Understanding layer-3 Interfaces

 

Understanding layer-3 Interfaces

Layer-3 interfaces are classified into two categories: Physical Layer-3 interfaces and logical layer-3 interfaces. logical layer-3 interfaces include layer-3 VLAN interfaces and routing sub-interfaces. layer-3 VLAN interfaces integrate the routing and bridging functions; the physical layer-3 interface has only the routing function, which is equivalent to a traditional router.

1. layer-3 logical VLAN Interface

The logical layer-3 VLAN interface provides a routing interface for the L2 Switch VLAN. Traditionally, a network requires a physical interface from a router to a vswitch to complete VLAN routing, switches like the Catalyst 4500 series support inter-VLAN (inter-VLAN) routing within a single Catalyst 4500 series switch through integrated routing and bridging functions.

Figure 9-6 shows how the routing and bridging functions of three physical devices in a traditional network are completed in a Catalyst 4500 series switch. The figure on the left shows the topology of Inter-VLAN routing in a traditional network. In this structure, two VLANs of a layer-2 switch are connected through a Router, the figure on the right shows that the Routing between two VLANs is implemented through the logical layer-3 VLAN interface of the Catalyst 4500 Series Switch (the Routing function is implemented by the Catalyst 4500 series switch itself ), this logic layer-3 VLAN Interface integrates both the routing and bridging functions. Obviously, this solution on the right is more cost-effective.

 

[Note] layer-3 switches with some module structures are specially configured with RSM (Routing Switching Module) or RSFC (routing switching function card ), the purpose is to implement layer-2 switching and layer-3 routing between different VLANs in the same switch. This will be detailed in Chapter 10th when introducing inter-VLAN routing.

There is also an interface type in the logical layer-3 interface, that is, the layer-3 Ethernet channel interface mentioned above, which will be detailed later in this chapter.

2. Physical Layer-3 Interface

The physical layer-3 interface supports functions equivalent to those of traditional routers. These L3 interfaces allow the Catalyst series switches to directly connect to the routable host. Figure 9-7 shows how the Catalyst 4500 series switches act as traditional routers. The Rourer in the figure is played by the Catalyst 4500 series switches. The interfaces connecting the two hosts are physical layer-3 routing interfaces.

 

 

 

 

Figure 9-7 physical nature of the Catalyst 4500 Series Switch
Example of a layer-3 interface routing application