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Router on a Stick Topology

Setup that consists of a Router and a Switch connected using one Ethernet link configured as an 802.1q Trunk Link. The Switch is configured with multiple VLANs and the Router performs all routing between the different networks/VLANs.

router-on-a-stick

  1. You need to create a subinterface.
    • interface x/y.sub, for example FastEthernet 0/0.10, to create subinterface 10 on FastEthernet 0/0 physical interface.
    • It is best practice to use VLAN ID.
  2. Assign encapsulation to subinterface.
    • encapsulation dot1q x, for example encapsulation dot1q 10.
  3. Each subinterface will be assigned the first IP address in each subnet.

CISCO Example​

YEDGE1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
NYEDGE1(config)#interface gigabitethernet 0/0.16
NYEDGE1(config-subif)#encapsulation dot1q 16
NYEDGE1(config-subif)#ip address 192.168.16.1 255.255.255.0
NYEDGE1(config-subif)#interface gigabitethernet 0/0.17
NYEDGE1(config-subif)#encapsulation dot1q 17
NYEDGE1(config-subif)#ip address 192.168.17.1 255.255.255.0
NYEDGE1(config-subif)#interface gigabitethernet 0/0.18
NYEDGE1(config-subif)#encapsulation dot1q 18
NYEDGE1(config-subif)#ip address 192.168.18.1 255.255.255.0
NYEDGE1(config-subif)#interface gigabitethernet 0/0
NYEDGE1(config-if)#no shutdown
NYEDGE1(config-if)#exit
NYEDGE1(config)#exit
NYEDGE1#

Troubleshooting​

  1. Physical Layer: Physical cabling and connectivity
  2. Data Link Layer: Encapsulation or VLAN ID mismatch
    • show run interface
    • show interface trunk
  3. Data Link Layer: Misconfiguration of the subinterfaces
    • Compare router configuration to that of its interfaces, show running-configuration
    • show interface
  4. Network Layer: Misconfiguration of the IP addresses
    • show ip interface brief