Tags:


Notable Replies

  1. Hi Rene

    Could you clarify the differences between this and OSPFv3 Prefix Suppression?

    I believe the main difference is that type 8’s and 9’s are suppressed rather than type 1 or 2’s.

    Are stub’s advertised as /128’s?

    Also how does OSPF detect that a prefix is part of a transit network?

  2. Hi @chrisnewnham17,

    In OSPFv3, there is no prefix information in LSA type 1 and 2 anymore, you can find those in LSA type 8 and 9 so yes, OSPFv3 prefix suppression removes it from LSA type 8 and 9.

    About the prefix / transit network. Here’s a quick example with three routers connected like this:

    R1-R2-R3

    All routers are in area 0 and R3 is the DR:

    R3#show ipv6 ospf database prefix self-originate  
    
                OSPFv3 Router with ID (3.3.3.3) (Process ID 1)
    
                    Intra Area Prefix Link States (Area 0)
    
      LS age: 8
      LS Type: Intra-Area-Prefix-LSA
      Link State ID: 0
      Advertising Router: 3.3.3.3
      LS Seq Number: 80000002
      Checksum: 0x7834
      Length: 52
      Referenced LSA Type: 2001
      Referenced Link State ID: 0
      Referenced Advertising Router: 3.3.3.3
      Number of Prefixes: 1
      Prefix Address: 2001:DB8:3333:3333::3
      Prefix Length: 128, Options: LA, Metric: 0
    
      LS age: 1729
      LS Type: Intra-Area-Prefix-LSA
      Link State ID: 3072
      Advertising Router: 3.3.3.3
      LS Seq Number: 80000001
      Checksum: 0x16FD
      Length: 44
      Referenced LSA Type: 2002
      Referenced Link State ID: 3
      Referenced Advertising Router: 3.3.3.3
      Number of Prefixes: 1
      Prefix Address: 2001:23:23:23::
      Prefix Length: 64, Options: None, Metric: 0
    

    Above you can see the referenced LSA type:

    * 2001: Associated with the router
    * 2002: Associated with the Network LSA

    The 2001:23:23:23:: prefix is configured on an Ethernet network, thus it shows up with LSA type 2002. You can also see the link state ID (3) of the advertising router (R3 our DR).

    R3#show ipv6 ospf database network self-originate 
    
                OSPFv3 Router with ID (3.3.3.3) (Process ID 1)
    
                    Net Link States (Area 0)
    
      LS age: 426
      Options: (V6-Bit, E-Bit, R-Bit, DC-Bit)
      LS Type: Network Links
      Link State ID: 3 (Interface ID of Designated Router)
      Advertising Router: 3.3.3.3
      LS Seq Number: 80000002
      Checksum: 0x41A3
      Length: 32
            Attached Router: 3.3.3.3
            Attached Router: 2.2.2.2
    

    About the stub, you mean a stub network? Loopbacks are advertised with a /128, similar to how OSPFv2 advertises them as a /32 by default.

  3. Hi Rene ,
    Can explain a bit about how the how ospf detect transit links when performing ospf prefix suppression . Also, you are mentioning that /32 link is considered an invalid link in the lsdb.In such case why aren’ t the Loopback networks considered invalid when prefix suppression is enabled? These are also /32 links.

    Regards,
    Ruwan

  4. Hello Ruwan

    There are two types of LSAs that we have to deal with in this lesson: The Router LSA, that is a type 1 LSA that contains prefix information about the 192.168.30.0/24 network, and the Network LSA< that is a type 2 LSA that contains prefix information about the 192.168.123.0/24 subnet.

    If you notice, in each of the LSAs described, the only networks that have a prefix other than /32 are the transit networks, so if you suppress prefixes, the transit networks will be removed.

    A subnet mask of /32 on a multi-access network is not possible because then it would not be a multi-access network. So if an OSPF router sees this, it knows that prefix suppression has been applied. It is not invalid in the sense that it cannot exist in the database.

    I hope this has been helpful!

    Laz

Continue the discussion forum.networklessons.com

Participants