General OSPF

• IPv4 – RFC 2328 v2
• IPv6 – RFC 5340 v3

• Uses Shortest Path First Algorithm.

• Guarantees a loop free topology by using SPF and Area 0.
• Standards
  • Can be used by multiple vendors.
• Convergence
  • Very Fast
  • Tracks Neighbor adjacencies actively.
  • Event driven incremental updates.

• Efficient updating
  • Non-OSPF updates do not process the updates on link.
  • Uses multicast and unicast for updates.
• Link metrics are based upon bandwidth of interfaces.

• Security
  • Supports clear text, MD5, SHA, IPsec for authentication.
• Extensibility
  • Application support through ‘opaque LSA’

OSPF Operations:

• Step1:
  • Uses Hello packets to find neighbors on links.
  • Protocol 89
  • Sends information via multicast
    • 224.0.0.5 – all OSPF speaking routers
    • 224.0.0.6 – DR/BDR communication
  • Hello packets have data that both speakers must agree upon to form adjacency
  • agreement and adjacency has formed, OSPF database gets exchanged between both parties.

Adjacency Attributes:

• Router-ID
  • Chosen via manual configuration, highest active loopback IP, then highest active interface IP.
• Interface IP addresses
  • OSPFv2 will use the interface primary IP (IPv4)
  • OSPFv3 will use the interface link local IP (IPv6)

Attributes that have to match:

• Area
• Hello and dead timers
• Network address (between two nodes)
• Interface MTU
• OSPF network type
• Authentication

Hellos:

• Sent out every hello interval (timer)
• Contains the following data
  • RID
  • Area
  • Subnet mask
  • Interface priority
  • Hello and Dead timers
  • Authentication information
  • DR/BDR addresses
  • RID of other neighbors on the link

Adjacency States

• Down
  • No hellos have been received.
• Attempt
  • Hello has been sent to neighbor, nothing received back.
• Init
  • Received hello, no acknowledgement of hello that’s been sent.
• 2-Way
  • Have received neighbor hello and acknowledgement of local hello already sent out.
• ExStart
  • Beginning of adjacency
  • Higher router ID is master and chooses DB sequence number
• Exchange
  • Link state database sent through DBD packets.
• Loading
  • Link state requests sent to ask for more info about particular LSA.
• Full
  • Adjacency has formed, DBs are synchronized.

• Step2:
  • Beginning of path selection.
  • Each LSA includes a cost attribute for each link.
  • Paths added to RIB are paths with lowest cost end to end.
  • ECMP is allowed.
  • Default cost value is 100Mbps / link bandwidth.
    • For bandwidth 10Gbps and higher, the reference bandwidth command needs to be used.

• Step3:
  • Neighbor tracking is from here on out completed via hellos and LSAs
    • Hellos for tracking neighbor changes.
      • HelloInterval is what defines how often hellos are sent out for tracking. 10 and 30 seconds depending on network/interface type.
      • DeadInterval
        • Time router declares neighbor dead if it does not receive a hello.
    • LSAs for tracking topology changes.
      • If new LSA is received, the LSDB sequence number, Age, and checksum are compared to make sure everything is up to date.
      • When change occurs, LSAs are flooded out all OSPF speaking interfaces – no split horizon. Self-originated LSAs are just dropped.