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  • 1.  OSPF Hello option bits on a capture versus show ospf neighbor detail options

    Posted 08-07-2023 11:32

    Hello,

    I maybe reading into this a bit much but I have not yet been able find an answer on the Juniper Users Guide or for that matter anywhere else on the Web. Here goes:

    I have two Juniper routers forming a OSPF neighborship between each other. Nothing special just a regular area. I do a Wireshark capture on the on the wire between them and the options field are set to 0x12. No biggie, I got that and understand what the information the are conveying with these option bits. My confusion is the following.

    When I do a "show ospf neighbor detail" on either router the opt field is set to 0x52 on the output. I would expect it to also be set to 0x12 as the capture of the hello packets showed. 

    What is the difference between the show command opt field and hello packet opt field?

    Thank you in advance,

    Raul



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    RAUL FERNANDEZ
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  • 2.  RE: OSPF Hello option bits on a capture versus show ospf neighbor detail options

    Posted 08-08-2023 02:54

    I think your are looking at different options, I suspect  0x12 is in the hello packet and 0x52 is in the database description.  let me lab quick ( its good practice anyway ). 
    Lucky you on having the All Access Pass !!, in my dreams LOL. 
    The hello packets is saying "to form a adjacency these must match .." 
    the DBD says " optional capabilities are ..."





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    Simon Bingham
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  • 3.  RE: OSPF Hello option bits on a capture versus show ospf neighbor detail options

    Posted 08-09-2023 10:50

    Simon,

    Thank you for the response and the clarification! Much appreciated.

    Raul



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    RAUL FERNANDEZ
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  • 4.  RE: OSPF Hello option bits on a capture versus show ospf neighbor detail options

    Posted 08-14-2023 08:39
    Edited by Simon Bingham (technical debt collector) 08-14-2023 08:46

    I've looked at this further as I wanted to understand this , I will not pretend to comprehend fully but I think the router is describing itself "There is a router 10.100.255.8" and it supports Opaque LSAs.    maybe this does not need to match for an adjacency to form so it not in the hellos.  
    I cannot find this in the

    show ospf database

    output on the Juniper though 





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    Simon Bingham
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  • 5.  RE: OSPF Hello option bits on a capture versus show ospf neighbor detail options

     
    Posted 08-14-2023 16:33

    As per RFC2370:

    When opaque-capable routers and non-opaque-capable OSPF routers are
       mixed together in a routing domain, the Opaque LSAs are not flooded
       to the non-opaque-capable routers. As a general design principle,
       optional OSPF advertisements are only flooded to those routers that
       understand them.
    
       An opaque-capable router learns of its neighbor's opaque capability
       at the beginning of the "Database Exchange Process" (see Section 10.6
       of OSPF, receiving Database Description packets from a neighbor in
       state ExStart). A neighbor is opaque-capable if and only if it sets
       the O-bit in the Options field of its Database Description packets;
       the O-bit is not set in packets other than Database Description packets.

    The O-bit in OSPF Hello is not set as  seen in the pcap, however Opaque capability is exchanged in Database Description (DD), thus 'show ospf neighbor' indicates the exchanged capabilities inclusive of both Hello & DD derived options.



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    Ashvin
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