Help - route to ADSL modem in bridge mode

RouterOS Beginner Basics
1

Hi - not sure why this won’t work. I have a RB750G as a pppoe client behind an ADSL modem in layer 2 bridge mode. Even in this mode I can still assign an ip to the Ethernet interface to view ADSL stats.

Internet-----[ADSL MODEM 1.1.1.1/24]----------(RB750G Gateway pppoe 192.168.10.0/24)

I have a route to 1.1.1.1 on the RB750G but I still can’t get to 1.1.1.1 from my 192.168.10.0 network. Do I need a route on the ADSL modem? I don’t know if I can even do that!

Any help appreciated.

2

If the ADSL modem and your WAN port on the 750 both have 1.1.1.1/24 assigned to them, then that’s an IP conflict and you won’t be able to access the modem. Give it a different IP address within the same subnet, and the 750 should handle the routing for you.

3

OK - no. I am not that much of a noob !!

Maybe I should draw the diagram better:

ADSL Modem [1.1.1.1/24] → [1.1.1.2/24] ** RB750G ** [192.168.10.1/24 ]–>Internal Network 192.168.10.0/24

So I have 1.1.1.2/24 assigned to ether1-gateway on the RB750G and 1.1.1.1/24 assigned to the ethernet port on the ADSL modem. Still doesn’t work.

4

I have also just done another test; I got a laptop and assigned the ethernet interface 1.1.1.2/24 with no default gateway etc. unplugged the ADSL modem from the RB750G and plugged the laptop direct to the ADSL modem. I could then use a web browser on the laptop to get to 1.1.1.1

Now when I plug it all back in I do a traceroute:

With no 1.1.1.2 address assigned to the RB750G I see that the trace gets routed out the ppoe interface (which I would expect):

First half of this screenshot:

Second half shows what happens when I assign 1.1.1.2/24 to the ether-1 interface on the RB750G. You can see the trace stops at 192.168.10.1.

Am I missing a route here somewhere ? Should I be NATTing the traffic back ? Is this because the modem does not know where to route the source packet 192.168.10.0/24 to ??

5

I hope this helps anyone else trying to do this !

So I set up a log and noticed the traffic was not being NAT’d:

The source IP was still 192.168.10.10

So I set up a simple NAT rule in the ether-1 interface to NAT outgoing traffic to 1.1.1.1 on the ether-1-gateway interface:

The new NAT rule is the first one in the list.

I think i could have also fixed this with a static route on the ADSL modem, but there was no where to put one in. This seems to work just fine !

6

Yes, sorry I wasn’t thinking right and was in a bit of a rush. I was thinking your ADSL modem was using the 750 as it’s default gateway and therefore had a route back to you. Yes setting up a NAT rule was what you wanted to do so you would appear to be coming off of the local subnet of the modem.