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Forum Replies

  1. Hi Rene
    just a little confused you did not advertise 2.2.2.2 network in R2 so how come it got populated in ospf database table ? and one more question using redistribute command makes a router an ASBR?? whats the difference between advertising a network with network command or using a redistribute command ?

    Many Thanks

  2. Let me explain them a bit more here:


    • Stub network link: this is a segment where we only have one OSPF router, a good example would be a loopback interface. An Ethernet interface where you don't have any neighbors also is a stub link. It doesn't have anything to do with stub areas.

    • Point to point network link: Used for serial connections (point-to-point) like PPP, HDLC or frame-relay point-to-point.

    • Transit network link: used for multi-access networks like Ethernet where you have more than one neighbor.

    • Virtual link network link: As the name implies, used for OSPF virtual links.

    Rene

  3. Hello sims.

    On Rene's example, he shows the output of the command show ip ospf database for R3. Here it is below as well:

    R3#show ip ospf database | begin Summary
    		Summary Net Link States (Area 1)
    
    Link ID         ADV Router      Age         Seq#       Checksum
    1.1.1.1         2.2.2.2         149         0x80000001 0x0033FB
    192.168.12.0    2.2.2.2         195         0x80000001 0x00219D
    
    		Summary ASB Link States (Area 1)
    
    Link ID         ADV Router      Age         Seq#       Checksum
    1.1.1.1         2.2.2.2         62          0x80000001 0x004DB9
    
    		Type-5 AS External Link States
    
    Link ID         ADV Router      Age         Seq#       Checksum Tag
    11.11.11.0      1.1.1.1         68          0x80000001 0x000F44 0

    In the above output, the Summary ASB Link States (Area 1) section displays the LSA type 4. A type for LSA has been received from 2.2.2.2 which is R2, which is indeed an ABR. With my last statement in the previous message, I wanted to confirm to you that type 4 LSAs come from ABRs, and R2 with an ID of 2.2.2.2 is indeed an ABR.

    If you have created a topology of your own and you cannot see any type 4 LSAs on R3, check to make sure the neighbour relationships are being created correctly between routers and make sure that R2 is indeed an ABR, that is, it should have at least one interface in each OSPF area you have created.

    I hope this has been helpful!

    Laz

  4. Hi Laz,

    For the below output, T3 and T4 LSA looks exactly the same (I have marked with arrow below) Therefore if we don't have T4 LSA, R3 should be able to find R1 with the help of T3 LSA?

    R3#show ip ospf database | begin Summary
    		Summary Net Link States (Area 1)
    
    Link ID         ADV Router      Age         Seq#       Checksum
    1.1.1.1         2.2.2.2         149         0x80000001 0x0033FB <<<<<<<<<
    192.168.12.0    2.2.2.2         195         0x80000001 0x00219D
    
    		Summary ASB Link States (Area 1)
    
    Link ID         ADV Router      Age         Seq#       Checksum
    1.1.1.1         2.2.2.2         62          0x80000001 0x004DB9  <<<<<<<
    
    		Type-5 AS External Link States
    
    Link ID         ADV Router      Age         Seq#       Checksum Tag
    11.11.11.0      1.1.1.1         68          0x80000001 0x000F44 0

    Best Regards,
    Neeraj

  5. The short answer:

    Implicit: this happens when a router sends a duplicate of the LSA that it receives. When the sender sees its own LSA, it knows that the other side has received it.

    Explicit: this happens when you send an ACK packet in response to an LSA that you have received.

    RFC2328 explains this in more detail:

    Each newly received LSA must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13).

    _If the LSA is listed in the Link state retransmission list for the receiving adjacency, the router itself is expecting an acknowledgment for this LSA. The router should treat the received LSA as an acknowledgment by removing the LSA from the Link state retransmission list. This is termed an "implied acknowledgment". Its occurrence should be noted for later use by the acknowledgment process (Section 13.5).

    OSPF can't do fragmentation but IP does. By default, OSPF will check for MTU mismatches but it's possible to disable this with the ip ospf mtu-ignore command. In this case, OSPF will work even with MTU mismatches and you can use IP fragmentation.

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