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Latest post 12-16-2016 12:03 AM by xterm13. 2 replies.
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  • 12-14-2016 1:28 AM

    OSPFv2 per neighbor/prefix AD

    Good day.

    The topology is as following.

    R1, R2, R3 participate in area 0, R2, R3, R4 - area 1, R2 and R3 - are ABRs.
    R1 advertises 1.1.1.2/32 as LSA1 in area 0, which is being translated into LSA3 on R2 and R3.
    R4#sh ip ospf da su 1.1.1.2

    OSPF Router with ID (4.4.4.4) (Process ID 1)

    Summary Net Link States (Area 1)

    Routing Bit Set on this LSA in topology Base with MTID 0
    LS age: 1079
    Options: (No TOS-capability, DC, Upward)
    LS Type: Summary Links(Network)
    Link State ID: 1.1.1.2 (summary Network Number)
    Advertising Router: 2.2.2.2
    LS Seq Number: 80000001
    Checksum: 0x2905
    Length: 28
    Network Mask: /32
    MTID: 0 Metric: 2

    Routing Bit Set on this LSA in topology Base with MTID 0
    LS age: 1089
    Options: (No TOS-capability, DC, Upward)
    LS Type: Summary Links(Network)
    Link State ID: 1.1.1.2 (summary Network Number)
    Advertising Router: 3.3.3.3
    LS Seq Number: 80000001
    Checksum: 0xB1F
    Length: 28
    Network Mask: /32
    MTID: 0 Metric: 2


    With all defaults R4 has two equal paths to 1.1.1.2/32.
    R4#sh ip route 1.1.1.2
    Routing entry for 1.1.1.2/32
    Known via "ospf 1", distance 110, metric 3, type inter area
    Last update from 10.0.24.2 on FastEthernet0/0.24, 00:00:04 ago
    Routing Descriptor Blocks:
    * 10.0.34.3, from 3.3.3.3, 00:00:04 ago, via FastEthernet0/0.34
    Route metric is 3, traffic share count is 1
    10.0.24.2, from 2.2.2.2, 00:00:04 ago, via FastEthernet0/0.24
    Route metric is 3, traffic share count is 1


    What I am trying to do is to set lower AD for this path advertised from R3 (RID 3.3.3.3):
    access-list 99 permit 1.1.1.2
    !
    router ospf 1
    distance 90 3.3.3.3 0.0.0.0 99
    !
    But what I've got is that this prefix from both ABRs has the same AD value of 99:
    R4#sh ip route 1.1.1.2 
    Routing entry for 1.1.1.2/32
    Known via "ospf 1", distance 90, metric 3, type inter area
    Last update from 10.0.24.2 on FastEthernet0/0.24, 00:00:43 ago
    Routing Descriptor Blocks:
    * 10.0.34.3, from 3.3.3.3, 00:00:43 ago, via FastEthernet0/0.34
    Route metric is 3, traffic share count is 1
    10.0.24.2, from 2.2.2.2, 00:00:43 ago, via FastEthernet0/0.24
    Route metric is 3, traffic share count is 1
    !

    Is there something wrong with my understanding of how this should work?

    IOS used is 15.2.4, but the same with 12 version.

    Configs are simple, but you can download them here - https://yadi.sk/d/duCjzCwe33Trnw

     

    • Post Points: 20
  • 12-15-2016 8:53 AM In reply to

    Re: OSPFv2 per neighbor/prefix AD

    The behavior that you are seeing is expected in OSPF.  This behavior change was introduced through the following bug ID:

    CSCeh46993 - distance should not affect ospf path selection within single process

    The distance command cannot be used to prefer one path over the other within the protocol.  'distance' is a part of 'ndb' structure and hence single value of distance is maintained globally.  This command should not be used to differentiate between multiple OSPF path.  OSPF should select the best path based on cost, removing comparison on distance for the same process as this affects other feature.

     

    • Post Points: 20
  • 12-16-2016 12:03 AM In reply to

    Re: OSPFv2 per neighbor/prefix AD

    Thanks, nice to know.

    They should wipe out mentioning this feature from CCNP track (:

    • Post Points: 5
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