Ccna routing and Switching Complete Study Guide

no encapsulation

*Feb 7 16:00:18.678:%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, 

changed state to up

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906

 

Chapter 21 

■

  Wide Area Networks

Notice the link came up because it now matches the encapsulation on the other end of 

the link! 

Always remember that you just can’t have PPP on one side and HDLC on 

the other—they don’t get along!

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 21.17 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.

f i g u r e   21.17     Mismatched IP addresses

Pod1R1

Pod1R2

hostname Pod1R1

username Pod1R2 password cisco

interface serial 0

ip address 10.0.1.1  255.255.255.0

clock rate 64000

bandwidth 512

encapsulation ppp

ppp authentication chap

hostname Pod1R2

username Pod1R1 password cisco

interface serial 0

ip address 10.2.1.2  255.255.255.0

bandwidth 512

encapsulation ppp

ppp authentication chap

This will never work. Let’s 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 appears to be work-

ing just fine. This is because PPP, like HDLC and Frame Relay, is a layer 2 WAN encapsula-

tion, so it doesn’t care about layer three addressing at all. So yes, the link is up, but you  

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Point-to-Point Protocol (PPP) 

907

can’t use IP across this link since it’s misconfigured, or can you? Well, yes and no. If you 

try to ping, you’ll see that this actually works! This is a feature of PPP, but not HDLC or 

Frame Relay. But just because you can ping to an IP address that’s not in the same subnet 

doesn’t mean your network traffic and routing protocols will work. So be careful with this 

issue, especially when troubleshooting PPP links!

Take a look at the routing table of Pod1R1 and see if you can find the mismatched IP 

address problem:

[output cut]

  10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       10.2.1.2/32 is directly connected, Serial0/0

C       10.0.1.0/24 is directly connected, Serial0/0

Interesting! We can see our serial interface S0/0 address of 10.0.1.0/24, but what is that other 

address on interface S0/0—10.2.1.2/32? That’s our remote router’s interface IP address! PPP 

determines and places the neighbor’s IP address in the routing table as a connected interface, 

which then allows you to ping it even though it’s actually configured on a separate IP subnet. 

For the Cisco objectives, you need to be able to troubleshoot PPP from the 

routing table as I just described.

To find and fix this problem, you can also use the 

show running-config

, 

show 

interfaces

, or 

show ip interfaces brief

 command on each router, or you can use 

the 

show cdp neighbors detail

 command:

Pod1R1#