ISDN to backup Microwave connection

Unanswered Question
Sep 11th, 2007

Is it possible to use ISDN BRI connection to backup a Microwave connection (using Ethernet port)?

I have 2 Cisco routers connected using Motorola Canopy microwave link through the Ethernet ports of the routers. The microwave link is working fine.

On both routers, I have ISDN BRI port. Is it possible to use the ISDN connection as backup when the microwave link is down?


I have this problem too.
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Richard Burts Tue, 09/11/2007 - 03:01


We would need to know a bit more about your environment and about your requirements to give a really accurate answer (are you using a dynamic routing protocol or static routes, what kind of routers, what addressing plan, how do you determine that the Microwave has failed and you need the backup, etc). But in general I would say that yes you can use BRI to back up the Microwave connection.



pllim Tue, 09/11/2007 - 17:53


I'm using static route for my Cisco 3660 and 3640 routers. Here's the addressing plan.

Cisco 3660 Cisco 3640

Eth0 Eth0

Eth1 Eth1


Cisco 3660 IP Route BRI3/0 150

Cisco 3640 IP Route 150

The Microwave device is connected to the Eth1 port at each router. Because the Eth1 Line Protocol is always UP, the only way I can determine that the Microwave link has failed is by pinging from 3660 to 3640.

amohabir1 Tue, 09/11/2007 - 03:36

It sounds like DDR - Dial on demand routing using the backup interface feature might work. However traditionaly the way it works is if a circuit is determined to be in a down state then DDR would kick in.

There is also the floating static route that would allow to assign a backup route in the event the primary route gets dropped from the routing table because the interface went down.

These options will only work if the ethernet interface being used does actually go down. I am not sure what happens to the ethernet interface when the microwave connection goes down. My assumption is that the ethernet interface is probably plugged into some sort of converter (that will probably never go down).

You can also probably use dialer watch. It works with routing protocols and there is no need to specify any interesting traffic.

I think this article might be helpful to you!!

pllim Tue, 09/11/2007 - 18:09

You're right amohabir1.

The Ethernet interface (Line Protocol) stays UP all the time even when the microwave link has failed. There is a converter between the Ethernet and Microwave device.

pllim Tue, 09/11/2007 - 20:01

I've look through the DDR backup methods you suggested. Don't think there is any suitable.

Backup Interface - Not possible.

The primary interface line protocol needs to be detected as down first.

Floating Static Route - Not possible.

When the primary route is a static route (which I am using now), the primary interface line protocol must go down for the floating static route to be used.

Dialer Watch - Not possible.

I'm using Static Route.

Object tracking route using rtr and sla - Not supported on my router. Using Cisco IOS 12.3(11)T3.

spremkumar Tue, 09/11/2007 - 20:39


can you create 2 GRE tunnel interfaces on top of microwave and isdn connection ?

But for auto fallback you need to have keepalives enabled on both the ends of the tunnel interfaces.

you can forward/send your traffic via the tunnel interface (route pointing towards tunnel instead of microwave interface).

similar kinda configuration at the remote end too so that you have the required reverse route available.


pllim Wed, 09/12/2007 - 00:52

I am not familiar with the GRE tunnel configuration.

Richard Burts Wed, 09/12/2007 - 02:38


GRE tunnels are not difficult. And the solution proposed by Edwin to use GRE tunnel keepalives is an interesting solution that should work. But before you start with that I have a different alternative to suggest. Your code may not support IP SLA but I believe that it does support Reliable Static Routes with Object Tracking. According to this link it was introduced in 12.3(8)T and your code is slightly newer that that.

The object tracking used here is similar to the approach used in SLA but a bit less complex and a bit less sophisticated. But it should do what you need. It can check for reachability of the next hop and remove the route if the next hop becomes unreachable - even if the interface remains up. This feature was introduced for exactly this problem. I believe that it will provide exactly the functionality that you need.



pllim Fri, 09/14/2007 - 20:07

Thanks for the advise Rick. Will try it out.

But right now, I have another problem to fix first. I have posted it ("Cisco - Microwave link - Cisco" connection drop) in this Forum.

pllim Tue, 09/25/2007 - 03:14

When I tried to use Reliable Static Routes with Object Tracking, my ISDN connection activates for 2 minutes even when my microwave connection is still running. Here is my configuration before any changes: -


IOS...:12.3(11)T3__________IOS 12.2(10d)



(MW - Microwave)

Static Route Router3661:

ip route

ip route Dialer3 150

Static Router Router3640:

ip route

ip route 150(ISDN)

Here are the configurations that I have made to Router3661:-

#conf t

Enter configuration commands, one per line. End with CNTL/Z.

#rtr 1

#type echo protocol ipIcmpEcho

#timeout 3000

#frequency 5

#threshold 5


#rtr schedule 1 life forever start-time now

#track 40 rtr 1 reachability

#show track 40

Track 40

Response Time Reporter 1 reachability

Reachability is Up

1 change, last change 00:03:41

Latest operation return code: Over threshold

Latest RTT (millisecs) 11

#access-list 101 permit icmp any host echo

#route-map BW-ROUTE-MAP permit 10

#match ip address 101

#set interface dialer 3


#ip local policy route-map BW-ROUTE-MAP

Once the command "ip local policy route-map BW-ROUTE-MAP" is entered, I can see that my ISDN connection is activated automatically (see below) even though the microwave connection is still running fine. It stays up for 120 secs before gets disconnected. This process is repeated until I remove the configuration "ip local policy route-map BW-ROUTE-MAP".

#show isdn hist




Call History contains all active calls, and a maximum of 100 inactive calls.

Inactive call data will be retained for a maximum of 15 minutes.


Call Calling Called Remote Seconds Seconds Seconds Charges

Type Number Number Name Used Left Idle Units/Currency


Out ---N/A--- 043892315 BW_ISDN 120 0

Out ---N/A--- 043892315 BW_ISDN 120 0

Out ---N/A--- 043892315 BW_ISDN 120 0

Out ---N/A--- 043892315 BW_ISDN 120 0

Out ---N/A--- 043892315 BW_ISDN 120 0

Out ---N/A--- 043892315 BW_ISDN 120 0


Does anyone know why the ISDN line is activated when my microwave connection is still working?

Richard Burts Tue, 09/25/2007 - 03:35


The reason is that your local policy routing says to send all ICMP echo (ping) packets over the ISDN. And ping is what you are using to track the microwave. I do not understand why you did this. If anything I would expect the policy should send the packets over the microwave since that is what you are attempting to track.



pllim Wed, 09/26/2007 - 03:48


What should I change to send the packets over the microwave instead of ISDN?

#route-map BW-ROUTE-MAP permit 10

#match ip address 101

#set interface dialer 3


#ip local policy route-map BW-ROUTE-MAP

Richard Burts Wed, 09/26/2007 - 06:56


If your are going to do the local policy based routing then I believe that you should change the set interface dialer 3 to set interface ethernet 1. This should send all pings over the microwave assuming that it is available.



pllim Tue, 10/16/2007 - 02:45

Just got back to work after a long holiday.

As per your advised, I changed the interface to ethernet 6/0 (which is connected to my fiber optic).

#route-map BW-ROUTE-MAP permit 10

#match ip address 101

#set interface ethernet 6/0


#ip local policy route-map BW-ROUTE-MAP

This time, my ISDN line works fine. It doesn't get activated when my fiber optic line is still working.

The next thing for me to test, is to see if the ISDN gets activated when the fiber optic line fails (by shutting down the interface ethernet6/0).

The 'ip route' on the 3661 router is changed -

ip route track 40

ip route Dialer3 150

When I shut my Ethernet6/0 interface, I can see that the ISDN is activated but I am no longer able to ping to When I run the 'sh ip route' command, I could see -

S is directly connected, Dialer3

Why can't I ping to anymore after I have shutdown my ethernet6/0 interface?

Richard Burts Tue, 10/16/2007 - 06:09


It looks to me like you have a route to and should be sending ping to them. I would suggest that you check and their end and see how their traffic response tries to get to you. Does it come back over the ISDN or does it still try to use the normal routing over the primary connection?

Remember that when ping fails it might mean that your packets do not get to the destination but it might also mean that responses from the destination do not get back to you.



pllim Fri, 10/19/2007 - 18:36

Before I switch to use the microwave link, my ISDN backup was working fine. I could ping from my 3660 ( router to 3640 ( when using the ISDN line.

The only change made to the 3640 router (to use the microwave) was the ip route -

no ip route (L/L)

ip route (M/W)

(L/L - leased line, M/W - microwave)

No change was made to the ip route for ISDN -

ip route 150

( is a loopback address at 3660 router)

Richard Burts Sat, 10/20/2007 - 09:33


You had to make several changes on your 3660 including object tracking and local policy based routing to get the routes over the mmicrowave to fail and use the routes over the ISDN. Have you made similar changes on the 3640? My guess is that you have not made those changes and that even when the microwave is not working that the static route still points to the microwave on the 3640.



pllim Thu, 10/25/2007 - 02:20

I did not know that I am suppose to make similar changes on the 3640.

I will have problem with if I need to do that because my IOS version is 12.2(10d) on the 3640. It does not support the feature. (I can create an RTR entry but will not be able to track it with my version)

Richard Burts Thu, 10/25/2007 - 07:24


The 3640 is facing the issue that your 3660 faced which is that even if the microwave stops working and does not transmit data the Ethernet interface stays up and the routing still points to the Ethernet/microwave path. The optimum solution is what you have done on the 3660 with object tracking for static routes. If the 3640 is running a version that does not have this feature then there is a problem. An alternative to consider might be to run a dynamic routing protocol. The advantage of a routing protocol would be that it could detect when the microwave path was no longer transmitting data and could converge to use the ISDN. Or you could consider upgrading the version on the 3640 to a version that does have the feature of object tracking.



pllim Fri, 10/26/2007 - 19:46

I don't have the new software version. I'll try to look around if I can find a new version for the 3640.


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