ip unnumbered

Hi folks,

Im a newbie to cisco hardware and configuration.

I was wondering if there was anyway i could do an ip unnumbered sort of set up for the following.

I am being presented with a leased line via ethernet, i wil terminate this on fa0/0, i want fa0/1 to be the LAN interface, however i need that to have the same ip as fa0/0, same sort of idea with a leased line provided via x21. Int s0/0 ip unnumbered fastethernet 0/1. I know i ban't do an ip unnumberered on a non point to point multi access interface.

Anyone any ideas? Maybe a loopback with some policy routing?

Im totally stumped, any pointers would be apprecited guys, thanks.


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It can be a bit confusung (I found) so I have guessed at a config - not typed it in to verify.

bridge irb bridge 1 protocol ieee bridge 1 route ip

! type in again - they are order sensitive and this fixes it. bridge irb bridge 1 protocol ieee bridge 1 route ip ! check you have no errors reported

int bvi 1 ip address whatever.you.want mask...

int fa 0/0 no ip address bridge group 1

int fa 0/1 no ip address bridge group 1

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Thanks i will give it a try, so routing wise i can point a defaul route to int fa0/0 etc?

Your a diamond for replying :)

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When i use the above config i cant send a default route to e0/0

int e0/0 no ip address bridge-group 1

int e0/1 no ip address bridge-group 1

int bvi1 ip address

Anyone any ideas?

Thanks for any input


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You will need to make the default gateway of your internal devices the address of the ISP's router. They will tell you what it is.

Perhaps you should really be doing NAT on this box. I did not think beyond your detailed request.

If you explain in detail what you are trying to do and what you have then perhaps we can assist.

You can mangle the iP addresses that you have if you like but leave any octets not covered with a 255 mask alone.


change it to something like

and keep changes consistent.

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Hi Bod thanks for your reply,

I have a leased line from site A to site B, site A address of router port terminating leased line is, the site B address is lets say, this used to run via ip unnumbered on the serieal interfaces on both routers, however the ISP have provided Fastethernet presentation, i cannot do ip unnumbered on these interfaces. So at site A i need fa 0/0 to be /24 however fa 0/1 must be in this range also as it hops traffic onto the /24 lan towards a firewall at

I want traffic destined for the /24 net to exit fa 0/0 interface and all other ( default ) traffic to exit the fa 0/1 interface towards the firewall. I need both my interfaces using the address for lan and wan access just like when using ip unnumbered on serial interface. Does this help?


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Forget the bridging.

Just use a new network (or subnet) for the new link. Anything that is not is use in your network.

e.g. for one end for the other

You will need to change the routes too.

ip route xxxxx xxxxx next.hop.address

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