adslethernetmodem
I've got my own network infrastructure already, but I'm tied to my ISPs modem which is a bit on the flaky side. The modem is in bridging mode, with the perimeter router behind it handling authorization via PPPoE.
I want to replace it with something as simple and no-frills as possible.
After happening upon these VDSL2 to Ethernet converters, I want to know if this general class of devices can be used, provided I handle the networking specifics on my router.
Is this possible or advisable?
OK, so assume you have a topology like:
SW1
/ \
/ \
/ \
PC A--SW2-----SW3--PC B
For some reason there is a bridging loop, STP is disabled or someone applied a filter in the wrong place or such.
PC A wants to communicate with PC B. It first ARPs for the MAC of PC B, the destination is a broadcast with MAC ffff.ffff.ffff. So the frame goes to both SW1 and SW3. The SRC MAC is PC A. SW1 then floods the frame towards SW3 and SW3 will flood the frame coming from SW2 to SW1.
SW1 and SW3 learned the MAC of PC A when the first frame came in. When the second one comes in from the opposite direction it has to relearn it. Because these events occur so fast and repeatedly you will see log messages complaining about MAC flapping. Something like "MAC FLAP 0000.0000.0001 is flapping between Gi0/24 and Gi0/23". This is a good sign that you have a loop.
What you could do then is to try to trace this MAC. Try looking in the ARP cache of a device in the same subnet and see what IP this device has. So with the MAC you could try to trace it with sh mac-address-table or with the IP maybe you have a list with all IPs and where they are connected.
If the host gets a IP address from a DHCP server you could also try there to find where the host is coming from. If you have option 82 enabled that would be a great help.
Other signs are that the CLI will be very sluggish. CPU load will be very high. Switches do almost everything in ASICs so if a switch has a CPU load over 50% it's probably not good. You should implement SNMP monitoring and watch for high CPU load. Also look for the MAC flap messages. If the switches have a loop the LEDs will probably be blinking like crazy.
Things you could do to protect against loops:
Based on this presentation some of those PAM5 codes (2 byte "SSD" and respectively 4 byte combo "csreset" and "ESD") mark the start and respectively end of frame, similar to how J/K and T/R do the same for 100BASE-TX.
The 802.3-2005 standard explains how these are coded relative to normal data:
During data encoding, PCS Transmit utilizes a three-state convolutional encoder. The transition from idle or carrier extension to data is signalled by inserting a SSD, and the end of transmission of data is signalled by an ESD. [...] During idle and carrier extension encoding, special code-groups with symbol values restricted to the set {2, 0, –2} are used. These code-groups are also generated using the transmit side-stream scrambler. However, the encoding rules for the idle, SSD, and carrier extend code-groups are different from the encoding rules for data, CSReset, CSExtend, and ESD code-groups. During idle, SSD, and carrier extension, the PCS Transmit function reverses the sign of the transmitted symbols. This allows, at the receiver, sequences of code-groups that represent data, CSReset, CSExtend, and ESD to be easily distinguished from sequences of code-groups that represent SSD, carrier extension, and idle.
CSReset means "Convolutional State Reset" and SSD/ESD mean Start-of-Stream/End-of-Stream Delimiter.
The actual codes are publicly available [for free] as ASCII table 40-1 and table 40-2. The codes are given per byte there, e.g. to send SSD, you send SSD1 and SSD2 in this order.
Best Answer
The particular device you have listed is required to be used as a pair. This is for someone who want to extend a network across voice cabling, or for a small ISP to use to connect to customers. It doesn't work as a general replacement for a DSL modem.
This is explained in the device description (my emphasis):
Normally, an ISP that allows you to use your own DSL device will have a list of compatible devices. There are multiple DSL standards, and you would need to use a device compatible with the standard used by your ISP.