"Roaming" 3G router w/ EIGRP

Unanswered Question
Dec 19th, 2008

Hi all,

I'm experimenting with backup solution for my customers that entails a 3G router.

We have multiple customers that are in a relatively small geographic area, we have purchased an 1841-3G bundle which includes an 1841 router with a 3G VWIC.

If we know a customer's T1 or private DSL (we're a DSL ISP too) is going to be down for more than a X hours or overnight until a tech can get onsite, a field tech would take the 3G router to the customer's site, and use it to create a VPN across the Cell connection back to our network for Email etc. In each case, this is a short-term solution.

I would envision being able to have some sort of generic configuration on the router so the field tech can just plug it in and it will work. In theory i would not have to do any site-specific configuration.

Our sites are all configured with some standardness:

All LAN IPs are: 10.220.X.X or 10.20.X.X

All Customer LAN routers are 2800s, 2600XMs

All Customer LAN routers run EIGRP

All Customer LAN routers are 10.x.x.1

The sites are setup with 1 broadcast domain in most cases. Flat architecture. Most sites do not have more than 30-40 users.

Gotcha: DHCP is hosted centrally, not at each site so we can't use DHCP to assign an IP to the router.

I was thinking we could plug in the 3G router, <do somthing with the IP>, and have it advertise a default route to the main router via EIGRP. The advertised route would have a high AD so the LAN router would fail back to its primary route once the connection is restored. Seems pretty feasible except for the LAN side IP of the 3G router.

Does anyone have any ideas how I can get an IP onto the LAN interface of the 3G router?

Could I use IP Un-numbered and just use the IP assigned to the 3G card and still have EIGRP work properly between the LAN router and the 3G router? I'm guessing yes, because they're on the same LAN...


Not scalable at all... but i could assing an IP from EVERY customer subnet as secondary IPs on the LAN router... what a headache. Not a good idea, i'm just throwing stuff out there at this point..

I'm still waiting for the hardware to arrive so i can play with it a bit, but I was just wondering if anyone had any ideas?

I have this problem too.
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Giuseppe Larosa Fri, 12/19/2008 - 13:00

Hello Ryan,

if the router is only one you can:

provision the same IP subnet on all customer sites.

This ip subnet is only used for buiding the EIGRP adjacency with the main router that will pass the site specific IP subnets.

the main routers need to filter this subnet on their updates on the primary link

The only router that can advertise it will be the 3G router.

In this way you can prepare one lan switch port on each site to connect the 3G router that will advertise the current site routes to the backbone via cellular network.

access-list 11 deny 10.z.z.0

access-list 11 permit any

main router on each site

router eigrp ASN

distribute-list 11 out


on net 10.z.z.0 the EIGRP neighborship is built with the 3G router

Hope to help



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