Mismatched IP Addresses

A tricky problem to spot is if you have HDLC or PPP configured on your serial interface but your IP addresses are wrong. Things seem to be just fine because the interfaces will show that they are up. Take a look at Figure 1 and see if you can see what I mean—the two routers are connected with different subnets—router Pod1R1 with 10.0.1.1/24 and router Pod1R2 with 10.2.1.2/24.

FIGURE 1 Mismatched IP addresses

This will never work. But as I said, take a look at the output:

Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
LCP Open
Open: IPCP, CDPCP

See that? The IP addresses between the routers are wrong but the link looks like it’s working fine. This is because PPP, like HDLC and Frame Relay, is a layer 2 WAN encapsulation and doesn’t care about IP addresses at all. So yes, the link is up, but you can’t use IP across this link since it’s misconfigured.

To find and fix this problem, you can use the show running-config or the show interfaces command on each router, or you can use what you learned in Chapter 5—the show cdp neighbors detail command:

Pod1R1#sh cdp neighbors detail
-------------------------
Device ID: Pod1R2
Entry address(es):
IP address: 10.2.1.2

You can view and verify the directly connected neighbor’s IP address and then solve your problem.