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2091 posts

Uber Geek
+1 received by user: 848

Topic # 95610 9-Jan-2012 13:57
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Hi All,
I've got a Cisco 857 in use at home - currently doing NAT and working fine.

I am keen to try out a half-bridge setup to my firewall box to muck around. I understand that it's a waste of an awesome device etc.. just want to have a play.

I have done a bunch of reading and I can't find a definitive guide. I understand I need to use half bridge (with the 857 handling the PPPoA auth), but I can't find an example config (unless you count examples using isdn/serial interfaces/pppoe etc)

Do you just need to bridge Dialer0 and vlan1?
How does the firewall get the IP passed on? DHCP?

Keen for anyone with experience to post their experiences and a possible config.

(Just to be clear I'm fine with networking concepts and using the cli, just need some help with this particular situation).


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449 posts

Ultimate Geek
+1 received by user: 125

  Reply # 566605 9-Jan-2012 19:17
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Be aware that in Cisco world half bridging refers to bridging ethernet frames and nothing like what it is commonly understood to mean here on Geekzone.  "ppp bridge ip" will not get you anywhere.

That said, I expect you probably can make this work as Cisco make a pretty flexible routing platform.  The only caveat I'd say is you'll likely need a static IP.

Off the top of my head I'd try the following

1. Set your dialler interface to be unnumbered.
2. Add a static default via the dialler interface
3. Create a small subnet around your static IP.  You might get away with a /31 eventually but start out with a /24 to keep things simple.
4. Pick another IP in that subnet and assign it to the ethernet interface of your 857
5. On the firewall configure the static IP as the external interface IP and the IP of the router as your default gateway.

2091 posts

Uber Geek
+1 received by user: 848

  Reply # 566620 9-Jan-2012 19:55
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thanks now those documents make a lot more sense - doesn't really help the situation though!

1984 posts

Uber Geek
+1 received by user: 133


  Reply # 566674 9-Jan-2012 22:03
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Try setting it up as a full bridge, then you can enter PPPoE details in your firewall and hope the Cisco converts that to PPPoA. Full bridge means your firewall always knows what its public IP is if its dynamic because doesnt have to wait for DHCP lease to expire. I have an ancient modem setup for bridge and been far more reliable than its original use as NAT router.

Qualified in business, certified in fibre, stuck in copper, have to keep going  ^_^

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