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If you have a point-to-point link, but the encapsulations aren’t the same, the link will never
come up. Figure 8.4 shows one link with PPP and one with HDLC.
FIGURE 8 . 4 Mismatched WAN encapsulations
Look at router Pod1R1 in this output:
Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is down
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 254/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
LCP REQsent
Closed: IPCP, CDPCP
hostname Pod1R1
username Pod1R2 password Cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
encapsulation ppp
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.0.1.2 255.255.255.0
encapsulation HDLC
PSTN/ISDN
Pod1R1 Pod1R2
8.3 Troubleshoot WAN implementation issues 367
The serial interface is down, and LCP is sending requests but will never receive any responses
because router Pod1R2 is using the HDLC encapsulation. To fix this problem, you would have to
go to router Pod1R2 and configure the PPP encapsulation on the serial interface. One more thing—
even though the usernames are configured and they’re wrong, it doesn’t matter because the command
ppp authentication chap isn’t used under the serial interface configuration and the username
command isn’t relevant in this example.
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 8.5, 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 8 . 5 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
hostname Pod1R1
username Pod1R2 password cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
encapsulation ppp
ppp authentication chap
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.2.1.2 255.255.255.0
encapsulation ppp
ppp authentication chap
PSTN/ISDN
Pod1R1 Pod1R2
368 Chapter 8 Implement and verify WAN links
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 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.
Troubleshooting Frame Relay Networks
Troubleshooting Frame Relay networks isn’t any harder than troubleshooting any other type
of network as long as you know what to look for, which is what I’m going to cover now. We’ll
go over some basic problems that commonly occur in Frame Relay configuration and how to
solve them.
First on the list are serial encapsulation problems. As you learned recently, there are two
Frame Relay encapsulations: Cisco and IETF. Cisco is the default, and it means that you have
a Cisco router on each end of the Frame Relay network. If you don’t have a Cisco router on
the remote end of your Frame Relay network, then you need to run the IETF encapsulation as
shown here:
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay ?
ietf Use RFC1490 encapsulation
<cr>
RouterA(config-if)#encapsulation frame-relay ietf
Once you verify that you’re using the correct encapsulation, you then need to check out
your Frame Relay mappings. For example, take a look at Figure 8.6.
So, why can’t RouterA talk to RouterB across the Frame Relay network? To find that out,
take a close look at the frame-relay map statement. See the problem now? You cannot use
a remote DLCI to communicate to the Frame Relay switch; you must use your DLCI number!
The mapping should have included DLCI 100 instead of DLCI 200.
Now that you know how to ensure that you have the correct Frame Relay encapsulation,
and that DLCIs are only locally significant, let’s look into some routing protocol problems
typically associated with Frame Relay. See if you can find a problem with the two configurations
in Figure 8.7.
8.3 Troubleshoot WAN implementation issues 369
FIGURE 8 . 6 Frame Relay mappings
FIGURE 8 . 7 Frame Relay routing problems
Hmmmm, well, the configs look pretty good. Actually, they look great, so what’s the problem?
Well, remember that Frame Relay is a NBMA network by default, meaning that it doesn’t
send any broadcasts across the PVC. So, because the mapping statements do not have the
broadcast argument at the end of the line, broadcasts, like RIP updates, won’t be sent across
the PVC.
Exam Objectives
Remember the two Frame Relay encapsulation methods. There are two Frame Relay
encapsulations: Cisco and IETF. Cisco is the default, and it means that you have a Cisco router
on each end of the Frame Relay network. If you don’t have a Cisco router on the remote end
of your Frame Relay network, then you need to run the IETF encapsulation.
Remember that DLCI numbers are considered locally significant. You cannot use a remote
DLCI to communicate to the Frame Relay switch; you must use your DLCI number.
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