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

Router-on-a-Stick Inter-VLAN Routing

Router-on-a-Stick Inter-VLAN Routing
5

Summary

This topic configure router-on-a-stick inter-VLAN routing. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Module 4 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.

Router-on-a-Stick Scenario

In the previous topic, three different ways to create inter-VLAN routing were listed, and legacy inter-VLAN routing was detailed. This topic details how to configure router-on-a-stick inter-VLAN routing. You can see in the figure that the router is not in the center of the topology but instead, appears to be on a stick near the border, hence the name.

In the figure, the R1 GigabitEthernet 0/0/1 interface is connected to the S1 FastEthernet 0/5 port. The S1 FastEthernet 0/1 port is connected to the S2 FastEthernet 0/1 port. These are trunk links that are required to forward traffic within and between VLANs.

Router-on-a-Stick Example
Router-on-a-Stick Example

To route between VLANs, the R1 GigabitEthernet 0/0/1 interface is logically divided into three subinterfaces, as shown in the table. The table also shows the three VLANs that will be configured on the switches.

Router R1 Subinterfaces

Subinterface VLAN IP Address
G0/0/1.10 10 192.168.10.1/24
G0/0/1.20 20 192.168.20.1/24
G0/0/1.30 99 192.168.99.1/24

Assume that R1, S1, and S2 have initial basic configurations. Currently, PC1 and PC2 cannot ping each other because they are on separate networks. Only S1 and S2 can ping each other, but they but are unreachable by PC1 or PC2 because they are also on different networks.

To enable devices to ping each other, the switches must be configured with VLANs and trunking, and the router must be configured for inter-VLAN routing.

S1 VLAN and Trunking Configuration

Complete the following steps to configure S1 with VLANs and trunking:

Step 1. Create and name the VLANs.

Step 2. Create the management interface.

Step 3. Configure access ports.

Step 4. Configure trunking ports.

Router-on-a-Stick Example
Router-on-a-Stick Example

Click each button for details of the configuration step.

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)#

Next, the management interface is created on VLAN 99 along with the default gateway of R1.

S1(config)# interface vlan 99
S1(config-if)# ip add 192.168.99.2 255.255.255.0
S1(config-if)# no shut
S1(config-if)# exit
S1(config)# ip default-gateway 192.168.99.1
S1(config)#

Next, port Fa0/6 connecting to PC1 is configured as an access port in VLAN 10. Assume PC1 has been configured with the correct IP address and default gateway.

S1(config)# interface fa0/6
S1(config-if)# switchport mode access
S1(config-if)# switchport access vlan 10
S1(config-if)# no shut
S1(config-if)# exit
S1(config)#

Finally, ports Fa0/1 connecting to S2 and Fa05 connecting to R1 are configured as trunk ports.

S1(config)# interface fa0/1
S1(config-if)# switchport mode trunk
S1(config-if)# no shut
S1(config-if)# exit
S1(config)# interface fa0/5
S1(config-if)# switchport mode trunk
S1(config-if)# no shut
S1(config-if)# end
*Mar 1 00:23:43.093: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
*Mar 1 00:23:44.511: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5, changed state to up

S2 VLAN and Trunking Configuration

The configuration for S2 is similar to S1.

Router-on-a-Stick Example
Router-on-a-Stick Example
S2(config)# vlan 10
S2(config-vlan)# name LAN10
S2(config-vlan)# exit
S2(config)# vlan 20
S2(config-vlan)# name LAN20
S2(config-vlan)# exit
S2(config)# vlan 99
S2(config-vlan)# name Management
S2(config-vlan)# exit
S2(config)#
S2(config)# interface vlan 99
S2(config-if)# ip add 192.168.99.3 255.255.255.0
S2(config-if)# no shut
S2(config-if)# exit
S2(config)# ip default-gateway 192.168.99.1
S2(config)# interface fa0/18
S2(config-if)# switchport mode access
S2(config-if)# switchport access vlan 20
S2(config-if)# no shut
S2(config-if)# exit
S2(config)# interface fa0/1
S2(config-if)# switchport mode trunk
S2(config-if)# no shut
S2(config-if)# exit
S2(config-if)# end
*Mar 1 00:23:52.137: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

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.

Router-on-a-Stick Example
Router-on-a-Stick Example
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
C:\Users\PC1>

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

Click each button for example output for these commands.

Verify that the subinterfaces are appearing in the routing table of R1 by using the show ip route command. Notice that there are three connected routes (C) and their respective exit interfaces for each routable VLAN. The output confirms that the correct subnets, VLANs, and subinterfaces are active.

R1# show ip route | begin Gateway
Gateway of last resort is not set
      192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.10.0/24 is directly connected, GigabitEthernet0/0/1.10
L        192.168.10.1/32 is directly connected, GigabitEthernet0/0/1.10
      192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.20.0/24 is directly connected, GigabitEthernet0/0/1.20
L        192.168.20.1/32 is directly connected, GigabitEthernet0/0/1.20
      192.168.99.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.99.0/24 is directly connected, GigabitEthernet0/0/1.99
L        192.168.99.1/32 is directly connected, GigabitEthernet0/0/1.99
R1#

Another useful router command is show ip interface brief, as shown in the output. The output confirms that the subinterfaces have the correct IPv4 address configured, and that they are operational.

R1# show ip interface brief | include up
GigabitEthernet0/0/1   unassigned      YES unset  up                    up
Gi0/0/1.10             192.168.10.1    YES manual up                    up
Gi0/0/1.20             192.168.20.1    YES manual up                    up
Gi0/0/1.99             192.168.99.1    YES manual up                    up
R1#

Subinterfaces can be verified using the show interfaces subinterface-id command, as shown.

R1# show interfaces g0/0/1.10
GigabitEthernet0/0/1.10 is up, line protocol is up
  Hardware is ISR4221-2x1GE, address is 10b3.d605.0301 (bia 10b3.d605.0301)
  Description: Default Gateway for VLAN 10
  Internet address is 192.168.10.1/24
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  10.
  ARP type: ARPA, ARP Timeout 04:00:00
  Keepalive not supported
  Last clearing of "show interface" counters never
R1#

The misconfiguration could also be on the trunking port of the switch. Therefore, it is also useful to verify the active trunk links on a Layer 2 switch by using the show interfaces trunk command, as shown in the output. The output confirms that the link to R1 is trunking for the required VLANs.

Note: Although VLAN 1 was not explicitly configured, it was automatically included because control traffic on trunk links will always be forwarded on VLAN 1.

S1# show interfaces trunk
Port        Mode             Encapsulation  Status        Native vlan
Fa0/1       on               802.1q         trunking      1
Fa0/5       on               802.1q         trunking      1
Port        Vlans allowed on trunk
Fa0/1       1-4094
Fa0/5       1-4094
Port        Vlans allowed and active in management domain
Fa0/1       1,10,20,99
Fa0/5       1,10,20,99
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/1       1,10,20,99
Fa0/5       1,10,20,99
S1#

Packet Tracer – Configure Router-on-a-Stick Inter-VLAN Routing

In this activity, you will check for connectivity prior to implementing inter-VLAN routing. Then you will configure VLANs and inter-VLAN routing. Finally, you will enable trunking and verify connectivity between VLANs.

Lab – Configure Router-on-a-Stick Inter-VLAN Routing

In this lab, you will complete the following objectives:

  • Part 1: Build the Network and Configure Basic Device Settings
  • Part 2: Configure Switches with VLANs and Trunking
  • Part 3: Configure Trunk-Based Inter-VLAN Routing

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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