881W Router WAN interface (dhcp assinged addr) cannot ping or respond to pings

[tgbazzie]

I am new to IOS and have thrown myself in head-first. I'm attempting to setup an 881W - IOS Version 12.4(24)T3 - for my learning attempt.

I want to configure the WAN interface (FastEthernet4) to use ISP-provided DHCP. I am testing this by connecting it to a consumer router that serves DHCP.

As the title suggests, the router cannot ping (or traceroute) out, and same-LAN hosts cannot ping the router address. The router can ping its own address. Examples below:

c881w01#ping 192.168.0.105 <-- on FastEthernet4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.105, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

c881w01#ping 192.168.0.101 <-- on my laptop

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.101, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

The interface config is:

interface FastEthernet4
  ip address dhcp
  ip virtual-reassembly
  no ip route-cache
  duplex full
  speed auto
  no cdp enable
end

This results in:

FastEthernet4 is up, line protocol is up
  Hardware is PQII_PRO_UEC, address is a44c.11c1.55b6 (bia a44c.11c1.55b6)
  Internet address is 192.168.0.105/24
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:06, output 00:00:58, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
    613 packets input, 40342 bytes
    Received 608 broadcasts, 0 runts, 0 giants, 0 throttles
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
    0 watchdog
    0 input packets with dribble condition detected
    616 packets output, 41479 bytes, 0 underruns
    0 output errors, 0 collisions, 3 interface resets
    595 unknown protocol drops
    0 babbles, 0 late collision, 0 deferred
    0 lost carrier, 0 no carrier
    0 output buffer failures, 0 output buffers swapped out

The only route is successfuly set by DHCP:

c881w01#show ip route
Default gateway is 192.168.0.1

Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty

I'm sure I've made some bonehead mistake, but I've stared at this too long without a breakthrough.

Thanks in advance for the help

[Georg Pauwen · Accepted solution]

Hello,

FastEthernet4 is the WAN port. Your LAN hosts cannot use that address as a default gateway. You need to configure an IP address on Vlan1 and use that as a default gateway. Have a look at the (partial) configuration below (for the sake of simplicity I have configured your router as DHCP server for your LAN hosts, so make sure your PCs have DHCP enabled):

ip dhcp excluded-address 192.168.1.1
!
ip dhcp pool vlan1
import all
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
!
interface FastEthernet0
!
interface FastEthernet1
!
interface FastEthernet2
!
interface FastEthernet3
!
interface FastEthernet4
ip address dhcp
ip virtual-reassembly
no ip route-cache
duplex full
speed auto
no cdp enable
!
interface Vlan1
ip address 192.168.1.1

Then, add a default route:

ip route 0.0.0.0 0.0.0.0 FastEthernet4

or

ip route 0.0.0.0 0.0.0.0 dhcp

[tgbazzie]

Thanks very much for the reply. I implemented your suggested configuration, but this did not resolve the issue. Out of desperation, I turned on debug all. When I repeated the ping test, I saw in the logs that ping was using 192.168.0.2 as a gateway. It was only then that I recalled setting ip address 192.168.0.2 255.255.255.0 on LoopBack0 (my purpose is for another discussion). Once I reconfigured to no ip address, things began to work as intended.

Again, thanks very much.

[Georg Pauwen]

Hello,

good to hear that you got it working. So without the Loopback address, your original configuration works ?

[tgbazzie]

I used your example, which was fairly close to what I had. The loopback address I had set earlier was disrupting the routing and preventing outbound traffic from egressing properly. I'm not sure of the precise failure mechanics, but having the loopback address on the same subnet as the WAN interface was certainly the root of it.