I have 2900 series legacy router being configured for small office environment with a single ISP connection, and are assigned a single IP address. I am using 10.0.0.0 network address space sent to a ASA5540 to use PAT with the ASA's outside interface address. On the router, I have configured the most basic config possible, and ping'd the local interfaces successfully but it cant ping the directly connected neighbors interfaces on the directly connected networks. No (default)routes are needed to ping interfaces on directly connected networks, the router knows were they are, they're directly connected. Both device IP's on the respective connected networks have the same subnet mask, so they have the same 'view of the world', ie - to prevent the problem of the ping goes out but can't get back. From the config below.... On Gi0/1, I should be able to ping any directly connected neighboring device in the range of 10.10.0.1 - 10.10.255.254, which is the directly connected network, but can't. On Gi0/0, I should be able to ping any directly connected device with IP's from, 10.20.0.2 - 10.20.255.254, but can't ping any. I am not an expert, but this is a very basic config. Assign some IP addresses/masks to interfaces, enable IP routing and you should at least be able to ping the local IP's and any on a directly connected subnet. The router config was wiped and started fresh adding the few items I described.
! Last configuration change at 03:01:47 UTC Thu Jul 26 2018
!
version 15.1
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Router
!
boot-start-marker
boot-end-marker
!
! card type command needed for slot/vwic-slot 0/0
!
no aaa new-model
!
no ipv6 cef
ip source-route
ip cef
!
no ip dhcp conflict logging
ip dhcp excluded-address 10.20.0.0 10.20.0.128
!
ip dhcp pool LAN-Ip's
network 10.20.0.0 255.255.0.0
domain-name My_Domain_Name
default-router 10.20.0.1
dns-server 208.67.222.222 1.1.1.1 8.8.8.8
!
no ip domain lookup
multilink bundle-name authenticated
!
crypto pki token default removal timeout 0
!
voice-card 0
!
hw-module pvdm 0/0
!
redundancy
!
interface GigabitEthernet0/0
ip address 10.20.0.1 255.255.0.0
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 10.10.10.2 255.255.0.0
duplex auto
speed auto
!
interface GigabitEthernet0/2
no ip address
shutdown
duplex auto
speed auto
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
control-plane
!
!gatekeeper
no shutdown
!
line con 0
logging synchronous
line aux 0
line vty 0 4
login
transport input all
!
scheduler allocate 20000 1000
end
