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OSPF route and loopback as next hop

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  • 1.  OSPF route and loopback as next hop

    Posted 05-25-2020 12:15

    Hi everyone,

    Please consider the following example;

    HOST1-subX-MX1-199.199.199.1--199.199.199.2 MX2--subY-HOST2

    Above MX1 has  lo0 :1.1.1.1, MX2 has lo0 :2.2.2.2

    Assume we are runnning ospf on MX1/MX2, on all ports, including lo0,  as a result, MX1 sees OSPF leaned route Y as 

    Y next hop 199.199.199.2

    Is there any way , I can force MX1 to see  Y reachable over MX2's next hop i.e

    Y next-hop 2.2.2.2

    Is there any way I can do the above without running BGP between MX1 and MX2 as explained below:

    Using BGP between MX1 and MX2:

    1)Run BGP between MX1 and MX2 using loopbacks

    2)Stop advertising Y in OSPF on MX2, instead use BGP to advertise it to MX1.

     

    Thanks and have a good day!!

     

    let



  • 2.  RE: OSPF route and loopback as next hop
    Best Answer

     
    Posted 05-25-2020 14:08

    Hi sarahr202,

     

     

    Howdy, unfortunately since the loopback interface is special a virtual interface this is not possible. A virtual interface is not able to physically transmit any data or physically receive any data. If the loopback can not send OSPF hello messages or receive OSPF hello messages then it is not possible to establish any neighbor relationship using loopback interfaces. Nevertheless, you can use an IRB for this purpose as the IRB is mapped to VLAN which at the same time will take advantage of the physical infrastructure from any layer 2 interfaces assigned to this VLAN if/when needed, in other words, the IRB would be able to send hellos and they will do It through the VLAN they are par of.

     

    As you can see here Lo0 is not even capable of sending hellos

     

    {master:0}[edit]
    root@LilDexx# run monitor traffic interface lo0 size 1500 no-resolve matching "host 224.0.0.5"
    verbose output suppressed, use <detail> or <extensive> for full protocol decode
    Address resolution is OFF.
    Listening on lo0, capture size 1500 bytes


    371 packets received by filter
    0 packets dropped by kernel

    {master:0}[edit]

     

    IRB interfaces are 🙂


    root@LilDexx# run monitor traffic interface irb size 1500 no-resolve matching "host 224.0.0.5"
    verbose output suppressed, use <detail> or <extensive> for full protocol decode
    Address resolution is OFF.
    Listening on irb, capture size 1500 bytes

    13:57:23.747812 In IP 12.34.56.1 > 224.0.0.5: OSPFv2, Hello, length 60
    13:57:26.944445 Out IP 12.34.56.2 > 224.0.0.5: OSPFv2, Hello, length 60

    2 packets received by filter
    0 packets dropped by kernel

     

     


    If this solves your problem, please mark this post as "Accepted Solution" so we can help others too \:)/

    Regards,

    Lil Dexx
    JNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB