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  1. Hello,

    I have a question regarding passive interfaces. From my understanding a “passive interface” under OSPF doesn’t send hello packets and makes adjacencies. I have the following config up on a router and would like clarification:

    Routing Protocol is "ospf 250"
      Outgoing update filter list for all interfaces is not set
      Incoming update filter list for all interfaces is not set
      Router ID 10.16.1.1
      Number of areas in this router is 1. 1 normal 0 stub 0 nssa
      Maximum path: 4
      Routing for Networks:
        10.0.0.0 0.255.255.255 area 28
      Passive Interface(s):
        Embedded-Service-Engine0/0
        GigabitEthernet0/2
        GigabitEthernet0/2.701
        Backplane-GigabitEthernet0/3
        GigabitEthernet0/0/0
        Serial0/3/0
    
    CORVG01#sh ip ospf interface brief 
    Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
    Lo0          250   28              10.16.1.1/29       1     LOOP  0/0
    SM1/0        250   28              Unnumbered Gi0/2.701 1     DOWN  0/0
    Se0/3/0      250   28              10.160.9.2/30      64    DOWN  0/0
    Gi0/2.701    250   28              10.0.8.250/24      1     DR    0/0<-------
    Gi0/1        250   28              10.16.1.10/31      1     P2P   1/1
    Gi0/0        250   28              10.16.1.8/31       1     P2P   1/1
    

    my question is why are we still seeing Gi0/2.701 as on OSPF interface? is it because Interface Gi0/2.701 has an ip address that falls under the network 10.0.0.0 0.255.255.255 area 28 command?

    router ospf 250
     router-id 10.16.1.1
     passive-interface default
     no passive-interface GigabitEthernet0/0
     no passive-interface GigabitEthernet0/1
     network 10.0.0.0 0.255.255.255 area 28 <---------
    

    Thanks

  2. Rohitendu,
    Cisco has five different options for OSPF over NBMA. Which one you should choose depends on several factors. One important consideration is whether the circuit supports multicast–this will determine whether you must define neighbors manually, or if they can discover each other. Additionally, you need to consider what is the topology of your WAN connection–full mesh, hub and spoke, …? This is important because you must ensure that any router eligible to be a DR or BDR must have reachability to all the other OSPF nodes.

    Here is some information on the five different OSPF NBMA network types:

    5 Modes of Operation
    -Non-Broadcast (NBMA) - RFC Standard
    -Point-To-Multipoint - RFC Standard
    -Point-To-Point - Cisco Proprietary
    -Broadcast - Cisco Proprietary
    -Point-To-Multipoint, Non-Broadcast - Cisco Proprietary

    Broadcast Mode
    -LAN-style operational mode over NBMA (there is no RFC standard for this, hence Cisco proprietary)
    -(config-if)# ip ospf network broadcast

    Non-Broadcast Mode
    -Default mode for X.25, FR, ATM
    -Neighbors are statically configured
    -Must be on one subnet
    -Acts like a LAN environment
    -DR/BDR are elected (DR/BDR must have full connectivity to all neighbors). This point will be a design constraint as your only option for a DR/BDR in a hub and spoke topology would be the hub
    -Typically used in full mesh networks

    Point-To-Multipoint Mode
    -Requires a single subnet
    -No DR/BDR elected
    -Neighbors form automatically as long as the “broadcast” option is chosen in configuring the link
    -Typically used in partial mesh networks
    -(config-if)# ip ospf network point-to-multipoint

    Point-To-Multipoint, Non-Broadcast
    -Exactly the same as P2P mode, but neighbors do not form automatically. They must be configured manually

    Point-To-Point Mode
    -Uses separate sub-interfaces
    -Requires different subnets
    -No DR/BDR elected
    -Neighbors form automatically

  3. Hi Rene,

    Please help! My topology is simple. R1–FW–R2. I am running ospf between R1 and R2. Firewall between R1 and R2 is transparent. R1 and R2 are able to ping each other through the firewall. However, I keep getting this error message. The hello and dead timer are identical on both R1 and R2. The neighbor already formed. However, it will disconnect and re-form back and forth…

    *Mar  1 03:11:10.743: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/1 from FULL to DOWN, Neighbor Down: Dead timer expired
     --More--
    *Mar  1 03:11:15.243: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/1 from LOADING to FULL, Loading Done
    
    
    R1#sh ip ospf nei
    
    Neighbor ID     Pri   State           Dead Time   Address         Interface
    2.2.2.2           1   FULL/DR         00:00:34    192.168.2.2     FastEthernet0/1
    
    
    ```
    R1#
    router ospf 1
     router-id 1.1.1.1
     log-adjacency-changes
     passive-interface default
     no passive-interface FastEthernet0/1
     network 192.168.2.0 0.0.0.255 area 0
    
    R2#
    router ospf 1
     router-id 2.2.2.2
     log-adjacency-changes
     passive-interface default
     no passive-interface FastEthernet0/1
     network 192.168.2.0 0.0.0.255 area 0
    ```
  4. Georgi,
    I think this question would need some clarification, and it depends on whether the two routers are the ONLY routers that are neighbors, or whether there are more than two routers.

    The situation you describe could happen where the router that is in 2-Way has not been elected as a DR or BDR. In this situation (assuming we are dealing with a multi-access network), this is perfectly normal to have a 2-Way state. Any router that is not a DR or BDR that has a neighbor relationship with another router that is also not a DR or BDR would show up as 2-Way/DROTHER.

    Now, if the person asking you this question was talking about where the two routers’ relationship with each other, that is a different story. In other words, let’s say there are two routers: A and B. If you issue “show ip ospf neighbor” on both A and B, and A shows as Full with B, but B shows as 2-Way with A, I don’t have an answer for that situation. In multi-access networks, OSPF only forms a Full relationship with either a DR or BDR, and that Full relationship goes both ways, so in this case A and B should see a Full relationship with each other. If only one side is seeing Full but the other is 2-Way, something is wrong, but I can’t think of what would cause that to happen.

    Do you know whether there were only two routers in the scenario presented to you?

  5. Hi Networklesson team ,

    I was searching for a issue that “why ospf neighbors will stuck at exstart/exchange state” , i got good explanation at below cisco link .

    please note at step 5 :
    ____________________________________________________________________________________
    Router 7 debug output:

    ***OSPF ENABLED ON ROUTER7, BEGINS SENDING 
    HELLOS AND BUILDING A ROUTER LSA  ////please note router building LSA Type 1////
    00:17:44: IP: s=170.170.11.7 (local), d=224.0.0.5 
    (Serial0.6), Len 64, sending broad/multicast, proto=89
    00:17:44: OSPF: Build router LSA for area 0, 
    router ID 170.170.11.7, seq 0x80000001
    

    ____________________________________________________________________________________

    after going through it i have an doubt that at exactly which ospf state which LSAs are sent/transmitted and when the LSDB database is assumed as complete before applying SPF algo ?

    Thanks on advance , contents and explanations is very clear crisp .

    Sameer.

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