for Unnumbered WAN Point-to-Point
Point-to-point numbered refers
to connections where the interfaces connecting the routers to the WAN have IP
addresses. On both WAN ports, the address of the WAN port is set to an IP address
on a common IP network, and the destination addresses are set to the IP address
of WAN port of the unit on the far side of the link.
====ETHERNET===================== 192.168.1.0/24 ===
| LAN IP: 192.168.1.100
| Router A |
| Serial0 |
| WAN IP: 192.168.100.1/32 (255.255.255.255)
[-] CSU/DSU (may be integrated on WAN card)
| PTP WAN
[-] CSU/DSU (may be integrated on WAN card)
| WAN IP: 192.168.2.200/32 (255.255.255.255)
| Serial0 |
| Router B |
| LAN IP: 192.168.2.200
====ETHERNET===================== 192.168.2.0/24 ===
Be sure to have two IP subnets for the Ethernets connected to
the routers. In an unnumbered setup, each WAN port connected to the point-to-point
WAN will share the IP address of a local Ethernet interface. In our configuration
above, the routers are using the Ethernet0 address (LAN IP) on the WAN IP for
Serial0 as well. Note that while Serial0 shares the IP
of Ethernet0 (LAN IP), it does not share the netmask. Always use a /32 (host)
netmask on an unnumbered WAN interface.
These IP network addresses
are given to you by your Internet Service Provider, your leased line carrier,
or by ARIN. If you
do not plan to connect you network to the Internet, you can use an IP network
address set aside for private use. The IP network address for private use can
be found in RFC 1597. Most people will use 192.168.0.0 as we have in this example.
This connection is common for interconnecting two points on the same
provider's network, but is rarely used for interconnections between providers.
See our Numbered WAN
example for a common setup between providers.
The WAN Ports
In this example, we are going to assume the following:
A has an Ethernet IP address of 192.168.1.100 with a netmask of 255.255.255.0
- Router A has a WAN IP address of 192.168.1.100 with a netmask of 255.255.255.255
- Router B has an Ethernet IP address of 192.168.2.200 with a netmask of
- Router B has a WAN IP address of 192.168.2.200 with a netmask
- Router A is providing Internet access to Router B.
- The WAN ports on both routers is Serial0.
The IP addresses
used in this Technical Note are examples only, you will need to use an IP network
given to you by your Internet Service Provider.
ip address 192.168.1.100 255.255.255.0
description Connection to Router B
ip address 192.168.1.100 255.255.255.255
ip route 0.0.0.0 0.0.0.0 192.168.1.1
ip address 192.168.2.200 255.255.255.0
description Connection to Router A
ip address 192.168.2.200 255.255.255.255
ip route 0.0.0.0 0.0.0.0 Serial0
#Alternate default route command
#ip route 0.0.0.0 0.0.0.0 192.168.1.100
the port does not have an "up" status for both hardware and protocol in the interface
statistics output (Option 2 from the Main menu) do the following:
hardware shows "down", check your cable connections and ensure that you have connected
to the correct port. If you have connected to the correct port, double-check the
telephone company smart jack or other telephone company-installed equipment for
your circuit. The smart jack should show no red alarms if the cables are connected.
If, after connecting the cables correctly, you have errors on the telephone company
equipment, contact your line provider for assistance.
If hardware shows
"up", but protocol shows "down", you should see the PPP negotiation and see the
status of the WAN port change to up. At this point you should be able to ping
the other side.
If you have an external CSU/DSU, put the CSU/DSU into a
local loopback. This will cause each packet sent to the CSU/DSU to reflect back
to the WAN port. This is for testing only, the line will not function while the
CSU/DSU is in a local loopback. If, in the interface statistics detail output
for the port, you see transmitted packets immediately received on the same port,
then you can determine the following facts:
- The WAN port on the router
is sending and receiving data.
- The cable and connectors between the router
and the CSU/DSU are functioning.
- The DTE port on the CSU/DSU is functioning.
- THE ROUTER IS FUNCTIONING AS IT SHOULD.
If you don't get
your packets back immediately on the same interface:
- For cards with
a software-selectable serial interface, check that you have specified the correct
wiring specification (V.35, RS232, RS422/X.21). The "dctype" command is typically
set to V.35 (the default) in North America and RS422 in Europe and Asia.
all cables are securely connected.
- Begin swapping hardware, cables, CSU/DSU,
If you get your packets back immediately, but the line protocol
status is not "up":
- Check the CSU/DSU settings such as clocking, etc.
Almost always the CSU/DSU should be set for external clocking. Your line provider
typically provides the clock. It's not a bad idea to verify any external CSU/DSU
settings with the manufacturer.
- Have their telephone carrier test the
line. Requesting a hard copy of the test results will often get you a better test.
If the line protocol status is "up" but you can't ping the other
- Double check settings. Make sure that the IP address for the
WAN ports on both sides use a host (/32, 255.255.255.255) netmask.
a traceroute from your workstation through the local (to you) router to the router
in trouble. Then do a traceroute from the router in trouble (assuming you can
get to it) back to your workstation. The problem will lie in the gap between the
- Make sure the default gateway is set to the upstream
router or the serial device used for the upstream connection, as in our example
above on Router B.