Load Balancing Multiple Cisco Routers with Multiple ADSL circuits

Discussion in 'Cisco' started by Big Phil, May 1, 2007.

  1. Big Phil

    Big Phil Guest


    I need to provide a large amount of bandwidth to one site. I'm
    thinking about having 20 or so ADSL circuits installed as opposed to a
    E1 or E3 circuit as this site will only be up and running for a few
    months so i need the flexablity of ADSL and SDSL and E1/E3 the costs
    are to prohibative. So i need a way to load balance these lines. I'm
    thinkiing of using a router from the Cisco2800 or 3800 series. I'm
    aware that these routers can only take a a maximum of 4 ADSL HWIC's.
    Our ISP will be bonding sets of 4 ADSL lines for us. This there a way
    that i can load balance 4 routers into one large virtual pipe? ie 4
    routers each with a bonded set of 4 adsl circuits.

    Also is it possible to to radius on the internal LAN to restrict
    outward traffic? ie like an ISP but on a LAN.

    Any thoughts welcome

    Big Phil, May 1, 2007
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  2. Big Phil

    net_expert Guest

    Hi Phil,

    It depends what kind of traffic you will need to carry over those
    links. Is it normal Internet traffic (large amount of TCP/UDP flows
    destinated to different IP addresses) ?

    If so, the solution is quite simple, you can use CEF load balancing to
    share the load on the 20 links.

    Here is a simple solution with 5 routers:

    R0 would need 5 FastEthernet ports, one connected to the LAN and one
    to each of the DSL routers. You can then simply use CEF load-
    balancing. You do not need to configure anything to enable it, it is
    enabled by default.

    The difficult part is to correctly configure the routing tables. If
    you do it statically:

    Each of the DSL routers should have 1 route to the LAN and 1 or 4
    default routes to the Internet (depending if your for links are
    bundled or not).

    On R0, you will need 5 default routes to the internet, one for each
    DSL routers.

    Keep in mind that CEF is doing per flow load-balancing, so if you do
    not have a large amount of flows load balancing will be unequal.

    Also, you will need to configure NAT on each DSL router to be sure
    that flows going to the internet through different routers have
    different source IP addresses (else, there wont be any load balancing
    in the Internet -> LAN direction).

    net_expert, May 1, 2007
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  3. Big Phil

    Trendkill Guest

    I don't think this is correct. If you use static routes, it will not
    load balance. Traffic will match the first route, and the first route
    only, until it fails. The only wait it would fail is if it had a
    destination of an interface instead of a next-hop, and that interface
    went down. Then and only then would it go to the next static route.
    If you want to load balance, you MUST run a routing protocol that
    knows about equal cost routes to the destination AND supports load
    Trendkill, May 1, 2007
  4. Big Phil

    NetExpert Guest

    Hi Trendkill,

    Thanks for jumping in.

    Equal-cost load-sharing with CEF works without any problem with static
    routes as long as you do not play with the administrative distance. By
    default this value is 1. If all the static routes have the same
    administrative distance, all of them will be added to the routing table.
    In latest IOS versions CEF supports up to 16 routes for load-balancing.

    What you are describing is only true if you tuned the administrative
    distance to use one static route as the backup of others.

    I would recommend reading ‘Routing TCP/IP Volume 1’ (Cisco Press). The
    third chapter explains CEF load-balancing with static routes in detail.

    Routing protocols are only needed in case you want to do unequal-cost
    load-sharing, which is not needed in Phil’s setup.

    Let me know if you need more details on this

    NetExpert, May 1, 2007
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