Router - TG582N - Using an external modem

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Revision as of 15:56, 8 March 2015 by Adsb (talk | contribs) (Add an interface to talk to servers on the external modem)
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One may wish to use the TG582n just as a router, and use an external ADSL modem. In your author's case, the performance of the TG582n as a modem was found to be very poor (3.2km from the exchange, 8km from a very powerful Medium Wave transmitter) and a different modem was much better (part of a modem/router which can't do IPv6...).

One could use the configuration wizard to setup the TG582n for FTTC, but some of the settings aren't appropriate - and my intention is to make it easy to switch back to the internal modem (although since doing this, I haven't needed to switch back!).

Sacrifice an Ethernet port

One has to dedicate an Ethernet port for the connection to the external modem. It's traditional to sacrifice port 4 for this, so

:eth bridge ifdelete brname bridge intf ethport4

Create a WAN interface

:eth intf=eth_WAN
:eth ifconfig intf=eth_WAN dest=ethif4 vlan=default
:eth ifattach intf=eth_WAN
:eth ifconfig intf=eth_WAN wan=enabled priotag=disable

Switch PPP over to use this new interface

:ppp ifdetach intf=Internet
:ppp menu

Select ifconfig and enter Internet as intf. Change dest to eth_WAN and then OK out of the menu.

Then bring PPP up:

:ppp ifattach intf=Internet

One can simply switch the dest field between atm_Internet and eth_WAN to change between internal and external modem.

Get the front panel LEDs working correctly

For external modem:

:system config WANMode=ETH WANEthPort=ethif4

For internal modem:

:system config WANMode=ADSL

Connect the external modem

Disconnect the TG582n from the phone line, connect an Ethernet cable from port 4 on the TG482n to ones external modem, connect the external modem to the phone line, and one should be back online.

MTU and baby jumbo frames

See also MTU

Using an external modem forces the TG582n to use PPPoE (rather than PPPoA). The MTU will drop by 8 octets as a result. It is possible to configure eth_WAN to use 1508 octet Ethernet frames ("baby jumbo frames") and give PPPoE a MRU/MTU of 1500 octets:

:eth ifconfig intf=eth_WAN mtu=1508
:ppp ifdetach intf=Internet
:ppp ifconfig intf=Internet mru=1500
:ppp ifattach intf=Internet

but (at least with firmware version 10.2.0.B) the TG582n doesn't support RFC4638 and won't negotiate 1500 octet MRU/MTU with the backhaul network.

[ One would need a PPPoE to PPPoA bridge running on the external modem to get a 1500 octet MRU/MTU with PPPoE. ]

Connecting to servers on the external modem

In your author's case, there are still servers (HTTP, Telnet, SSH) running on the external modem - it has an RFC1912 address. It's very useful to be able to connect to these servers from the LAN. There's already an Ethernet link to the external modem (it's carrying the PPPoE traffic), so add an interface on eth_WAN to talk TCP/IP to the external modem:

:ip ifadd intf=ModemLink dest=eth_WAN
:ip ifconfig intf=ModemLink group=lan ipv6=disabled
:ip ifattach intf=ModemLink

and add an IP address on that interface. In this case the external modem is on 192.168.0.1/24:

:ip ipadd intf=ModemLink addr=192.168.0.2/24 addroute=enabled

Other pages regarding this router:

<ncl style=bullet maxdepth=5 headings=bullet headstart=2 showcats=1 showarts=1 showfirst=1>Category:Router TG582N</ncl>