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 A Frame Relay network offers a few additional benefits that a leased-line implementation
does not offer. But with these benefits comes a bit more complexity. The addition of
concepts such as nonbroadcast multiaccess (NBMA), Local Management Interface (LMI),
Inverse Address Resolution Protocol (ARP), and Frame Relay maps requires that an
administrator have a fundamental knowledge of these concepts to better troubleshoot
connectivity issues that can arise.
Components of Troubleshooting Frame Relay
Troubleshooting Frame Relay requires a step-by-step approach that identifies and tests each
of the major components. Figure 8-31 outlines this approach in a flowchart.
The major components of Frame Relay troubleshooting are described as follows:
■ Troubleshooting a Frame Relay link that is down, which could be a Layer 1 or
Layer 2 issue
■ Troubleshooting Frame Relay remote router connectivity, which is the connectivity
between the Frame Relay peer routers
348 Chapter 8: Extending the Network into the WAN
■ Troubleshooting Frame Relay end-to-end connectivity, which is the connectivity
between workstations across a Frame Relay network
Figure 8-31 Flowchart for Troubleshooting Frame Relay
Troubleshooting Frame Relay Connectivity Issues
The first step in troubleshooting Frame Relay connectivity issues is to check the status of
the Frame Relay interface. Figure 8-32 gives a flowchart for troubleshooting these issues.
Figure 8-32 Troubleshooting Connectivity Issues
Use the show interface serial number[/number] command to check the status of the Frame
Relay interface.
Which symptom
best describes
the problem?
Is the
Frame Relay
link down?
Troubleshoot a
Frame Relay
link that is down.
Troubleshoot
Frame Relay
remote router
connectivity.
Troubleshoot
Frame Relay
end-to-end
connectivity.
N
Y Y Y
N
Is there no
connectivity
with the remote
Frame Relay
router?
Is there
connectivity
end-to-end through
the Frame Relay
network?
Status of
show interface
serial
command?
Serial
interface down
and line protocol
down?
Troubleshoot
Frame Relay
remote router
connectivity.
Check for matching
LMI type
sh frame-relay lmi.
Check for matching
encapsulation type and
keepalive intervals.
Check for cable
sh controller
serial.
Loopback test to
troubleshoot
cable and hardware.
Check for
correct DLCI
sh frame-relay pvc.
N
Y Y Y
N
Serial
interface up
and line protocol
down?
Serial
interface up
and line protocol
up?
Troubleshooting Frame Relay WANs 349
If the output of the show interface serial command displays a status of “interface down/
line protocol down,” this typically indicates a problem at Layer 1, the physical layer. This
output means that you have a problem with the cable, CSU/DSU, or the serial line.
First, use the show controllers serial [slot/port] command to verify that the cable is present
and recognized by the router.
Next, you might need to troubleshoot the problem with a loopback test.
Follow these steps to perform a loopback test:
Step 1 Set the serial line encapsulation to High-Level Data Link Control (HDLC) and
keepalive to 10 seconds. To do this, use the encapsulation hdlc and keepalive
10 commands in the interface configuration mode of the interface you are
troubleshooting.
Step 2 Place the CSU/DSU or modem in local-loop mode. Check the device
documentation to determine how to do this. If the line protocol comes up
when the CSU/DSU or modem is in local-loop mode, indicated by a “line
protocol is up (looped)” message, this suggests that the problem is
occurring beyond the local CSU/DSU. If the status line does not change
status, a problem could be in the router, connecting cable, CSU/DSU, or
modem. In most cases, the problem is with the CSU/DSU or modem.
Step 3 Execute a ping command to the IP address of the interface you are
troubleshooting while the CSU/DSU or modem is in local-loop mode.
No misses should occur. An extended ping that uses a data pattern of
0x0000 is helpful in resolving line problems because a T1 or E1
connection derives clock speed from the data and requires a transition
every 8 bits. A data pattern with many 0s helps to determine whether the
transitions are appropriately forced on the trunk. A pattern with many 1s
is used to appropriately simulate a high 0 load in case a pair of data
inverters is in the path. The alternating pattern (0x5555) represents a
“typical” data pattern. If your pings fail or if you get cyclic redundancy
check (CRC) errors, a bit error rate tester (BERT) with an appropriate
analyzer from the telephone company (telco) is needed.
Step 4 When you are finished testing, ensure that you return the encapsulation
of the interface to Frame Relay.
An incorrect statically defined DLCI on a subinterface can also cause the status of the
subinterface to appear as “down/down,” and the PVC status might appear as “deleted.” To
350 Chapter 8: Extending the Network into the WAN
verify that the correct DLCI number has been configured, use the show frame-relay pvc
command, as demonstrated in Example 8-8.
In the output, the DLCI number shows “100” and reports a status of “deleted.” This might
indicate that the DLCI you configured was incorrect.
If the output of a show interface serial command displays a status of “interface up/line
protocol down,” this typically indicates a problem at Layer 2, the data link layer. If this is
the case, the serial interface might not be receiving the LMI keepalives from the Frame
Relay service provider. To verify that LMI messages are being sent and received and to
verify that the router LMI type matches the LMI type of the provider, use the show framerelay
lmi command, as demonstrated in Example 8-9.
Example 8-8 Verifying DLCI Configuration
RouterX# show frame-relay pvc
PVC Statistics for interface Serial0/0/0 (Frame Relay DTE)
Active Inactive Deleted Static
Local 0 0 1 0
Switched 0 0 0 0
Unused 0 0 0 0
DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = DELETED, INTERFACE = Serial0/0/0
input pkts 9 output pkts 8 in bytes 879
out bytes 1024 dropped pkts 0 in pkts dropped 0
out pkts dropped 0 out bytes dropped 0
in FECN pkts 0 in BECN pkts 0 out FECN pkts 0
out BECN pkts 0 in DE pkts 0 out DE pkts 0
out bcast pkts 2 out bcast bytes 138
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
pvc create time 00:00:27, last time pvc status changed 00:00:27
Example 8-9 Verifying LMI Traffic Delivery/Receipt and LMI Type Matches Between Router
and Provider
RouterX# show frame-relay lmi
LMI Statistics for interface Serial0/0/0 (Frame Relay DTE) LMI TYPE = CISCO
Invalid Unnumbered info 0 Invalid Prot Disc 0
Invalid dummy Call Ref 0 Invalid Msg Type 0
Invalid Status Message 0 Invalid Lock Shift 0
Invalid Information ID 0 Invalid Report IE Len 0
Invalid Report Request 0 Invalid Keep IE Len 0
Troubleshooting Frame Relay WANs 351
The output shows that 236 LMI status inquiry messages have been sent (Num Status Enq.
Sent), 31 LMI status messages have been received (Num Status msgs Rcvd), and the
LMI type is set to “Cisco.”
For a Frame Relay router to reach a peer router across the Frame Relay network, it must
map the IP address of the peer router with the local DLCI it uses to reach that IP address.
Figure 8-33 shows the steps involved in troubleshooting remote router connectivity issues.
Figure 8-33 Troubleshooting Remote Router Connectivity with Frame Relay
Num Status Enq. Sent 236 Num Status msgs Rcvd 31
Num Update Status Rcvd 0 Num Status Timeouts 206
Last Full Status Req 00:00:38 Last Full Status Rcvd 00:00:38
Example 8-9 Verifying LMI Traffic Delivery/Receipt and LMI Type Matches Between Router
and Provider (Continued)
Does the
router have a
Frame Relay map
sh frame-relay
map?
Is the
router using
Inverse ARP?
Is the
IP address-to-DLCI
mapping
correct?
Verify ACL rules and
temporarily remove ACL
from interface.
Check IP addresses
and DLCI
assignments.
Add or edit the static
Frame Relay map.
Clear the map table
clear framerelay-
inarp.
Y Y
Y
N N
N
Y
Are there
any ACLs applied
to the Frame Relay
interfaces?
352 Chapter 8: Extending the Network into the WAN
As demonstrated in Example 8-10, the show frame-relay map command shows the IP
address–to–DLCI mappings and indicates whether the mapping was statically entered or
dynamically learned using Inverse ARP.
If you have recently changed the address on the remote Frame Relay router interface, you
might need to use the clear frame-relay-inarp command to clear the Frame Relay map of
the local router. This will cause Inverse ARP to dynamically remap the new address with
the DLCI.
If the IP address of the peer router does not appear in the Frame Relay mapping table, the
remote router might not support Inverse ARP. Try adding the IP address–to–DLCI mapping
statically by using the frame-relay map protocol protocol-address dlci [broadcast]
command.
Additionally, access control lists (ACL) might be applied to the Frame Relay interfaces that
affect connectivity. To verify whether an ACL is applied to an interface, use the show ip
interface command.
To temporarily remove an ACL from an interface to verify whether it is affecting
connectivity, use the no ip access-group acl_num {in | out} command in interface
configuration mode.
For end-to-end connectivity to exist between two workstations across an active Frame
Relay network, general routing requirements must be met. Figure 8-34 shows the
troubleshooting steps for verifying end-to-end connectivity.
If you are experiencing end-to-end connectivity problems in your Frame Relay network,
check the routing tables to see whether the routers have a route to the destination with
which you are having connectivity problems. To check the routing table, use the show ip
route command, as demonstrated in Example 8-11.
Example 8-10 Verifying IP Address–to–DLCI Mappings and Static or Dynamic Configuration
RouterX# show frame-relay map
Serial0/0/0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic,
broadcast,
CISCO, status defined, active
Troubleshooting Frame Relay WANs 353
Figure 8-34 Troubleshooting End-to-End Connectivity with Frame Relay
If only directly connected routes appear in the routing table, the problem might be that
the Frame Relay network is preventing the routing protocol updates from being
advertised across it. Because of the NBMA nature of Frame Relay, you must configure
Example 8-11 Confirming End-to-End Connectivity
RouterX# show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.2.0 is directly connected, Loopback1
10.0.0.0/24 is subnetted, 3 subnets
C 10.23.23.0 is directly connected, Serial0/0/1
C 10.2.2.0 is directly connected, FastEthernet0/0
192.168.1.0/24 is variably subnetted, 3 subnets, 3 masks
C 192.168.1.64/28 is directly connected, Loopback0
Can you ping
the remote node
from the router?
Is the
Frame Relay
router configured
for dynamic
routing?
Verify the broadcast
parameter in the
Frame Relay map.
Configure the
Frame Relay interface
as point-to-point.
Verify static
route configuration.
Is
split horizon
enabled?
Check the
default gateway
of the source node.
Y
Y
N
Y N
Y
N
N
Is the
Frame Relay
interface configured
as multipoint?
354 Chapter 8: Extending the Network into the WAN
the router to pass routing protocol broadcasts or multicasts across the Frame Relay
network. With the use of Inverse ARP, this capability is in effect automatically. With a
static Frame Relay map, you must explicitly configure the support for broadcast traffic.
The show frame-relay map command displays whether the broadcast capability is in
effect, allowing routing updates to be passed across the Frame Relay network, as
demonstrated in Example 8-12.
Summary of Troubleshooting Frame Relay WANs
The following summarizes the key points that were discussed in the previous sections:
■ There are three aspects of troubleshooting frame relay: troubleshooting the link,
troubleshooting the mapping from one router to another, and troubleshooting routing
across a Frame Relay network.
■ Use the show interface serial and show frame-relay lmi commands to verify Layer
1 and Layer 2 link failures. Use the show frame-relay map and show frame-relay pvc
commands to test connectivity between routers.