Layer-3 Switches

Contributed by indianboy February 25, 2004  
Tagged: Cisco switch

Overview of how it layer-3 switches work. Catalyst Switches are normally used as Layer-3 switches. Thus, the switch can have a SUP engine ,a redundant SUP engine , a RSM module and layer-2 (ethernet or fast ethernet) ports.

Functions of different modules ( for new guys like me )

RSM ( Route Switch Module ): Performs Routing mostly Inter-Vlan routing
SUP ( Supervisor Engine ): The switch part of the Layer-3 Switch mostly Vlan creation etc, redundant engines are commonly used ….
Layer-2 Ports : Ethernet or Fast Ethernet Ports through which machines can be connected .

Overview of Operation ( Again for guys like me ) ( Examples on Catalyst 5500 series )

1. Vlans are configured on the SUP Engine
Console> (enable) set vlan 10 name example-1
Console> (enable) set vlan 20 name example-2

2. The configuration can be viewed using the command
Console> (enable) sh vlan

3. Log into the RSM module using the command
Console> (enable) session 15 or ( module number of RSM module )

4. Goto the enable mode and then the configure mode the final result is
RSM(config)#

5. Create the virtual interfaces on RSM for the Vlans created in SUP engine .
RSM(config)#interface vlan 10
RSM(config)#interface vlan 20

6. Enter into the specific interfaces and add ip addresses
RSM(config)#interface vlan 10
RSM(config-if)#ip address 10.10.10.1 255.255.255.0
RSM(config-if)#description ***Example-1_Interface ***
RSM(config-if)#no shut
RSM(config)#interface vlan 20
RSM(config-if)#ip address 10.10.20.1 255.255.255.0
RSM(config-if)#description ***Example-2_Interface ***
RSM(config-if)#no shut

7. Configure the necessary Routing Information
RSM(config)#router ospf 100
RSM(config-router)#network 10.10.10.0 0.0.0.255 area 0
RSM(config-router)#network 10.10.20.0 0.0.0.255 area 0

8. Once this much is set you should be able to ping the addresses
RSM#ping 10.10.10.1
RSM#ping 10.10.20.1

9. Configure the ethernet ports to the particular vlan on specific ports
eg: for the 18 port on the 3rd module the command will be
Console>(enable)set vlan 3/18 10

10. Connect machines to the specified ports . For machines in the example-1 network the default gateway will be 10.10.10.1 and the port on the switch will be in vlan number 10 and for the example-2 network the default-gateway for machines will be 10.10.20.1 and the port on the switch to which the machine will be connected will be vlan number 20 .

11. You should be able to ping and exchange info between the two subnets ….