CCNP BSCI Notes – Understanding Simple Single-Area OSPF

Ensurepass

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 224.0.0.5 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:

         Down

         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:

         224.0.0.5 – All OSPF routers

         224.0.0.6 – 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.

Areas

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

         Type

         Length

         Router ID

         Area ID

         Checksum

         Authentication type (none/plaintext/md5)

         Authentication data

         Data

Configuring OSPF in a Single Area

Necessary information:

        OSPF process ID (locally significant)

        Participating interfaces

        Area ID

        Router ID

Enable OSPF

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Configure Included Networks

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A single interface can be specified by supplying its IP address and a null wildcard mask: network 192.168.0.1 0.0.0.0 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:

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

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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:

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

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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.

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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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