Introductions of OSPF

OSPF

• OSPFv3 is used for IPv6 networks.
• OSPF is a link state routing protocol
• A link is an interface on a router
• Link-state information includes the network prefix, prefix length and cost

Components of OSPF

• Routing Protocol Messages
  • Hello packet
  • Database description packet
  • Link-state request packet
  • Link-state update packet
  • Link-state acknowledgment packet
• Data structures
  • Adjacency Database (This creates the neighbor table)
    • show ip ospf neighbor
  • Link-state Database(LSDB)(This creates the topology table)
    • All routers within an area have identical LSDB.
    • List information about all other routers in the network.
    • show ip ospf database
  • Forwarding Database(This creates the routing table)
    • The routing table is unique.
    • show ip route
• Algorithm
  • Dijkstra shortest-path first (SPF) algorithm

Link-state operation

1. Establish neighbor adjacency (Hello Packet)
2. Exchange Link-state advertisement (LSAs)
   • LSAs contain the state and cost of each directly connected link
   • Flood the LSAs to adjacent neighbors until all the routers in the area have all the LSAs
3. Build the link state database
   • Based on the received the LSAs
4. Execute the SPF algorithm
5. Choose the best route

Single-area and Multi-area OSPF

• Single-area OSPF
  • All routers are in one area.
  • Best practice is to use area 0
• Multi-area OSPF
  • In a hierarchical way
  • All areas must connect to the backbone area (area 0)
  • Routers interconnecting the areas are referred to as Area Border Routers(ABR)

  • Advantage of Multi-area OSPF

• Smaller routing tables
  • Fewer routing table entries
  • Network addresses can be summarized between areas. (Route summarization is not enabled by default)
  • Reduced link-state update overhead
  • Reduced frequency of SPF calculations

Link-state Packets

• Link-state packets are the tools used by OSPF to help determine the fastest available route for a packet.
  • Type 1: Hello Packet
    • This is used to establish and maintain adjacency with other OSPF routers.
  • Type 2: Database Description (DBD) Packet
    • This contains an abbreviated list of the LSDB of the sending router
    • It is used by the receiving routers to check again the local LSDB
    • The LSDB must be identical on all link-state routers within an area
  • Type 3: Link-State Request (LSR) Packet
    • The receiving router can request more information about any entry in the DBD by sending an LSR
  • Type 4: Link-state Update (LSU) packet
    • This is used to reply to LSRs
    • LSUs contain several different types of LSAs
  • Type 5: Link-State Acknowledgment (LSAck) Packet
    • When an LSU is received, the router sends an LSAAck to confirm receipt of the LSU.

LSU (link-state update)

• LSU contains one or more LSAs
• LSA Type
  1. Router LSAs
  2. Network LSAs
  3. Summary LSAs
  4. Summary LSAs
  5. Autonomous System External LSAs
  6. Multicast OSPF LSAs
  7. Defined for Not-SO-Stubby Areas
  8. External Attributes LSA for Border Gateway Patrol (BGPs)