implementing a 56k backup link for a wireless ethernet bridge

[pblower]

Is there a way to configure my routers to use a 56k serial link as a backup to my wireless ethernet bridge?

My problem is that when the radio link fails, the interface on the routers don't actually go down.

My remote end was forwarding on Eth0, blocking on S0. When the radio link fails, S0 does go into forwarding mode, but since the Eth0 interface hasn't gone down and it's still in forwarding mode and has a better weight, it still forwards out the Eth0 interface rather than using the serial interface.

On the root end, both the ethernet and serial interface are in forwarding. So I down the ethernet interface so only the serial is available for use, but it still doesn't work.

Of course, if I could shutdown Eth0 on the remote router, then the serial interface is the only forwarding interface on both routers and it would work. But since the link is down I don't have access to the remote router...

Just for fun, in my test environment, I changed the bandwidth on all interfaces so the Serial link was used as the primary link and the ethernet link was the backup. In this case, pulling out the serial cable caused the serial interfaces on both sides of the link to go down and the ethernet link went from blocking to forwarding and everything worked fine.

Any suggestions as to a workable solution?

[rokibbe]

You'd need some mechanism that would check to see if the packets were actually making it across the ethernet link. A dynamic routing protocol and a floating static route might do the trick, although I have personally never tested such a configuration with wireless.

For a place to start, consider a situation similar to that posed in TAC Sample Config "Using Floating Static Routes and Dial-On-Demand Routing" at http://www.cisco.com/warp/public/123/5.html and substitute the wireless bridges in the equation for the WAN link.

[pblower]

I managed to get it to work using irb. I created a Bridge Virtual Interface (BVI) on each router and included both the serial and ethernet interfaces in the bridge-group.

I presume that this works since now it is the BVI that detects whether or not packets are arriving from one interface or the other. My testing has shown that the serial link does get used when the Ethernet link breaks.

Phil.

[ajarina]

How did you go about it?

Please take alook if this will work on my end:

Branch 1:

s0: 192.168.1.0/24

e0: 192.168.0.0/25

e0: 192.168.2.0/24 (secondary address for aironet)

Branch 2:

s0: 192.168.1.0/24

e0: 192.168.0.0/25

e0: 192.168.2.0/24 (secondary address for aironet)

and I configrured to have this static routes on both routers:

ip route 192.168.0.0 255.255.255.128 192.168.2.1 100

ip route 192.168.0.0 255.255.255.128 192.168.1.1 200

Using IRB can this be done by, using IRB with another network number like 192.168.3.1 and include all interfaces to the same bridge-group number? Thnks in advance for the help.

[rcrisostomo]

we currently have the same problem with our serial link as a backup to our wireless link... if you will be kind enough to send details/sample config on how you were able to resolved the issue using irb...

thanking you in advance,

[arodko]

I was working on the same exact problem and found that gre tunneling works perfectly. If you setup tunneling over the wireless link, the tunnel interface will go down when the wireless connection goes down. You then setup the serial interface as a backup interface for the tunnel interface. Just make sure the tunnel does not re-establish itself over the serial link or else the interfaces will flap.

[t.theodoropoulos]

Try using a dynamic routing protocol for the bridge-link (such as rip, igrp, eigrp) runing exclusively over the ethernet interface (the serial interface must be in passive mode). Then configure static routes for the serial interface giving them bigger administrative distance than the one that the routing protocol has.

when the ethernet goes down, due to dynamic protocol failure the dynamic routes will be disscarted from the routing table and all the traffic will pass through the serial interface

cheers

thanos

[ajarina]

In my place, we have 2 offices connected over 64kbps using 2 2610. Each one is using different subnets. I want to connect them using the new 350 bridges. I want to create another subnet for the WLAN. I tried to add a secondary address on the ethernet port but it cant even ping the bridge? Im stuck there...

If i can get the secondary ethernet address to ping the bridge and even ping to the secondary ethernet address of the next router, then i should be able to have a floating static addrress for both routers for redundancy. I hope im on the right track. Anybody care to help?

[fcapra]

I had a similar request for a backup and used the dialer watch to trigger the backup interface to come up. Check out

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/dial_c/dcdbakdw.htm

This method watches a specific route in the routing table instead of interesting traffice or interface status. If the route drops out of the table, the router assumes the primary link is down and initiates the dialup.