Router with wireless fallback

Answered Question
Nov 4th, 2008
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This is going to be a tricky one!


So we're deploying c881 with GSM to our stores and the idea is that if the DSL wan link goes down the GSM radio interface will kick in.


The first obvious problem is that a router can only have one default gateway right? The second is the implementation of VRF and perhaps SAA. I've heard that VRF doesn't work well with NAT. I can see myself spending a lot of time on the phone with TAC so. Does anyone in here have any experience with a similar setup?


Thanks in advance.

Correct Answer by tcordier about 8 years 5 months ago

A router can have several default gateways. In your situation, just as an example, you could define a static route pointing to the wireless interfaces


ip route 0.0.0.0 0.0.0.0 250


This default route has an administrative distance (AD) of 250 which makes it less preferred than routes (default and other) from other routing protocols (which all have an AD of 200 or less).


You would configure the 3G card to be used as dial-backup should the DSL fail. Typical dial-backup configurations will bring the 3G connection up, once the primary connection fails, and route traffic down the 3G card using a "backup" static route as mentioned above (called a "floating static"), or a dynamic routing protocol over the 3G link to the other end of the dial-up connection. See e.g. http://www.cisco.com/en/US/docs/routers/access/1800/1861/software/feature/guide/3ghwic.html#wp1237893


for some configuration examples.


I am not sure about the NAT and the VRF. Where do you use a VRF, and where NAT?

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Correct Answer
tcordier Wed, 11/05/2008 - 00:01
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A router can have several default gateways. In your situation, just as an example, you could define a static route pointing to the wireless interfaces


ip route 0.0.0.0 0.0.0.0 250


This default route has an administrative distance (AD) of 250 which makes it less preferred than routes (default and other) from other routing protocols (which all have an AD of 200 or less).


You would configure the 3G card to be used as dial-backup should the DSL fail. Typical dial-backup configurations will bring the 3G connection up, once the primary connection fails, and route traffic down the 3G card using a "backup" static route as mentioned above (called a "floating static"), or a dynamic routing protocol over the 3G link to the other end of the dial-up connection. See e.g. http://www.cisco.com/en/US/docs/routers/access/1800/1861/software/feature/guide/3ghwic.html#wp1237893


for some configuration examples.


I am not sure about the NAT and the VRF. Where do you use a VRF, and where NAT?

I have experience with this design and found that BGP was the way to go. Basically, the remote office is a bgp neighbor with corporate's Edge router. In many cases the primary link is MPLS or VPN, doesnt really matter. On the remote side you can use a combination of "Weight" making one neighbor / path preferred and prepending which will take care of the hub side in our case corporate, for return route determination. At one point we had EIGRP / BGP redistribution going on which really caused a problem for the Hub side often not reusing the primary link after the link returned and was stable but we used a conditional advertisement (Advertise-map)which only advertised to the backup edge router when the primary went down and removed the advertisment when the primary came up. This way only one route was being offered so no problems with BGP / EIGRP metrics.


Now, with this scenario, the GSM is always active but not sending any data. I dont have any experience with Demand Dialing with the design.

hope this helped

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