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Forum Replies

  1. Hello Sims!

    The network topology that you are describing would not COMPLETELY simulate the frame relay situation in this lesson. What you are describing would require all communication between R1 and R3 to go through R2 just like in the lesson, however, this also means that the two interfaces on R2 that connect R1 and R3 would have to be on the same subnet, something that is not possible. So you would just be creating two point to point links on different subnets, however, in the frame relay example in the lesson, all three routers are in the same subnet.

    What you require is to create a frame relay switch on GNS3. You can find some frame relay examples in GNS3 from Rene’s GNS3 Vault here: http://gns3vault.com/labs/frame-relay/

    I hope this has been helpful!


  2. Hello Sims!

    Although I’m sure you know most of this, let me start by stating the network types for OSPF and stating an example for each:

    Point to Point - a serial connection between two routers
    Broadcast - two or more routers on the same subnet connected to an Ethernet switch

    The next two types of network are used to accommodate multi access technologies that don’t support broadcast. These technologies include Frame Relay and ATM.

    Non Broadcast Multi Access - This is used when you have multiple routers connected on the same subnet over a non-broadcast technology such as frame relay. OSPF must emulate a broadcast by sending multiple unicast packets to all destinations. In this case, a DR must be elected and it must be the HUB router that is the DR. Otherwise OSPF will not function correctly.

    Point to Multipoint - This is a network where the topology is the same as in the Non Broadcast Multi Access, however, the HUB router creates multiple point to point connections to each remote router, thus emulating multiple point to point connections. Each point to point connection has its own subnet. A DR/BDR is not necessary in this topology.

    So, to specifically answer your questions:

    "can you provide a non multi-access ospf network ?"
    A non-multi access network is just a point to point network. Multi access just means that there are more than two routers involved in a specific topology.

    " And why it is called non multi access ?"
    There is no specific term “non multi access” used for OSPF network topologies. Non multi access just means point to point. If you are referring to Non Broadcast, then that just means that you are using a technology that doesn’t allow broadcasts to occur such as Frame Relay.

    "The broadcast and non-broadcast network types require a DR/BDR
    So which type does not require a DR/BDR ?"

    Specifically, out of the four types of networks, the Broadcast network and the Non Broadcast Multi Access require DR/BDRs. The Point to Point and Point to Multipoint do not since the basis of their topologies are point to point connections. Note that in a Broadcast network the DR can be any router based on the DR elections, while in the Non Broadcast Multi Access, the DR must be the HUB router for OSPF to function.

    I hope this has been helpful!


  3. Hello Nanu

    It all has to do with the way OSPF as a link-state protocol functions as compared with EIGRP which is a distance vector protocol.

    OSPF advertises a large amount of topological information about the network. Each router must learn what every metric is for every link in the OSPF area. When LSAs are exchanged, each router shares with other routers their perspective of the WHOLE network. EIGRP routers on the other hand share information about a portion of the network. So, first off, LSAs are much larger than the data exchanged by EIGRP, so a traffic regulator like the DR/BDR is necessary when there are multiple OSPF routers in the same subnet.

    Secondly, and probably most importantly, OSPF creates neighbor relationships with ALL OSPF routers in its subnet whereas EIGRP creates neighbor relationships only with DIRECTLY CONNECTED EIGRP routers. So information exchange between EIGRP routers is drastically reduced as compared to OSPF.

    I hope this has been helpful!


  4. Hello Laz,

    Thank you so much for your guidance, this absolutely cleared my doubt.


  5. Personnaly ; I got a little bit confused about the mapping possibilities between OSPF and Frame-Relay. Drawing the table below does help me get a better understanding of the subject.

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