Sorry for the newb question, I am trying to figure out how to best link up an subinterface on a Cisco 1921 router, with an external network, by connecting the router and the cable from the external network, to a Cisco Catalyst 2960 switch and isolating them into their own VLAN. The external network recommended I use a 172.A.A.102 255.255.252 for my router IP, and ping 172.A.A.101 on their network. I was able to ping successfully by connecting the external network's cable to my laptop and configuring their IP information, but I cant ping from my router after what I setup:

  1. I created the subinterface for the external network on my router with:

    interface GigabitEthernet0/0.3 encapsulation dot1Q 802 ip address 172.A.A.102

  2. On the switch I configured two ports: f0/5 for the router, and f0/6 for the external network cable with:

interface FastEthernet0/5 switchport mode trunk spanning-tree portfast !
interface FastEthernet0/6 switchport access vlan 802 switchport mode access spanning-tree portfast
But unfortunately I cannot ping 172.A.A.101. Please what am I doing wrong?

  • Is the VLAN 802 up? What is the result of a "show vlan" on the switch? Is the interface GigabitEthernet0/0.3 on the router up?
    – JFL
    Jun 26, 2017 at 8:59
  • Yes VLAN 802 is up, show vlan presents 802 VLAN_NAME active Fa0/6, interface is up on the router: GigabitEthernet0/0.3 172.A.A.102 YES manual up up
    – Unpossible
    Jun 26, 2017 at 9:05
  • Wow. Seems to have worked, I can ping the external network IP from the router now. main1#ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.A.A.101, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms But from the catalyst switch, I can't ping 172.A.A.101 or 172.A.A.102, is this normal?
    – Unpossible
    Jun 26, 2017 at 9:19
  • The switch doesn't have an IP in this network, and it can't since it's a /30 network which allow only 2 IPs - a waste, since the provider could use a /31 for the same result. So yes , it is normal. Glad you found out. You could post what you did as an answer and accept it so the question doesn't pop up for ever.
    – JFL
    Jun 26, 2017 at 9:25

1 Answer 1


Actually I did the right thing, I suppose I wasn't patient enough for the switch to start accepting packets. Everything works as expected.

  • you should accept your answer so that the question doesn't keep popping up forever, looking for an answer.
    – Ron Maupin
    Jul 26, 2017 at 9:44

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