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Inter-VLAN Routing

Chapter Description

In this sample chapter from Switching, Routing, and Wireless Essentials Companion Guide (CCNAv7) for Cisco Networking Academy students, you will learn how to troubleshoot common inter-VLAN configuration issues.

Router-on-a-Stick Inter-VLAN Routing (4.2)

In this section, you configure router-on-a-stick inter-VLAN routing.

Router-on-a-Stick Scenario (4.2.1)

In the previous section, three ways to create inter-VLAN routing were listed, and legacy inter-VLAN routing was detailed. This section 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 Figure 4-5, 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.


Figure 4-5 Router-on-a-Stick Topology

To route between VLANs, the R1 GigabitEthernet 0/0/1 interface is logically divided into three subinterfaces, as shown in Table 4-2. The table also shows the three VLANs that will be configured on the switches.

Table 4-2 Router R1 Subinterfaces



IP Address







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 (4.2.2)

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

  • Step 1. Create and name the VLANs. First, the VLANs are created and named, as shown in Example 4-1. VLANs are created only after you exit out of VLAN subconfiguration mode.

    Example 4-1 Create and Name VLANs

    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
  • Step 2. Create the management interface. Next, the management interface is created on VLAN 99 along with the default gateway of R1, as shown in Example 4-2.

    Example 4-2 Create the Management Interface

    S1(config)# interface vlan 99
    S1(config-if)# ip add
    S1(config-if)# no shut
    S1(config-if)# exit
    S1(config)# ip default-gateway
  • Step 3. Configure access ports. Next, port Fa0/6 connecting to PC1 is configured as an access port in VLAN 10, as shown in Example 4-3. Assume PC1 has been configured with the correct IP address and default gateway.

    Example 4-3 Configure Access Ports

    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
  • Step 4. Configure trunking ports. Finally, ports Fa0/1 connecting to S2 and Fa05 connecting to R1 are configured as trunk ports, as shown in Example 4-4.

    Example 4-4 Configure Trunking 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 (4.2.3)

The configuration for S2 is similar to S1, as shown in Example 4-5.

Example 4-5 S2 Configuration

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)# interface vlan 99
S2(config-if)# ip add
S2(config-if)# no shut
S2(config-if)# exit
S2(config)# ip default-gateway
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 (4.2.4)

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 configuration in Example 4-6, the R1 G0/0/1 subinterfaces are configured for VLANs 10, 20, and 99.

Example 4-6 R1 Subinterface Configuration

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
R1(config-subif)# exit
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
R1(config-subif)# exit
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
R1(config-subif)# exit
R1(config)# interface G0/0/1
R1(config-if)# description Trunk link to S1
R1(config-if)# no shut
R1(config-if)# end
*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

Verify Connectivity Between PC1 and PC2 (4.2.5)

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, as shown in Example 4-7.

Example 4-7 Verify Windows Host Configuration

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                     :
  Subnet Mask                      :
  Default Gateway                  :

The output confirms the IPv4 address and default gateway of PC1. Next, use ping to verify connectivity with PC2 and S1, as shown in Figure 4-5. The ping output successfully confirms that inter-VLAN routing is operating, as shown in Example 4-8.

Example 4-8 Verify Inter-VLAN Routing by Pinging from PC1

C:\Users\PC1> ping
Pinging with 32 bytes of data:
Reply from bytes=32 time<1ms TTL=127
Reply from bytes=32 time<1ms TTL=127
Reply from bytes=32 time<1ms TTL=127
Reply from bytes=32 time<1ms TTL=127
Ping statistics for
    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> ping
Pinging with 32 bytes of data:
Request timed out.
Request timed out.
Reply from bytes=32 time=2ms TTL=254
Reply from bytes=32 time=1ms TTL=254 |
Ping statistics for
    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 (4.2.6)

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

As shown in Example 4-9, 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.

Example 4-9 Verify Subinterfaces Are in Routing Table

R1# show ip route | begin Gateway
Gateway of last resort is not set is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/1.10
L is directly connected, GigabitEthernet0/0/1.10 is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/1.20
L is directly connected, GigabitEthernet0/0/1.20 is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/1.99
L is directly connected, GigabitEthernet0/0/1.99

Another useful router command is show ip interface brief, as shown in Example 4-10. The output confirms that the subinterfaces have the correct IPv4 address configured, and that they are operational.

Example 4-10 Verify Subinterface IP Addresses and Status

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

Subinterfaces can be verified using the show interfaces subinterface-id command, as shown in Example 4-11.

Example 4-11 Verify Details of the Subinterface

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
  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

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 Example 4-12. The output confirms that the link to R1 is trunking for the required VLANs.

Example 4-12 Verify Trunk Link Status

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
6. Inter-VLAN Routing using Layer 3 Switches (4.3) | Next Section Previous Section

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