Isolating Data Link Layer Issues

Once the physical layer is happily doing its job, the DSL connection will train up and negotiate upstream and downstream channels. With PPPoA and PPPoE, additional functions must take place at Layer 2 to get the connectivity established.

The DSL router will negotiate an ATM PVC. It may be more accurate to say that the DSL router will be automatically configured with an ATM PVC. It is not so much a negotiation function, but simply a provisioning function. Most routers have a debugging capability that can be used in watching this function take place. To debug ATM connectivity functions, use the debug atm events and debug atm packet commands. The output from the debug atm events command shows the current VPI and VCI on which traffic is being received, as demonstrated in Example 7-4. In this case, 8/35 is the VPI/VCI pair.

Example 7-4 debug atm events Output

Router#debug atm events

ATM events debugging is on 2d18h: RX interrupt: conid =

0, rxBd

= 0x80C7EF74

length

52

2d18h:

Data Cell received on

vpi = 8

vci = 35

2d18h:

RX interrupt: conid =

0, rxBd

= 0x80C7EEC0

length

52

2d18h:

Data Cell received on

vpi = 8

vci = 35

The output from the debug atm packet command shows the current state of ATM traffic hitting the interface and the encapsulation type. In Example 7-5, the highlighted line shows that the SAP is AAAA. This is both DSAP (AA) and SSAP (AA). CTL is set to 03 and OUI to 0080C2. From previous discussions of ATM, this is readily recognizable as LLC SNAP encapsulation (also known as AAL5SNAP encapsulation). This means that there is the potential for multiple protocols to be transported over this ATM circuit.

Example 7-5 debug atm packet Output

Router#debug atm packet

03:21:32: ATM0(I):

VCD:0x2 VPI:0x1 VCI:0x1 Type:0x0 SAP:AAAA CTL:03 OUI:0080C2

TYPE

0007

Length:0x30

03:21:32: 0000 0050 7359 35B7 0001 96A4 84AC 8864 1100 0001

0 0 0

C021

09AB 000C 0235

03:21:32: 279F 0000 0000

03:21:32:

If the VPI and VCI are received and the circuit looks good and is trained up, the connectivity of the local ATM segment can be tested through the use of an ATM ping, as demonstrated in Example 7-6. This is an additional extension to the Cisco Extended Ping capability and is specific to ATM interfaces.

Example 7-6 ATM Ping Results

Router#ping atm interface atm 0/1 8 35 seg-loopback

Type escape sequence to abort.

Sending Seg-Loopback 5, 53-byte OAM Echoes to a neighbor, timeout is 5 seconds: !!!!!

Success rate is 100 percent (5/5)

The segment loopback sends ATM Operation, Administration, and Maintenance (OAM) cells across the circuit to the next ATM device encountered and then the cells are looped back to return in the same manner. This verifies the configuration of the local PVC.

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