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OSPF Multi-area Routing

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  • 1.  OSPF Multi-area Routing

    Posted 01-18-2020 04:48

    I have a question: 

     

    If the R2-R4 link fails, which way will traffic route from R1 to R4? I am thinking it will follow the lwoest metric, but as it is across areas, is it correct to go R1 - R2- R3 -R4? 

     

    Thanks 

     

    Screenshot 2020-01-18 at 12.45.33.png


    #ospf
    #metric
    #areas
    #IGP


  • 2.  RE: OSPF Multi-area Routing

     
    Posted 01-18-2020 08:06

    There is too less information in the exhibit to answer this for sure. However, if the links R1 - R2 - R3 - R4 are all inside area 0, then the packets will go this way, as the traffic will not leave area 0 to transit another area to go back to area 0 again.



  • 3.  RE: OSPF Multi-area Routing
    Best Answer

    Posted 01-18-2020 20:26

    Hello,

     


    @NetworkMan23 wrote:

    is it correct to go R1 - R2- R3 -R4? 

     


     

    No. If this is pure IP forwarding (no MPLS and no tunnels of any sort: GRE, IPIP, IPSec etc) then when packet destined to R4 hits R2, then R2 has 2 choice routes:

    - INTRA area 10 route from R4 via R5

    - INTER area 0 route from R3

    According to RFC 2328 section 11, INTRA area route is preferred and R2 will send this packet to R5 and R5 will forward to R4.

     

    HTH

    Thx

    Alex

     



  • 4.  RE: OSPF Multi-area Routing

     
    Posted 01-18-2020 21:08

    Intra-area routes are preferred over inter-area routes regarless of the metric. If you wanted traffic going to R4 to be sent via R3 when the link R2<=>R4 fails, you can configure multi-area adjancency between R2 and R3 as shown:

    multi-area adjacency.png 

    protocols ospf {

    area 0.0.0.0 {

       interface ge-0/0/0.0;

    }

    area 0.0.0.10 {

       interface ge-0/0/0.0 {

             secondary;

    }