An OSPF area is a logical grouping of routers that runs OSPF with identical topological databases. An area is a subdivision of the OSPF routing domain. Each area runs SPF separately and summaries are passed between each area.
Problems associated with OSPF in a Single Area
Consider a growing OSPF network with a single area. Several problems come out in relation to capacity capabilities:
- The SPF algorithm runs more frequently the larger the network gets, the greater the probability of a network change and a recalculation of the entire area (iow the more resources OSPF chews up). Each of these recalculations in a large network takes longer and involves more “work” with each recalculation for a small area (the expenditure of scarce resources time, cpu, memory, etc).
- The larger the OSPF area, the greater the size of the routing table (duh). The routing table is not sent out (like in Distance Vector Routing Protocols). In OSPF this means that the the greater the size of the table the longer the lookup becomes. The memory requirements on the router also increase as the size of the routing table increases.
- In a large network, the routers topological database increases in size and eventually becomes unmanageable (the topological database is exchanged between adjacent routers at least every 30 minutes).
As the various databases (Routing Table, Topological Database, Neighbor Table) increase in size and the calculation increase in frequency the CPU utilization increases and memory availability decreases (inverse relationship). This can affect network latency or cause link congestion, resulting in various additional problems (convergence times, loss of connectivity, loss of packets, system hangs) which is bad for networks.
OSPF creates a two-level hierarchy of areas.
Area Zero (Naught) a.k.a the backbone are or transit area. This is always the central area; all the other areas (stub areas that move towards the edge) attach to Area Zero. Area Zero forms the top level in the hierarchy and remaining areas form the bottom level of the hierarchy. This hierarchical design supports summarization and minimizes routing table entries.
Routers within Area Zero are called backbone routers. Routers that link to Area Zero and another area are called Area Border Routers (ABR). OSPF routers that redistribute routing information from another protocol are called Autonomous System Boundary Routers (ASBR).
OSPF Type Packets
As OSPF link-state information is shared between areas, an intricate set of mechanisms is followed, relying on a number of different OSPF packet types. All OSPF traffic is transmitted inside IP Packets. Receivers recognize OSPF traffic because it is marked as IP Protocol (89).
OSPF includes five packet types:
- Hello Packets – Establish communication with directly attached neighbors.
- Database Descriptor (DBD) - Sends a list of router IDs from whom the router has an Link State Advertisements (LSA) and the current sequence number. This information is used to compare information about the network.
- Link State Requests (LSR) – Follow the Database Descriptors (DBDs) to ask for any missing Link State Advertisements (LSAs)
- Link State Update (LSU) – Replies to a link-state request with the requested data.
- Link State acknowledgments (LSAck) - Confirm receipt of link-state information.
All OSPF packets have a common format that contains the following nine fields:
- Version – All packets are assumed to be Version 2 (at least for this part of Cisco stuff)
- Type - There are five packet types, numbered 1 to 5
- Packet Length - The length in bytes
- Router ID – 32-bit identifier for the router
- Area ID – 32-bit identifier for the area
- Checksum - Standard 16-bit check sum
- Authentication Type - OSPFv2 supports three authentication methods:
- no authentication
- plain text passwords
- MD5 hashes
- Authentication Data – 64-bit data, either empty, with a plain-text word, or with a “message digest” of a shared secret
- Data – Values being communicated
And this took me almost 2 weeks. Shame on me.
Notes and Notices: This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I cannot lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.