There are many options available for use with the ping utility in IOS. One of the more useful features is the ability to specify the source interface in the ping request. This is useful when you need to accurately test end to end reachability. For example, if you try an end to end test from the router, accurate results will almost never occur by default. These unwanted results occur because the router will use the source interface that is directly connected to the next hop. In the scenario where a WAN is involved, the source interface will likely be a serial interface. If you wanted to test from a workstation’s point of view behind the router, a source of the WAN interface will not produce accurate results. This is because the serial interface and the Lan interface is on separate networks. Consider the following:

Router configuration at SiteA:

interface FastEthernet0/0
ip address 1.1.1.1 255.255.255.0
interface Serial1/0
ip address 192.168.1.1 255.255.255.252

Routers configuration at SiteB:

interface FastEthernet0/0
ip address 2.2.2.1 255.255.255.0
interface Serial1/0
ip address 192.168.1.2 255.255.255.252

A workstation at Site A on the 1.1.1.0 subnet can’t reach a server at SiteB on the 2.2.2.0 subnet. An engineer who is responsible for troubleshooting the problem works from a location other than either site. The engineer logs into the router at SiteA to try and ping the server at SiteB. The command issued on routerA is:

ping 2.2.2.100 (address of server).

The router will use a source address of 192.168.1.1 (Serial Interface) because it’s route table has a route via 192.168.1.2 (routerB’s serial interface). This would only test the network assigned to the serial interfaces (192.168.1.0), which is not where the workstation is located.

The proper way to accurately test would be to use the command:

ping 2.2.2.100 source 1.1.1.1

Using the source ip of the same network the workstation is connected to means that routing will be properly tested between the client and server subnets.