docinthebox
join:2003-01-25
Laurel, MD
| reply to nwrickert
Re: SpeedStream 4100: "Bridged mode" vs "PPP on the computer"
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.
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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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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.
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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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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.
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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 | 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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59126125
Premium
join:2006-01-21
clubs:
| reply to docinthebox
My understanding is when the 4100 is fully bridged the router is then assigned a WAN IP (69.xxx.xxx.xxx), but if the 4100 is configured to "PPP on the computer" it still hands out a LAN IP (192.xxx.xxx.xxx) to the router.
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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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nwrickert
sand groper
Premium,MVM
join:2004-09-04
Geneva, IL | If the 4100 is fully bridged, it is talking ethernet, not TCP/IP. It doesn't have a WAN IP address. The WAN IP address is assigned over PPPoE, so is assigned to the system that actually does the PPPoE. |
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59126125
Premium
join:2006-01-21
clubs:
| Ok, still confused. Here is my understanding on how the three different configurations for the 4100 work and would appreciate any corrections.
1. PPP on the modem - modem authenticates, is first IP device in contact with the WAN network and is assigned the WAN IP from the AT&T Redback router. 4100 hands out private IP to first device behind it. 4100 acting as a router.
2. PPP on the computer - any device behind the 4100 can do the authentication, yet the 4100 is still the first IP device in contact with the WAN and still gets assigned the WAN IP from the AT&T Redback router. 4100 hands out private IP to first device behind it. 4100 acting as a router.
3. Bridged Mode - 4100 ceases to exist as far as IP goes and merely translates protocols and forwards packets. The router is now assigned the WAN IP from the AT&T Redback router since it is the first IP device in contact with the WAN. The router is now on the same network as the Redback Router. 4100 acting as a bridge.
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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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wayjac
Premium,MVM
join:2001-12-22
Indy
·AT&T Midwest
| The PPPoE client reguardless of where it is gets the wan IP info
PPP on the modem and PPP on the computer have a lot in common, the modem's lan DHCP server is active, with the correct configuration another device can do the PPPoE.
In Bridged mode the modem's lan DHCP server is disabled.
I agree fully with statement 1 and 3 statement 2 should read just like statement 3 with a few "buts"
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God bless our troops |
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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
In mode 2 (from your list), the 4100 is not an IP device on the WAN. It has no WAN IP, and it does not know what the public IP should be. It merely accepts ethernet frames from the redback, and bridges them to the local ethernet. It likewise picks up ethernet frames on the local ethernet, and bridges those to the redback.
In addition, it assigns itself an IP address of 192.168.0.1 on the local ethernet, and can exchange IP packets with other systems on the local ethernet. It does not exchange IP packets with WAN hosts - it only bridges ethernet frames but does not look inside those frames. It does give out a DHCP address of 192.168.1.64 to one client on the local ethernet. This would normally be used only for getting the modem statistics.
A local host on the ethernet can do the authentication. But then that same local host has to use the assigned WAN address, and has to extract IP packets from the PPPoE transmissions that it receives. It can't just authenticate - it has to do all of the PPPoE. If that local host wants to act as a router, it can. But then it needs a second ethernet interface.
Mode 3 (bridged mode) differs in that it does not do DHCP. I think (but I'm not sure) that it will still accept packets to IP=192.168.0.1 for requesting statistics. But the computer that wants to see the statistics would need to be manually configured with IP=192.168.1.64, netmask=255.255.0.0.
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Let me describe my WAP (Wireless Access Point). The WAP is, in effect, a bridge between the WiFi LAN and the wired LAN. It acts on ethernet frames, and transfers them between the wired LAN and the wireless LAN. It does not do any routing, so both wired and wirless LANs have to share the same range of IP addresses. The WAP happens to also have an IP address. It is a smart bridge, and uses its own IP address so that you can configure it. But, except when you are configuring the WAP (or getting info on settings from the WAP), it is acting only as a bridge.
The 4100 in "PPP on computer" mode is likewise a smart bridge. It is mostly doing bridging, but it has its own IP address on the LAN so that it can be configured and so that you can get info from it. It happens to also give out a DHCP address to make that configuration a little easier to do. |
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59126125
Premium
join:2006-01-21
clubs:
| Ah, I think it is getting clearer now. So basically, whatever device does the authentication gets assigned the IP from the Redback. So, when the 4100 is set to "PPP on the computer", the modem has to "bridge" straight to a LAN IP for the connection to work to the MAC address. Yet, it keeps a LAN IP so it can be accessed via regular ethernet. Do I finally have it right?
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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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