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d3phoenix's profile



9 Messages

Sunday, September 11th, 2011 8:13 PM

How to fake bridged mode with U-Verse

I have worked out a way to simulate full bridged mode between a U-Verse RG and an enterprise grade router.


It's not identical to uverse in bridged mode, but it is pretty close.



  • AT&T U-Verse VDSL service with the Static IP add-on. (I'll be using the 8-ip block in this case)
  • A router capable of running a routing redundancy protocol, such as HSRP, VRRP, or GLBP.
  • A device connected to the internet that is NOT using your u-verse service (For testing functionality from the outside, where it matters. Smart phones work great for this, or just call a friend who has a PC handy)

The problem:

The AT&T RG abuses ARP to ensure that a static IP has to be assigned by MAC address. One MAC = One static IP. This effectively prevents you from routing the whole block to an internal router or firewall, because such devices will respond for all usable addresses using the same MAC. Why the RG chooses to do this is beyond me, but it is what it is and we have to work around it.

(When the RG sees what it thinks is the internal device changing its IP while keeping the same MAC (due to multiple IPs on one interface), the RG will update the device's "current IP address" in the IP Address Allocation tab, and sever all TCP connections to the old address, and the firewall will block all attempts to connect to the old address as it does not see it being registered to any internal device.

Ultimately, this means that if we intend to use our own internal device to protect and control our static IP block, we have to get our device to lie to the RG and present itself as several unique MAC / IP pairs.

This is difficult because by definition, most routers and firewalls only have one MAC and are not capable of generating more. Even high-end Cisco routers do not allow you to add an arbitrary number of MAC addresses to an interface because it shouldn't be necessary -- they assume other devices know how to use ARP.)

The solution:

To solve this, we have to fake out the RG into cooperating. We can accomplish this fakery by doing some protocol abuse of our own -- running a routing redundancy protocol even though we have no peer router to balance with. In this example I'll be using Cisco's HSRP, although in theory this will work with any redundancy protocol that creates virtual IPs and MACs. Routing redundancy protocols use these fake MAC + IP Pairs as floating "virtual interfaces" that would normally flip between two routers running the protocol, so that in the event of a failure none of the clients have to re-learn a MAC and IP. Nobody has to ARP and you get a very quick failover. We're going to use that to fake out the RG.


  • The static IP range AT&T gave us is (.1 through .5 usable, .6 is the RG)
  • We will use .5 as our router's actual address, .1 through .4 will be virtual.
  • When I say "LAN IP range" I am NOT talking about the RG's default LAN range of, I mean the addressing space we choose to use behind our router handling the static IPs.
  • The LAN IP range behind our router is
  • We have two computers on the LAN:
  •         PC1 =, on which we want to expose all services to the world as
  •         PC2 =, on which we want to expose JUST a web server to the world as
  • We want to provide outbound internet access to all hosts in using the router's actual static IP, We will use a NAT overload (or PAT) to accomplish this.


Step 1) Log in to the RG and make the following settings changes:

    * Settings
        * Broadband
            * Link Configuration
                * Supplementary Network
                    * Check Enable
                    * Place the router's address (Last usable of the subnet) into the Router Address field.
                    * Fill in the subnet mask
                    * DO NOT check Auto Firewall Open
                    * Save
        * LAN
            * DHCP
                * Make sure "New Device DHCP Pool" is set to "Private Network"
                * Save
        * Firewall
            * Advanced
                * Enhanced Security
                    * Disable Stealth Mode (Useful for troubleshooting)
                    * Disable Block Ping (Useful for troubleshooting)
                    * Disable Strict UDP Session Control (I have found that this can interfere with VOIP apps)
                * Attack Detection
                    * Disable Excessive Session Detection (The RG's definition of "excessive" is a bit small)
                    * Disable Invalid Source/Destination IP Address (For some reason this seemed to interfere)
                    * Disable Invalid ICMP Detection (Seems to block ALL ICMP??)
                    * Save

Step 2) Unplug all but your configuring PC from the RG.

Step 3) Under Diagnostics -> Resets, Clear the RG's device list.

Step 4) Unplug your PC from the RG and reconnect.

Step 5) The RG is now ready to accept statically assigned addresses from our router.

Step 6) Configure the router. This part will require familiarity with your chosen device. I'll provide the commands as a Cisco IOS configuration file with comments. If your device is not a cisco box, hopefully this will give you enough information to configure it properly.

    ! Cisco IOS Configuration File
    ! Version 12.4

! Set up the inside interface
    interface FastEthernet0/0
        description LAN (TO SWITCH)
        ip address
        ip nat inside
        no shutdown

! Set up the outside interface
    interface FastEthernet1/0
        description WAN (TO RG)
    ! Assign the router the last usable IP in the range (right before the RG)
        ip address
        ip nat outside
        no shutdown
    ! Slow the HSRP timers down (Don't need to check for a non-existent peer every 1 second)
        standby timers 254 255
    ! This router should be the master (It will never come up otherwise)
        standby preempt
    ! Create a virtual HSRP IP+MAC pair for each usable ip address (Except the one already assigned above)
    ! Make sure that your MACs are unique. I like to start them with 0000 to signify a locally-administered address, and end them with 1 + the three digit last octet of the IP it's associated with. This makes it easier to figure out which is which in the RG's configuration later if anything gets messed up.
        standby 1 ip
        standby 1 mac 0000.0000.1001
        standby 2 ip
        standby 2 mac 0000.0000.1002
        standby 3 ip
        standby 3 mac 0000.0000.1003
        standby 4 ip
        standby 4 mac 0000.0000.1004

! Configure our default gateway and default route to be the 3600HGV's address in the static block
    ip default-gateway
    ip route

! Use NAT to expose PC1,, to the internet on all ports as
    ip nat inside source static

! Use NAT to expose JUST the web service on PC2,, as
    ip nat inside source static tcp 80 80

! Create a NAT overload (PAT) to allow listed devices share our router's address for internet access:
    ip nat inside source list ACL-INET interface FastEthernet1/0 overload

! Create the ACL-INET Access List and configure it to allow all PCs to use the overload:
    ip access list standard ACL-INET
        10 permit





4 Messages

13 years ago

Hi d3phoenix.  Ty for the reply and suggestions.  Yes I have a budget of 500.00 for cpe equipment and I will learn whatever it takes to get this right as it can be.  The cisco 887v I mentioned does vdsl2 but att won't let us use anything but the 2wire. That seems like the best router for this type connection but I know many others can suffice.  My op is that uverse biz should be just like a T1 to set up.  It isn't so now I wish others would chime in with what they have done like you did.


Here's my idea goal:.


 ip block =  5th IP >  gateway IP  x.x.x.x  is the public ip that is seen from internet that routes to other 4

                   4th IP >   1st IP segment behind firewall this segment runs the servers and it's serverices/apps

                   3rd IP >   2nd IP segment    "             "                    "                           "                               "

                   2nd IP >  3rd IP segment                   "                                                  "                               "

                   2 Ip's left in reserve for future


To me this is quite standard and sane design.   The other way I can do this is to just hang each domains server box and it's nic mac off each of the lan ports of the 2wire RG  and harden them and use the 2wire for all firewalling.  That seems to be what ATT would have me do by the 2wires design (supplemental network).


I was under the understanding that the last usable IP addy is the gateway IP used on the WAN port of whatever cpe is chosen.  Right now I get a att dhcp assigned IP to me from it's pool and it's outside of my block which I don't want.


On using basic dlink etc routers:  I have 6 meg down/ 1 meg up service from uverse.  I don't see a bottleneck happening until and  If I ever get a higher rate like 12meg up to say 100 meg someday.  I know using a business router is the way to go.  I will buy that now if the solution is right from ebay or new.


Please critque freely...I'n not tied to any specific design right now.     Thanks , Tom










9 Messages

13 years ago

@trw2012 wrote:


I was under the understanding that the last usable IP addy is the gateway IP used on the WAN port of whatever cpe is chosen.  Right now I get a att dhcp assigned IP to me from it's pool and it's outside of my block which I don't want.

Actually, when you have static IP addressing, the AT&T RG functions like a full L3 router for the subnet block that you bought.You will still have that WAN address on the outside, and then the RG provides the highest usable IP of the subnet block you bought as a default gateway for your internal static IP devices to talk to. The problem then is that it restricts it further, such that each static IP must be assigned to a unique MAC address.


Imagine it like this:

  • You have a router with three ports on it -- WAN0, LAN0, and LAN1.
  • You have an internet T1 plugged in to WAN0.
  • WAN0 Pulls the internet address from DHCP from the provider.
  • The router uses address on LAN0.
  • The router provides DHCP for 192.168.1.*/24 on LAN0.
  • The router provides NAT translation between the 192.168.1.*/24 addresses and on the LAN0 Interface.
  • The router uses address on LAN1
  • The router provides L3 routing to the internet for devices on the LAN1 interface.
  • The router CAN provide DHCP to the LAN1 devices, but only if you set up a reservation (assign statics by DHCP) or tell it to give out WAN addresses to ALL new devices (Set the DHCP scope to "public pool").

Now, imagine what happens if you plug both LAN0 and LAN1 into the same L2 segment on a switch, and give the end user access to 4 other ports on said switch.


This is how the RG is behaving. So yes, even with your Static IP's, the RG will still have a WAN address that is outside your static IP range. It doesn't matter though -- because the RG is Routing and not NATing, they will still show up as their rightfully assigned addresses on the internet.


All that said, if you only have 6Mb from U-Verse, then I don't see any problem using several gateway devices to NAT once for each of the three LANs. You can hard-code each NAT device's WAN port to one of your static addresses. Make sure to ping the devices from the RG after you set them up, or else they can take a long time to show up and work properly.


Just keep in mind that even though most consumer grade gear has 100Mb ports, their CPU can't usually keep up with true wire speed. 100Mb port does not necessarily mean 100Mb throughput. At most I would expect to get about 15-30 Mb, depending on the device and feature set enabled. Even Cisco gear will cap out well below interface speed if you run too many CPU intensive operations.


For example, here's a link to a PDF that shows the rated throughput of various Cisco devices. 2600's cap out at around 15Mb, which is why I strongly recommend getting nothing less than a 3640 if you do go Cisco. 2800's are even better:


I hope this answers your question, otherwise feel free to PM me.



644 Messages

13 years ago

This whole thread is above my brain functions. In simple terms why and what for would someone do this...



9.4K Messages

13 years ago

@Kong57 wrote:

This whole thread is above my brain functions. In simple terms why and what for would someone do this...


For business service, it is a de-facto standard that a static IP block you obtain from the provider would come with no restrictions on how it is to be used.  AT&T unfortunately does not provide this functionality with their static IP blocks due to the design of the 2Wire router.  Specifically, it is impossible to properly run a static IP block behind your own router, which a business would likely want to do in order in implement their own firewall or VPN setup.


This thread is discussing a novel way to work-around this restriction using an unrelated functionality that a Cisco router can implement.




4 Messages

13 years ago

I'm beginning to understand the 2wire 3801 firmware design:


"This is how the RG is behaving. So yes, even with your Static IP's, the RG will still have a WAN address that is outside your static IP range. It doesn't matter though -- because the RG is Routing and not NATing, they will still show up as their rightfully assigned addresses on the internet."


It is doing both nat (att IP) and route(att static block) jobs actually.  Your explanation is helpful but I figured that out after it was all setup.  It nat's the att provided IP to my home network on my chosen LAN subnet or 172.x/192.x in the setup page ... which is kinda cool and useful because it gives that network everything it needs (fwall) and the supplemental setup is fairly close to what I would setup if I had a enterprise router.   I hope no problems come from the supp network setup.. i.e.  ... packets aren't  free and clear of interference from the 3801.  That remains to be seen.  I've read of the 3801 interfering with traffic i.e. firewalling when it shouldn't do anything to the packets or only one subnet is allowed to use the full open features.  My config page shows the 3 wide open though. The logs show it is doing some weird stuff  right now to udp traffic.  I might have to monitor things with a packet mon. to be sure like wireshark-tcpdump later. 


Thanks d3p~.






1 Message

12 years ago

Any chance this could be done with a DD-WRT router?



9 Messages

12 years ago

I'm not sure. DDWRT appears to lack the required routing redundancy protocol capabilties that I'm abusing to work around the RG's ARP issues.


You might be able to pull it off if you can assign multiple virtual MAC addresses on the DDWRT's WAN interface, but I haven't used it enough to know if that is possible or not.


It boils down to this: You have to present your WAN interface to the RG as a set of unique MAC address + IP pairs that will respond to the RG's ARPs as such. If you can pull that off (and then NAT from there), it should work.

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