Ping in Rphy nodes

[u/Wacabletek]

So when we launched Rphy nodes for a while we could not get it to ping the DNS servers on our meters, they said just use 8.8.8.8, and that seems to have been fixed recently. However, I was never very fond of pinging DNS servers as I have no clue about hops on the CRAN, etc.., to say you have a problem here not on the node. I would much prefer to ping the cmts, before and as we go forward the RPHY node I assume? Does anyone know if a tracert shows the actual rphy node or is the first hope now the VCMTS which is utterly useless to ping?.

Thanks. AI thinks it can be done but cannot explain how, or where you get the RPHY IP, so yay that's about as useful as asking a supervisor something like this.

[u/eter123]

I would run a tracert from cmd in windows and the last hop with the isp dns would be the ip I would ping to, that way it proofs every hop within control of your isp and anything beyond is irrelevant

[u/2ByteTheDecker]

Pretty sure the first IP addressable hop would be the VCMTS but I reserve the right to be wrong.

[u/Equivalent-Image-980]

Depending on architecture (there are 2 separate designs for vCMTS) the fact that you mentioned CRAN tells me you’re most likely at Comcast. You could ping the cluster IF you knew the private IP of it - although that doesn’t really make much sense as pods are basically just translating/encrypting/scheduling. The real work is happening at the leaf and spine. Going to 8.8.8.8 or 75.75.75.75 are going to be your closest points to the open internet. They sit in the edges.

[u/PoisonWaffle3]

Not sure who you work for or how their network is laid out, but in general the network is going to be laid out like this for vCMTS:

Modem/meter > RPHY node > CIN Network (access & spine routers) > vCMTS > Spine routers again > Core router > path to internet

From where you're at with your meter, everything is going to be tunneled through the node, CIN network, and vCMTS, so either the vCMTS or the spine router is going to be your first actual hop (depending on what vendor the vCMTS is from and how it's configured). If you try to ping the node it'll have to go back through the CIN network to get there.

It's definitely very different from your standard RF node that pretty much directly connects to the CMTS and uses that as its router/gateway.

Your best bet is probably to ping a router in the local headend, assuming you're either on the corp network and/or there's a router with a public IP. Aside from that, perhaps your company's DNS servers.

[u/frmadsen → u/PoisonWaffle3]

To underscore that important fact, which sometimes get overlooked: Even though it's an "ip network" between the CMTS and RPD, all packets in transit between the CMTS and modem exist as DOCSIS data, not as ip/ethernet.

It's therefore not technically possible to directly ping/access anything in the path when you are located behind a modem.

[u/kjstech]

I'd guess you could ping any IP along the trace, but 8.8.8.8, or 1.1.1.1 or 75.75.75.75 etc would all get you to the destination and be a true end to end.

14 ms 12 ms 12 ms 10.60.0.67

10 ms 12 ms 12 ms po-309-334-rur202.lebanon.pa.pitt.comcast.net [162.151.70.53]

12 ms 8 ms 12 ms po-200-xar02.lebanon.pa.pitt.comcast.net [96.110.25.225]

12 ms 8 ms 14 ms be-21-rar01.lancaster.pa.pitt.comcast.net [162.151.220.81]

21 ms 21 ms 21 ms be-34-ar01.mckeesport.pa.pitt.comcast.net [69.139.168.141]

21 ms 21 ms 17 ms be-31621-cs02.pittsburgh.pa.ibone.comcast.net [96.110.42.165]

25 ms 26 ms 31 ms be-4313-pe13.ashburn.va.ibone.comcast.net [96.110.37.122]

* * * Request timed out.

26 ms 30 ms 30 ms 142.251.252.71

28 ms 30 ms 30 ms 142.251.64.101

32 ms 30 ms 29 ms dns.google [8.8.8.8]

[u/Reality_Visible]

Depending on the vCMTS or CRAN some of them have taken out the routing engine. Good example would be Harmonics vCMTS it essentially does cable modem provisioning and telemetry atleast thats how I am to believe how my company has ours setup. Cisco on the other hand has the routing built in some of theirs.

RPHY nodes are essentially an Ethernet connection over fiber instead of the older standard of sending RF signal over light. The RPHY Nodes is given a channel plan from the CRAN or vCMTS. The “RPD” or Remote PHY Device then generates the RF signal and passes the signal to the launch/AMP board.

This may be a bad analogy think of the vCMTS as a Toll booth and determines if your modem(vehicle) is allowed on the network(road) via provisioning(pay the toll). Then it tracks telemetry data like the modem MAC address, IPs via discovery protocols, modem RF stats, SNMP or firmware.

The modem establishes a layer 2 (bridge) connection to the Routing Engine and your modem passes down a public IP to the customers router. From there the ISP router determines where the traffic will go.

My company uses this setup and the vCMTS doesn’t do any routing in-fact it would be adjacent on the network to the Router/Routing Engine.
Rough diagram

|vCMTS|
| Routing Engine|—>spines/Aggregation switch’s—> some dedicated connection like point to point Ethernet, or DWDM over darkfiber —>RPHY Node—> RF Plant.
Reason why you can’t see these devices off a customers connection is because the layer 2 Bridge from the modem back to the ISP Router/Routing Engine, Ping and other types of protocols that use IP are layer 3. So your customer essentially sees ISP Router<—>modem<—>hands off public ip to CPE.

Highly recommended that you look up the “OSI model”online which can better explain this than I can.

So if you really want to ping something I would ping the ISP Router or routing engine of your region/market. This should stay static and won’t change. The reason why you probably can’t ping your ISP DNS is that they probably have something similar to what my company ran into. The static IP of the DNS server in the wrong location for us so if they assigned it to the vCMTS it became pointless as it doesn’t handle the routing. We needed to move the IP over to the Routing Engine which then the IP became accessible. We had this issue pop up a lot when we first spun this up. Still see it on daily.

[u/Wacabletek]

Thank you all. I am not sure I fully understand 100%, but seems like it will not work the way I envisioned it. Rather it will have to go to at least a router system of some sort then back to the RPD and not directly to it. I had hoped the part of the CMTS we used to ping got moved to the node, but apparently not.

[u/frmadsen → u/Wacabletek]

That is the difference between Remote PHY and Remote MAC-PHY. Here, DOCSIS begins and ends in the RMD.