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Is it possible to bridge a data and voice network with a Cisco 871W-G-A-K9 cable modem/router?

Posted on 2006-07-12
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Last Modified: 2010-04-12
I want to bridge a data network of 192.168.1.x with a VoIP network of 192.168.2.x.  I do not have a server that supports dual NICs to do the bridging through Windows and buying a Layer-3 router is out of the question right now.  However, I do have a Watchguard Firebox x500 and this Cisco 871W-G-A-K9 Cable Modem/Router (funny, we use a T1 here and there is this cable modem laying around)...  I have not used bridging before and I realize this could top off a dumb question list, but, I am hoping someone could help me out here.

Thanks
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Question by:draracle
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9 Comments
 
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grblades earned 500 total points
ID: 17097485
You cant bridge between two different logical networks.
You could use the firebox to route between the two networks. You just need to setup the firewall to permit all traffic in both direction and disable NAT.
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by:draracle
ID: 17098784
I use NAT for various reasons on the Firebox.  Could I use a Windows XP machine with dual NICs and point the two together (forming a bridge)?  This is the same location with Layer-2 switches setup with VLAN 1 and VLAN 2.  I can get communication where it needs to go but I would like the bridge so that I can connect to the VoIP subnet (192.168.2.x) from the data (192.168.1.x).  For example, I might want to tap into the PBX or a switch to change some settings and I won't be able to get to the other subnet from my data without a bridge.  I believe routing is the way to go down the road but our network, servers, and other things are not at the warm-fuzzy-feeling level so we will have to fix some things before that.  

Thanks for your interest on this one.
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by:grblades
ID: 17099024
You cant bridge unless both networks use the same IP address range.

If you just wish to use it for administration purposes then the easiest way might be to just get a computer with a couple of network cards and connect it to both networks. Dont bother configuring routing.
Then install VNC (http://www.tightvnc.com/) on the server to you can make a remote connection to the PC from your computer and then once on there can use it to perform any reconfiguration you require.
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Author Comment

by:draracle
ID: 17099234
I plan on changing the subnet from 255.255.255.0 to 255.255.248.0 so that both networks are running on the same subnet.  The VoIP range will be assigned a scope of 192.168.2.50 to 192.168.2.200 (my data already has a scope of 192.168.1.10 to 192.168.1.255 (but I can increase once the subnet mask gets changed).  Perhaps, I am going at this the wrong way because you are saying I cannot bridge unless both networks use the same IP address range, but what if they fall under the same subnet?
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Expert Comment

by:grblades
ID: 17099319
Yes if you make that change you can bridge but why do you want to bridge anyway?
If you are going to bridge you might aswell just connect the networks together directly. The only porpose of bridging is to connect different types of network together.
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Author Comment

by:draracle
ID: 17099654
I have two buildings, one which has the network setup correctly and efficiently which allows VoIP to run smoothly on it, and then there is this other one that has a variety of dasiy-chained switches and I haven't had the time to rebuild the network for this building.  So, new building has a PBX (ip campus), PoE switch, and data switches that connect, via fiber, old building which has main PBX, servers, data switches, about a million various switches (some probably hidden in drop-ceilings even), firebox, T1 modems, etc.  The idea is to have the building appear as one, which is easy to do with data, but now I will be mixing digital, analog, and IP phones.  IP phones are in the new building and not the old one.

New Building Switch        Old Buidling Switch
[settings]                       [settings]                               [VLAN0 DATA]->(can see)->\
[VLAN0 DATA]->[fiber]->[VLAN0 DATA]                                                                 |
[VLAN1 VOIP] ->[fiber]->[VLAN1 VOIP]  -> [PBX] <(the)---------------------------------/

If my data connection is bridged with the 192.168.2.x range, could this work?  I think the VoIP network will need to see the data gateway in order to have outside VoIP capabilities via NAT IP pointers.

Thanks,
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LVL 36

Expert Comment

by:grblades
ID: 17099778
All pridging will do is effectivly the same as plugging one end of a network cable into the data VLAN and the other end into the VOIP VLAN. Getting a separate computer do do this is therefore pointless.

The best thing you can do is keep the two networks separated and get a router to route between them.
In the old building everything will connect to the data network and if it needs to communicate with VoIP the traffic will go via the router. You may have minor issues with this due to lack of QOS.

Then as you sort out the networking in the old office you can upgrade the switches to VLAN capable ones so phones and computers are directly connected to the correct VLAN.

As far as getting access to the PBX from the internet this can be problematic and SIP does not work well through NAT devices. You would be best off getting a 2nd network interface on the PBX and connecting it directly to the internet.
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