLAN interfaces provide logical routing interfaces to VLANs on Layer 2 switches.
A traditional network requires a physical interface from a router to a switch to perform inter-VLAN
routing. The Catalyst 4500 series switch supports inter-VLAN routing by integrating the routing and
bridging functions on a single Catalyst 4500 series switch.
Figure 23-1 shows how the routing and bridging functions in the three physical devices of the traditional
network are performed logically on one Catalyst 4500 series switch.
Figure 23-1 Logical Layer 3 VLAN Interfaces for the Catalyst 4500 Series Switch
Physical Layer 3 Interfaces
The physical Layer 3 interfaces support capabilities equivalent to a traditional router. These Layer 3
interfaces provide hosts with physical routing interfaces to a Catalyst 4500 series switch.
Figure 23-2 shows how the Catalyst 4500 series switch functions as a traditional router.
Figure 23-2 Physical Layer 3 Interfaces for the Catalyst 4500 Series Switch
L2 Switch L2 Switch
Router
Host 1 Host 2
Traditional network topology for routing
between VLANS
Host 1 Host 2
Logical Inter-VLAN routing on a single
Catalyst 4500 series switch
Routing
VLAN1 VLAN2
VLAN1 VLAN2
94169
Interface Ethernet
1.1.1.1
Interface VLAN1
1.1.1.1
Interface VLAN2
2.1.1.1
Interface Ethernet
2.1.1.1
Router
Host 1 Host 2
Physical Inter-VLAN Routing on a Catalyst 4500 series switch
Interface Ethernet
1.1.1.1
Interface Ethernet
2.1.1.1
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