Cisco 1841 ADSL active/active failover

Posted on 2008-11-15
Last Modified: 2012-05-05
Cisco 1841 ADSL active/active failover

Dear colleagues,

I am wondering if anyone has done this before, but I am trying to configure a Cisco 1841 dual WAN interface router to offer active/active failover on DSL.

The router is running Cisco IOS software release 12.4(15)T7 and the interface cards are Cisco HWIC-ADSL-B/ST.

We use RIP (v2) as a dynamic routing protocol between the CPE and L2TP Network Server (LNS).

We have successfully achieved active/passive using DSL+DSL and DSL+ISDN.  The DSL+DSL active/passive setup uses an offset-list command in the RIP configuration whereas the DSL+ISDN active/passive setup uses a floating static route via the ISDN backup interface  dialer 1, e.g.

ip route dialer1 200

The problem is that ADSL is an "always on" technology so both DSL links come up by default and the RIP default route is given to one link at the time not both.

It is worth mentioning that both DSL links terminate on different end-point discriminators or LNS. So the ideal solution is to have RIP route over both links at the same time to their respective end-point discriminators so that traffic can be evenly spread across both channels.

How can the routing configuration be done to have routing on both channels at the same time? Could the following command do the trick?

ip route dialer1 120

Thanks very much in advance
Question by:koudry
    LVL 32

    Expert Comment


    If you ran a dynamic roouting protocol over the links and assigned equal cost routes. Who is at the other end of the DSL circuits, your provider? You should be able to run BGP with them.

    harbor235 ;}
    LVL 10

    Author Comment

    We have the provider on the other end.  The DSL links terminate on separate LNS.  
    We use RIP version 2 on the CPE and this works fine for the active / passive setup where we use the offset-liist command.  
    However, for the active/active setup, I don't know what to do.  Although we do use BGP on the core network, we don't have much control over that. I am working on the CPE where we use RIP.
    I tried using static routes, i.e.
    ip route dialer1 120
    ip route dialer2 120
    but this does not work either because when you send QoS traffic, only one link gets the traffic.
    The idea is to get both link to share the load equally on a round robin fashion.
    LVL 32

    Accepted Solution


    You run RIPV2 where? Not with the provider because you are trying to use dual static defaults.
    You need to run a dynamic routing protocol (BGP) with your provider. Even if they just send you a default via BGP, your router will have two equal cost routes via bgp with the seperate next-hops.
    You will have to tell BGP to allow multiple routes but this works.

    harbor235 ;}
    LVL 10

    Author Comment

    We run RIPv2 on the CPE at the customer site.
    OK if I understand what you are saying, some BGP configuration needs to be done on the remote LNS to to allow multiple routes. As I said, we don't have much control over the head-end.  So that I can simulate this in the lab, how would I do the BGP configuration to allow multiple paths?
    On the CPE, where we use RIPv2, below is the config:
    router rip
     version 2
     no validate-update-source
     timers basic 5 20 20 25
     redistribute connected
     redistribute static
     passive-interface Loopback10
     network X.X.X.X   --> loopback IP address network
     network X.X.X.X  ---> negotiated IP address network
     no auto-summary
    We use IP negotiated in the dialer 2 configuration.
    So if I know what BGP configuration needs to go on the head-end, I can simulate this in the lab on another router using Ethernet interface as opposed to DSL/ATM interface.
    LVL 10

    Author Closing Comment

    Thanks Harbor, for taking time to look at this question for me. Unfortunately, I did not have the chance to try out the solution. I don't have much control of what is happening at the core level of our network so I was unable to try out the BGP solution. If I have the chance to revisit the requirement and try the solution, I will come back here to post a feedback.

    Many thanks.

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