Where to Begin

The easiest way to determine which layer you should begin to troubleshoot is to issue the command show ip interface brief, as demonstrated in Example 7-1.

Example 7-1 show ip interface brief Command Output Helps You Discern at Which OSI Layer the Problem Originate

Router#show ip interface brief

Interface

IP-Address

OK?

Method

Status

Protocol

ATM0

unassigned

YES

manual

up

up

ATM0.1

unassigned

YES

unset

up

up

Ethernet0

10.10.10.1

YES

manual

up

up

The output of this command differs slightly depending on your configuration. However, the essential information provided is all there.

The interface Status and Protocol columns provide excellent information. If the status of ATM0 and ATM0.1 are up and the Protocol is up, this indicates that the interface is properly functioning. The Status column is a Layer 1 indicator, whereas the Protocol column is a Layer 2 indicator. If, as in this example, both show as up, Layers 1 and 2 can be generally be ruled out as causes of trouble. If the ATM interfaces show Status as down, or if they flap (that is, continuously transition between up and down state), the likely issue is at Layer 1. Example 7-2 illustrates a flapping interface.

Example 7-2 Flapping Interface

BM2821#

000217: Apr 2 21:59:01.536: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down

000218: Apr 2 21:59:01.536: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/1 from FULL to DOWN, Neighbor Down: Interface down or detached

000219: Apr 2 21:59:01.544: %LDP-5-NBRCHG: LDP Neighbor 3.3.3.3:0 (1) is DOWN (Interface not operational)

BM2821#

000220: Apr 2 21:59:11.536: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

BM2821#

000221: Apr 2 21:59:21.536: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/1 from INIT to DOWN, Neighbor Down: Interface down or detached

000222: Apr 2 21:59:21.536: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down

Example 7-2 Flapping Interface (Continued) BM2821#

000223: Apr 2 21:59:31.536: %LINEPROTO-5-UPDOWN: Line protocol on Interface

GigabitEthernet0/1, changed state to up BM2821#

In Example 7-2, the interface state changes are logged to the console by default or some other configured logging facility.

Obviously, a Status of down will be accompanied by Protocol down. If Layer 1 is down, Layer 2 cannot be up. However, there are cases where Status can be up and Protocol is down. This is indicative of a Layer 2 problem.

If the status shows the interface to be administratively down, the interface has been manually placed in a shutdown state, as demonstrated here:

Router#show interface atm 0

ATM0 is administratively down, line protocol is down <output omitted>

To remedy this issue, you must enter the no shutdown command in the router's interface configuration for the interface in question:

Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z. Router(config)#interface atm 0 Router(configif)#no shutdown

Once this is completed, check the status of the interface once again by reissuing the show interface command:

Router#show interface atm 0

ATM0 is up, line protocol is up

Hardware is PQUICC_SAR (with Alcatel ADSL Module)

Now issue the show dsl interface atm0 command and check to ensure that the status shows "Showtime," which indicates successful training, as demonstrated in Example 7-3.

Example 7-3 show dsl interface interface Command Output Verifies Successful Training

Router#show dsl int atm 0

ATU-R (DS)

ATU-C (US)

Modem Status: Showtime (DMTDSL_SHOWTIME)

DSL Mode: ITU G.992.1 (G.DMT)

! Output omitted for brevity

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