Fundamental OSPF Concepts

  • What is the difference between EIGRP and OSPF?
    • EIGRP is a distance vector protocol while OPSF is a link-state protocol (all routers have a link-state database which has a “map” of the network)
  • What is an ABR?
    • The Area Border Router is between 2 or more area. Inside an area, routers exchange detailed topology information, but these detailed information is not advertised between areas… only briefer.
  • What is the difference between ABR and ASBR?
    • ABR is between OSPF areas while ASBR between OSPF and another routing protocol (redistribution happens here).
  •  What is a LSU and an LSA?
    • Link-State Updates contain detailed information specially LSAs. Link-State Advertisement are structures which contain topology information.
  • What is a Backbone Router?
    • An OSPF router which is connected to Area 0
  • What is an Internal Router?
    • Non ABR/ASBR router.
  • What commands do you use to verify OSPF?
    • sho ip ospf interface brief
    • sho ip protocols
    • sho ip ospf neighbors
    • sho ip ospf database
    • sho ip route ospf
  • How do you call the timers in OSPF (like EIGRP hello and hold)?
    • hello and dead timer
  • What other tool can you use to monitor OSPF neighbor connection?
    • BFD
  • What are the OSPF update packets’ destination?
    • (all SPF routers)
    • (all designated routers)
  • How does OSPF neighborship form? 
    • When OSPF is enabled it sends Hello packets to
    • If the required parameters match in the Hello packet the neighborship forms
  • What parameters need to match to become routers?requirements.PNG
  • What happens if 2 routers have the same OSPF router ID?
    • they cannot become neighbors
  • What happens if the MTU is not matching between neighbors?
    • The 2 routers will be neighbors but will not exchange topology information. (in addition the neighborship will flap)
      Neighbor ID Pri State Dead Time Address Interface 1 EXSTART/DR 00:00:38 FastEthernet0/1
      debug ip ospf adj
      *Feb 1 14:17:14.811: OSPF: Nbr has larger interface MTU
  • What does “FULL/” state means in sho ip ospf neighbor?
    • No attempt was made to elect a DR.
  • What solution do you know for the following problems?ospf-issue.PNG
    • use Virtual Links
  • How does a Virtual Link work?
    • The virtual link allows 2 ABR to form a neighbor relationship through a nonbackbone area. ABR need to be part of it’s own area and the transit area.
    • The ABR sends all OSPF messages as unicast (over the virtual link).
    • The router marks the LSA with a Do Not Age (DNA) bit which means that the LSAs will not be reflooded on the usual refreshment interval (30 min). This reduces overhead.
  • What do the parts of virtual link command mean?
    • area area-number virtual-link remote-RID
      • remote-RID: the other router’s RID
      • area-number: the transit area where the packets flow between the 2 routers
    • In the above example a virtual link is built between R4 and R5:
    • !R4
      router ospf 1
        area 20 virtual-link
      router ospf 1
        area 20 virtual-link
      R4#sho ip ospf virtual-links
      Virtual Link OSPF_VL3 to router is up
       Run as demand circuit
       DoNotAge LSA allowed.
       Transit area 20, via interface Serial2/0
       Topology-MTID Cost Disabled Shutdown Topology Name
       0 64 no no Base
       Transmit Delay is 1 sec, State POINT_TO_POINT,
       Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
       Hello due in 00:00:01
       Adjacency State FULL (Hello suppressed)
       Index 2/3, retransmission queue length 0, number of retransmission 0
       First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
       Last retransmission scan length is 0, maximum is 0
       Last retransmission scan time is 0 msec, maximum is 0 msec

      With this virtual-link both R6 and R9 is reaching the backbone.

  • What other key features do you know about virtual link?
    • the transit area must not be stubby
    • the OSPF authentication configured on the area virtual link command
    • the hello and dead timers configured on the area virtual-link
    • the virtual link cost is set as if it were a P2P link
  • How do you verify the virtual links?
    • sho ip ospf virtual-links
    • sho ip ospf neighbor 

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