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OSPF: Where are the equal costs?

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  • OSPF: Where are the equal costs?


    I started with OSPF. Well, thereīs ECMP load sharing mentioned. Iīm asking myself where those equal cost paths are.
    I added a picture.

    - We have OSPF routers from A to I
    - All black lines are 1 Gbit/s fiber lines
    - Letīs say we have a host behind I and G
    - The host behind I sends several data to the host behind G

    Now, where are thos ECMPs?

    - The whole path from I to G, i.e.GFABI (orange) and GEDCI (green)?
    - Is it only between each router, e.g. we have 3 times 1 Gbit/s connections between E and D. Will there be a kind of load balancing?
    - Is it both?

  • #2
    Re: OSPF: Where are the equal costs?

    Its a common misconception that routing protocols do load balancing. The actual load balancing determines what kind of switching mechanism you are running (CEF,Fast, Process) So in short the switching mechanism determines the load balancing not the routing protocol.The routing protocol is responsible to get multiple routes in the RIB. CEF by default does per destination load balancing. So in your situation if you are sending data from a host behind I to the same host behind G and you are doing per destination load balancing then only one of those links will be used. CEF does support per packet load balancing so if there are multiple OSPF routes in the routing table and you are running per packet load balancing then one packet will go one way the next the other and so on. If you run voip inside your network be wary of per packet load balancing as packets may arrive out of order and create jitter and delay. Cisco recommends per destination load balancing and thats why its the default. Also since you are using gig interfaces you may want to change the ospf bandwidth calculation. By default OSPF uses the forumual 100000000bits/bandwidth. So if you have a 100Mbps interface the cost will come out as 1. Since your running gig interfaces the cost will be one for them as well. You can change the defaults by using the command:

    auto cost reference bandwidth under the ospf process

    int g0/1
    ip load-sharing per-packet
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