docinthebox
join:2003-01-25
Laurel, MD
| SpeedStream 4100: "Bridged mode" vs "PPP on the computer"
I've read past threads on this issue but I'm still not clear. What's the difference between the two? I know if you set "PPP on the modem", then the modem acts as a router, with firewall, NAT and everything. If you set "Bridged mode", then the modem acts as a dumb modem, and your DSL router (WRT54G for example) does the PPPoE authentication, firewall etc. But what about "PPP on the computer"? If you set this mode, is the modem acting as a router or as a bridge? |
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59126125
Premium
join:2006-01-21
clubs:
| With PPP on the computer you could actually have the 4100 work in either mode, as a router or as a bridge. You just need something to do the authentication, whether it is the computer, a router, or the modem. However, it's advisable to only have one device do the login.
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There is a reason the wires are twisted together, it's called a pair. It defeats the whole purpose of twisted pair cabling by using the solid orange and solid green to wire the jack. |
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docinthebox
join:2003-01-25
Laurel, MD | So if I set the 4100 to "PPP on the computer" under "PPP location" in the 4100 menu, and have my router do the authentication, would the 4100 be acting as a router or as a bridge in this case? |
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59126125
Premium
join:2006-01-21
clubs: | As a router, since it would still be doing NAT until it is fully bridged. |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL
 ·AT&T U-Verse
 ·AT&T Midwest
| reply to docinthebox
If you set the modem for PPP on the computer, it is basically set as a bridge.
I'm not sure, but possibly with "PPP on the computer" it might still give out a DHCP address of 192.168.1.64, if requested, and that could be used to direct connect to the modem for reading the statistics. Note that I am really guessing here - I haven't actually tested that.
If modem is set to "PPP on computer", and is connected to a router that does PPPoE, then you are using the modem as a bridge. |
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d_l
Barsoom
Premium,MVM
join:2002-12-08
Reno, NV
1 edit | Your guess was correct. With "PPP on the computer", the modem will give out its only DHCP address. I use it this way all the time and the modem is definitely acting as a bridged modem this way. |
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docinthebox
join:2003-01-25
Laurel, MD
| reply to nwrickert
So then what's the difference between the "PPP on the computer" mode and the "Bridged mode"? In the Bridged mode, I suppose all the packets just pass through the modem from the WAN to the router and vice versa, right? Is this also the case in the "PPP on the computer" mode? |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL | Basically it is a dumb bridge in "bridge mode", and a smart bridge in "PPP on computer" mode. The main difference is that it is easier to get to the modem statistics page in "PPP on computer" mode. |
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docinthebox
join:2003-01-25
Laurel, MD | So there shouldn't be any speed difference between the Bridge Mode and the "PPP on computer" mode, right? |
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59126125
Premium
join:2006-01-21
clubs:
1 edit | reply to docinthebox
Well, according to this »kb.efficient.com/article.asp?art···82&p=351
The Router supports two fundamental modes of operation with respect to connectivity between the Local Area Network (LAN) and the Wide Area Network (WAN): bridge/routing mode and bridge mode.
The default mode of operation is bridge/routing mode. With bridge/routing mode, the Router provides typical routing functionality between the WAN side and the LAN side. However, all LAN-side interfaces are "bridged."
The second mode of operation provides only "bridging" functionality. This applies to both WAN-to-LAN connectivity as well as to all LAN-side interfaces. Point-to-Point (PPP) connections are not available under the bridge mode of operation.
Important! If you switch to Bridge mode, you will lose access to the Web management interface and can only return to Router mode by resetting the Router to factory defaults.
So my understanding is that the 4100 is technically acting as a router regardless of what is doing the login since it is still doing NAT, until it is put in bridge mode, at which point it is a true bridge and not a router.
--
There is a reason the wires are twisted together, it's called a pair. It defeats the whole purpose of twisted pair cabling by using the solid orange and solid green to wire the jack. |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL
·AT&T U-Verse
·AT&T Midwest
| reply to docinthebox
Right, no speed difference. Unless you are trying to get statistics, you should not notice any difference.
As far as I know, the "PPP on computer" is the recommended setting if you are using a separate router and want that router to handle the PPPoE. |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL
·AT&T U-Verse
·AT&T Midwest
| reply to 59126125
Note that your link is the documentation for the full retail version of the 4100, not for the SBC version with its more restrictive firmware. |
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59126125
Premium
join:2006-01-21
clubs: | Sorry about that, I should have included that in my post. Though it still describes the functionality of the modem. |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL | In my quick browse through that documentation, I didn't find the "PPPoE on the computer" option. So maybe that's an option only for the SBC version. The SBC version of the 4100 is not doing NAT when set for PPPoE on the computer. |
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docinthebox
join:2003-01-25
Laurel, MD | Thanks for all your help! |
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59126125
Premium
join:2006-01-21
clubs:
| reply to nwrickert
Ok, now that has me confused. How can a device be handing out a private IP and yet not be doing NAT? If that's the case, that would mean the 4100 is always a bridge since it only hands out one private IP? My understanding is that if a device is doing NAT, it is a router, even if it is a very simple one and only hands out one private IP. Also, my understanding is that a true bridge only translates different protocols, like ATM to Ethernet, without doing NAT. The PPP login's sole purpose is authenticating the user on the network so a WAN IP gets assigned to the circuit to access the Internet. If this is incorrect, someone please let me know.
--
There is a reason the wires are twisted together, it's called a pair. It defeats the whole purpose of twisted pair cabling by using the solid orange and solid green to wire the jack. |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL
·AT&T U-Verse
·AT&T Midwest
| How can a device be handing out a private IP and yet not be doing NAT? It is only doing NAT if it is also mapping that private IP to a different (usually public) IP. If it is merely giving out a private IP for communication on the LAN, and is handling the DSL traffic purely as an ethernet bridge would, then it is not acting as a router and is not doing NAT.
Also, my understanding is that a true bridge only translates different protocols, like ATM to Ethernet, without doing NAT. Right. But an ethernet can sustain multiple traffic. There is no reason that you could not use LAN tcp-ip communication on the local ethernet at the same time that the bridged ethernet packets are being transferred between the local ethernet and the remote ATM network. |
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59126125
Premium
join:2006-01-21
clubs:
| said by nwrickert  :How can a device be handing out a private IP and yet not be doing NAT? It is only doing NAT if it is also mapping that private IP to a different (usually public) IP. If it is merely giving out a private IP for communication on the LAN, and is handling the DSL traffic purely as an ethernet bridge would, then it is not acting as a router and is not doing NAT. Well, that's what has me confused the most. Wouldn't the 4100 still be acting as a router since it is connecting two different networks? For an example of why I believe it would still be doing NAT, lets say the WAN side is assigned an IP of 69.xxx.xxx.xxx while the LAN side is 192.xxx.xxx.xxx, wouldn't there have to be a translation from address to address / network to network and some type of routing table used? I always thought that a bridge could only forward packets to devices assigned on the same network / IP range.
--
There is a reason the wires are twisted together, it's called a pair. It defeats the whole purpose of twisted pair cabling by using the solid orange and solid green to wire the jack. |
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docinthebox
join:2003-01-25
Laurel, MD | I thought the 4100 is forwarding packets from the WAN side (69.xxx.xxx.xxx) to the WAN port of the router which is still 69.xxx.xxx.xxx. It is the router that's connecting 69.xxx.xxx.xxx with 192.xxx.xxx.xxx |
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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL
·AT&T U-Verse
·AT&T Midwest
| reply to 59126125
Let's remember that the 4100 has several modes of operation. In some of those modes, it is acting as a router.
We were, in particular, discussing the "PPP on computer" mode. There, it is simply taking the ethernet frames it receives encapsulated in the ATM data, and placing them on the local ethernet. It is not examining the content of those frames at all. It is not seeing the IP address encoded in the PPPoE data that is encapsulated in the ethernet frames.
Quite separately from its action as above, it also gives out a single DHCP address purely for communication on the LAN.
For LAN communication, it will receive ethernet frames addressed to the 4100 MAC address. For PPPoE communication it will handle frames addressed to the ISP equipment at the other side of the bridge (which tunnels through the ATM network). There is no difficulty distinguishing between these two classes of frames. |
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