I can't seem to get to set a gateway of last resort on a router. I want it to forward anything whom's ip address is not in the routing table to router HQ through interface s0/0/0 by doing the following:

Branch1#config t
Branch1(config)#ip route s0/0/0

When I check the routing table it doesn't appear there...

Branch1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
Gateway of last resort is not set is variably subnetted, 8 subnets, 4 masks
C is directly connected, GigabitEthernet0/0.107
L is directly connected, GigabitEthernet0/0.107
C is directly connected, GigabitEthernet0/0.37
L is directly connected, GigabitEthernet0/0.37
C is directly connected, GigabitEthernet0/0.157
L is directly connected, GigabitEthernet0/0.157
C is directly connected, Serial0/0/1
L is directly connected, Serial0/0/1

Although it does appear in the running-config as

    ip classless 
    ip route Serial0/0/0

The ip interface brief:

    Branch1#sh ip interface br
    Interface              IP-Address      OK? Method Status                Protocol 
    GigabitEthernet0/0     unassigned      YES unset  up                    up 
    GigabitEthernet0/0.37   YES manual up                    up 
    GigabitEthernet0/0.107    YES manual up                    up 
    GigabitEthernet0/0.157   YES manual up                    up 
    GigabitEthernet0/1     unassigned      YES unset  administratively down down 
    Serial0/0/0            unassigned      YES unset  up                    up 
    Serial0/0/1     YES manual up                    up 
    Serial0/1/0            unassigned      YES unset  administratively down down 
    Serial0/1/1            unassigned      YES unset  administratively down down 
    Vlan1                  unassigned      YES unset  administratively down down

The problem is going to be that the interface is down. When an interface goes down, any routes that are pointed to the interface are withdrawn from the routing table, but they will reappear when the interface comes back up.

  • The interfaces are up already
    – A Alw
    Dec 7 '18 at 3:19
  • Give us a show ip interface brief in your question. You do not have any routes attached to S0/0/0, meaning it is down or you have not assigned any addressing.
    – Ron Maupin
    Dec 7 '18 at 3:20
  • Sorry, added them.
    – A Alw
    Dec 7 '18 at 3:26
  • 2
    You do not have IPv4 enabled on the interface, so you cannot route to the interface. For IPv4, the interface is down. You must either assign an IPv4 address, or you must use the ip unnumbered <interface> command on the interface to enable IPv4 for the interface.
    – Ron Maupin
    Dec 7 '18 at 3:29

I have faced the same issue. In my case one of the interfaces was down. I have activated it and it solved the issue.

Step 1:

Initially, check your routing table

If you are enable mode, use

show ip interface brief

If you are in configuration mode, use

do show ip int bri ( here do is added )

Step 2:

Check the topology of your network and set the ip address accordingly.

  1. interface (interface name)

int g0/2 ( interface is gi 0/2)

  1. set ip address

ip address

NOTE: use the ip addresses, interface related to your topology

  • 1
    The OP was asking about a serial interface which works numbered or unnumbered - as long as IP has been activated on it. Otoh, an Ethernet interface requires an IP address as a router interface.
    – Zac67
    Jul 8 at 17:18

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