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Cisco Troubleshooting ADSL/DSL/VDSL issues

There’s a few basic troubleshooting steps that should be followed if there’s an issue with a DSL connection. Broadly speaking, there’s three networking layer levels that may require troubleshooting. Depending on the type of issue, troubleshooting might begin at a layer 1, or at layer 2.

Layer 1 – at the lowest level troubleshooting involves  checking the physical connectivity to the Digital Subscriber Line Access Multiplexer (DSLAM) at the ISP.

Layer 2 – troubleshooting involves looking at ATM connectivity and the PPPoA or PPPoE protocol

Layer 3 – troubleshooting IP connectivity

Where to Begin Troubleshooting?

I would recommend starting with the simple show ip interface brief command – the output of which might look a bit different depending on your device and configuration.

In my case here the ADSL connection happens to be configured on Dialer0 and the status and Protocol are up/up so it would make sense to assume Layer 1 is okay and that troubleshooting can start at layer 2.

If you have a different configuration you could be looking for interfaces ATM0 and ATM0.1 for example.

router#sh ip interface brief
Interface         IP-Address  OK?  Method  Status  Protocol
Ethernet0         10.1.1.1    YES  NVRAM   up      up
Dialer0           123.1.1.7   YES  IPCP    up      up

If your DSL interface is not up/up or if they seem to be alternating between up and down then begin troubleshooting at layer 1.

Layer 1 Testing

Some things to try…

  • Is the carrier detect (CD) light on the front panel of the router on or off? If it is on then this indicates layer 1 is okay and you can probably start troubleshooting at layer 2
  • If the CD light is off, check out the physical cabling to ensure that the DSL interface is connected.
  • If the CD light is off and the cabling seems correct try checking to see if the interface is administratively down by issuing a no shutdown command in the DSL interface configuration.
  • Starting to get more desperate? On the DSL interface try dsl operating-mode auto
  • At this stage there is probably some physical issue with cabling, the provisioning of the DSL service, or the hardware – and troubleshooting this might be up to a cabling expert or the ISP.

Layer 2 Testing

One thing to check is that you have the correct PVC values for VPI and VCI for your ISP. This is rarely a problem these days but is here for completeness.

First, let’s check to see if PPP is trying to negotiate with your ISP. We’ll check the input and output packets to see if the values are changing. In the sample command below I’m filtering the output (which is a page or more long) to just show lines that contain the string “packets”.

Wait a few moments and run the command again and see if the input and output packets change.

router#sh int dialer0 | i packets
  5 minute input rate 8959000 bits/sec, 924 packets/sec
  5 minute output rate 568000 bits/sec, 640 packets/sec
    711015331 packets input, 2847957132 bytes
    459512972 packets output, 2531036441 bytes
  • If input packets are NOT incrementing then you are not receiving PPPoE negotiation packets from your ISP, so probably give them a call before continuing with any further steps.
  • If output packets are not incrementing then check your PPP configuration. Even if your ISP is broken you should ordinarily still be sending outbound packets trying to negotiate.

Is the PPPoE session up?

PPPoE is a two-phase process – PPPoE established first, and then PPP second.

Some useful commands to debug PPPoE

  • show vpdn
  • debug vpdn pppoe-events

Some useful commands to debug PPP

  • show ppp all
  • debug ppp negotiation

Layer 3 Testing

You might actually have layer 3 connectivity, but some ping packets are lost. To check the speed in kbps that your are syncing with the DSLAM at try the following command – the output is snipped to show the relevant part.

Depending on your router, or type of HWIC/EHWIC installed you might need to enter show controllers vdsl 0 command. In my example below I’m synced at 21406 kilobit/s down and 1074 kilobit/s up.

router#show dsl int atm0/0/0
              DS Channel1   DS Channel0   US Channel1   US Channel0
Speed (kbps):          0         21406             0          1074

If some sites (particularly web sites) work well and others do not, the MTU size or MSS-Adjust might need changing. Generally MTU of 1492 or MSS adjustment of 1360 would be fine.