Connecting to NBN HFC with a linux router

Internode recently migrated me from an ADSL connection to an NBN HFC connection. Here’s how I configured the connection using my own Debian GNU/Linux router instead of the TP-Link VR1600v internode supplies…

NBN Co supplies an Arris CM8200 NTD, which is a modem that bridges local ethernet to the ISP via DOCSIS over the coaxial cable.

Additionally, Internode’s configuration requires PPPoE encapsulation with 802.1q VLAN tagging.

I’m using physical interface eth1, so in /etc/network/interfaces I have:

# See interfaces(5)
auto eth1
iface eth1 inet static
    address 192.168.1.2/24

# VLAN ID 2 for Internode's NBN HFC.
auto eth1.2
iface eth1.2 inet manual

auto nbn
iface nbn inet ppp
    pre-up /bin/ip link set eth1.2 up
    provider nbn

Then in /etc/ppp/peers/nbn I have:

# See pppd(8).
user "xxx@internode.on.net"
noipdefault
usepeerdns
defaultroute
replacedefaultroute
hide-password
lcp-echo-interval 20
lcp-echo-failure 3
connect /bin/true
noauth
persist
noaccomp
default-asyncmap

# Connect NBN using PPPoE over the eth1 interface with VLAN ID 2.
mtu 1492
plugin rp-pppoe.so eth1.2

In /etc/ppp/pap-secrets, add a dummy password:

"xxx@internode.on.net" * "not used for NBN"

To bring it up:

sudo ifup ppp0=nbn

A note on the MTUs here. From a Mac OS X device on the local network, the largest IP datagram that you can send without fragmentation or errors is 1492 octets (tested via ping -D -s 1464 www.dslreports.com). The ethernet frame arriving at the NTD can be broken down as:

  • 1464 octet ICMP echo data payload
  • + 8 octet ICMP header = 1472
  • + 20 octet IPv4 header = 1492. This is the IP datagram size enforced by the “MTU” settings I have on my local ethernet connections (e.g. wifi) and also in /etc/ppp/peers/nbn
  • + 2 octets PPP + 6 octets PPPoE = 1500. This is the size of the ethernet payload
  • + 4 bytes 802.1Q header for the VLAN tag, + 18 802.3 ethernet overhead = 1522. This is the ethernet frame size, and it’s the maximum layer 2 frame the NBN will accept.

Unlike ADSL (using ATM), there’s no need to worry about fragmentation with smaller frame sizes at layer 2; everything is ethernet, so choosing the maximum end-to-end MTU (1492) is the best strategy for minimizing overheads.

One thought on “Connecting to NBN HFC with a linux router”

  1. Thanks for your post. The important information was that it was possible.

    For Tangerine Telecom I found some differences as follows:
    A password , being the first (numeric) part of the user id, was necessary.

    The peculiar effect of having a dummy password, is that I gained authentication and a
    dynamic IP with a 15 minute turnover. This was blocked to port 80 requests, but open
    for ssh etc.

    The installation proved in the end to be the same as the former installation with ADSL.

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