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    CCNP BSCI Notes – Understanding Simple Single-Area OSPF

    By Raiy Wong | November 19, 2009

    OSPF Fundamentals

    Link-state routing protocols utilize more internal resources in favor of reducing bandwidth consumption.

    All OSPF routers in an area share the same Link State Database (LSDB).

    Link State Advertisements (LSAs) are flooded to all neighboring routers.

    OSPF tables:

           Neighbor table

           Topology database

           Routing table

    Forming adjacencies

    Routers multicast hellos to every 10 seconds on a broadcast link and every 30 seconds on a nonbroadcast link.

    Once hellos are exchanged, neighboring routers add one another to their neighbor tables.

    Contents of a hello packet:

             Router ID – 32-bit unique number (IP address)

             Hello/dead intervals – Timers

             Neighbor list – List of neighboring router IDs

             Area ID

             Priority – Used in electing the DR and BDR

             DR and BDR

             Authentication (if enabled)

             Stub Area Flag – On if this is a stub area

    Neighbor states:


             Attempt – Used for manually configured neighbors on an NBMA link; unicast hellos sent to neighbor from which hellos have stopped being received

             Init – Hello packet received from neighbor, but without the recipient’s router ID

             2-Way – Bi-directional communication has been established

             Exstart – The DR and BDR have been elected, link-state exchange starting

             Exchange – Exchange of database descriptor (DBD) packets

             Loading – Exchange of link-state information

             Full – Full adjacency established

    Example OSPF packet capture

    Cisco OSPF will load balance over up to four equal-cost links; configurable up to six.

    Designated Routers

    Neighbors on a broadcast segment elect a designated router (DR) and backup designated router(BDR), which peer with all other routers on the segment. All non-designated routers peer only with the DR and BDR.

    Multicast destinations:

    – All OSPF routers

    – All designated OSPF routers (DR and BDR only)

    DRs are chosen based on priority (0 – 255). 1 is default; routers with 0 priority will never be elected.

    Priority ties are broken by choosing the higher router ID.

    DRs are elected on point-to-point Ethernet links even though this is unnecessary (Ethernet is always seen as a broadcast medium). Interfaces can be configured to operate in point-to-point mode to prevent this.

    (B)DRs are not preempted. New election will take place only when a current (B)DR goes offline or its

    OSPF process is administratively restarted.


    All routers in an area maintain an identical topological database.

    Areas are defined to logically segment a network and reduce routing table size and complexity.

    All areas connect to area 0 (the backbone area).

    Router types:

            Backbone routers – Routers in area 0

            Area Border Routers (ABRs) – Routers in multiple areas

            Autonomous System Boundary Routers (ASBR) – Routers which redistribute information from another AS

            Internal – Routers which have all interfaces in a single area

    Routers can fill multiple roles.

    Packet Types

    OSPF is IP protocol 89.

             Hello – Used to establish communication with directly connected neighbors

             Database Descriptor (DBD) – Lists router IDs from which the router has an LSA and its current sequence number

             Link State Request (LSR) – Request for an LSA

             Link State Update (LSU) – Reply to an LSR with the requested information

             Link State Acknowledgment (LSAck) – Used to confirm receipt of link-state information

    Packet Fields

             Version – Version of OSPF being run



             Router ID

             Area ID


             Authentication type (none/plaintext/md5)

             Authentication data


    Configuring OSPF in a Single Area

    Necessary information:

            OSPF process ID (locally significant)

            Participating interfaces

            Area ID

            Router ID

    Enable OSPF


    Configure Included Networks


    A single interface can be specified by supplying its IP address and a null wildcard mask: network area 0

    Router ID

    If no router ID has been administratively declared, a router will choose the highest loopback IP address.

    If no loopback addresses are present, the highest IP address of the first active interface will be used.

    A router ID can be manually specified:


    Best practice dictates the creation of a loopback address to be used as the router ID for stability and continuity:


    Default Cost

    Link cost is a 16-bit value (0-65535); default cost is calculated as 100Mbps/interface bandwidth.(Interfaces 100Mbps and faster are assigned a cost of 1.)

    OSPF cost can be manually specified per interface:


    An alternative to defining static costs per interface is to change the numerator bandwidth (default100Mbps):


    Reference speed is a 32-bit value (1 – 4294967). If reference speed is modified, the same modification should be performed on all routers within the area.

    Router Priority

    Default DR election priority is 1, and a router with a priority of 0 will not become a DR. Priority range is 0 – 255.


    Verifying OSPF Configuration

            show ip ospf – OSPF process details

            show ip ospf database – Contents of the topology database

            show ip ospf interface – Interfaces participating in OSPF

            show ip ospf neighbor – Neighbor information

            show ip protocols – Displays all active routing protocols

            show ip route

            debug ip ospf events

            debug ip packet


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    Topics: CCNP, CCNP Notes | No Comments »


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