Router-on-a-Stick Inter-VLAN Routing

Step 1. Create and name the VLANs.

Step 2. Create the management interface.

Step 3. Configure access ports.

Step 4. Configure trunking ports.

1

First, the VLANs are created and named. VLANs are only created after you exit out of VLAN subconfiguration mode.

S1(config)# vlan 10

S1(config-vlan)# name LAN10

S1(config-vlan)# exit

S1(config)# vlan 20

S1(config-vlan)# name LAN20

S1(config-vlan)# exit

S1(config)# vlan 99

S1(config-vlan)# name Management

S1(config-vlan)# exit

S1(config)#

2

3

4

R1 Subinterface Configuration

The router-on-a-stick method requires you to create a subinterface for each VLAN to be routed.

A subinterface is created using the interface interface_id.subinterface_id global configuration mode command. The subinterface syntax is the physical interface followed by a period and a subinterface number. Although not required, it is customary to match the subinterface number with the VLAN number.

Each subinterface is then configured with the following two commands:

• encapsulation dot1q vlan_id [native] - This command configures the subinterface to respond to 802.1Q encapsulated traffic from the specified vlan-id. The native keyword option is only appended to set the native VLAN to something other than VLAN 1.

• ip address ip-address subnet-mask - This command configures the IPv4 address of the subinterface. This address typically serves as the default gateway for the identified VLAN.

Repeat the process for each VLAN to be routed. Each router subinterface must be assigned an IP address on a unique subnet for routing to occur.

When all subinterfaces have been created, enable the physical interface using the no shutdown interface configuration command. If the physical interface is disabled, all subinterfaces are disabled.

In the following configuration, the R1 G0/0/1 subinterfaces are configured for VLANs 10, 20, and 99.

R1(config)# interface G0/0/1.10
R1(config-subif)# description Default Gateway for VLAN 10
R1(config-subif)# encapsulation dot1Q 10
R1(config-subif)# ip add 192.168.10.1 255.255.255.0
R1(config-subif)# exit
R1(config)#
R1(config)# interface G0/0/1.20
R1(config-subif)# description Default Gateway for VLAN 20
R1(config-subif)# encapsulation dot1Q 20
R1(config-subif)# ip add 192.168.20.1 255.255.255.0
R1(config-subif)# exit
R1(config)#
R1(config)# interface G0/0/1.99
R1(config-subif)# description Default Gateway for VLAN 99
R1(config-subif)# encapsulation dot1Q 99
R1(config-subif)# ip add 192.168.99.1 255.255.255.0
R1(config-subif)# exit
R1(config)#
R1(config)# interface G0/0/1
R1(config-if)# description Trunk link to S1
R1(config-if)# no shut
R1(config-if)# end
R1#
*Sep 15 19:08:47.015: %LINK-3-UPDOWN: Interface GigabitEthernet0/0/1, changed state to down
*Sep 15 19:08:50.071: %LINK-3-UPDOWN: Interface GigabitEthernet0/0/1, changed state to up
*Sep 15 19:08:51.071: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0/1, changed state to up
R1#

Verify Connectivity Between PC1 and PC2

The router-on-a-stick configuration is complete after the switch trunk and the router subinterfaces have been configured. The configuration can be verified from the hosts, router, and switch.

From a host, verify connectivity to a host in another VLAN using the ping command. It is a good idea to first verify the current host IP configuration using the ipconfig Windows host command.

C:\Users\PC1> ipconfig
Windows IP Configuration
Ethernet adapter Ethernet0:
  Connection-specific DNS Suffix . :
  Link-local IPv6 Address          : fe80::5c43:ee7c:2959:da68%6
  IPv4 Address                     : 192.168.10.10
  Subnet Mask                      : 255.255.255.0
  Default Gateway                  : 192.168.10.1
C:\Users\PC1> 

The output confirms the IPv4 address and default gateway of PC1. Next, use ping to verify connectivity with PC2 and S1, as shown in the figure. The ping output successfully confirms inter-VLAN routing is operating.

C:\Users\PC1> ping 192.168.20.10
Pinging 192.168.20.10 with 32 bytes of data:
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127 
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127
Ping statistics for 192.168.20.10:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss).
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms
C:\Users\PC1> 
C:\Users\PC1> ping 192.168.99.2
Pinging 192.168.99.2 with 32 bytes of data:
Request timed out.
Request timed out.
Reply from 192.168.99.2: bytes=32 time=2ms TTL=254
Reply from 192.168.99.2: bytes=32 time=1ms TTL=254	|
Ping statistics for 192.168.99.2:
    Packets: Sent = 4, Received = 2, Lost = 2 (50% loss). 
Approximate round trip times in milli-seconds:
    Minimum = 1ms, Maximum = 2ms, Average = 1ms

Router-on-a-Stick Inter-VLAN Routing Verification

In addition to using ping between devices, the following show commands can be used to verify and troubleshoot the router-on-a-stick configuration.

• show ip route

• show ip interface brief

• show interfaces

• show interfaces trunk