We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is Why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 588 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)


312 New Members signed up the last 30 days!


100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!


Forum Replies

  1. Hi Rene
    just a little confused you did not advertise 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.


  3. 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.

  4. Hello Brian

    Redistribution of E1 and E2 types is actually found within the realm of CCNP Route. Take a look at this Cisco link to see more.

    I hope this has been helpful!


  5. Hi Network lessons team,

    Thanks for your topic, it helped me making sense about LSA operation. But acctually I’m having a little bits confuse about LSA Type 4 and LSA Type 5.
    In my point, I thought LSA Type 4 just only notice the router in difference area about ASBR router. But why it needs to know ASBR position? Because when ABR advertise LSA Type 5, it’s already having a Advertising Router information (it’s the ASBR IP), and router in other area can base on this field to choose next-hop.

    I hope that you can make sense to me. Thank you so much!

    Thinh Cao Minh

101 more replies! Ask a question or join the discussion by visiting our Community Forum