OSPF FEATURES AND CHARACTERISTICS

OSPF Features And Characteristics Complete Guide

OSPF is a popular multivendor, open-standard, classless link-state routing protocol. OSPFv2 is used for IPv4 networks, while OSPFv3 is used for IPv6 networks. In addition, OSPFv3 with address families supports both IPv4 and IPv6.

 OSPF is a link-state routing protocol that was developed as an alternative for the distance vector protocol Routing Information Protocol (RIP). RIP was an acceptable routing protocol in the early days of networking and the internet.

 However, RIP’s reliance on hop count as the only metric for determining the best route quickly became problematic. Using hop count does not scale well in larger networks with multiple paths of varying speeds. OSPF has significant advantages over RIP in that it offers faster convergence and scales to much larger network implementations.

 OSPF is a link-state routing protocol that uses the concept of areas. A network administrator can divide the routing domain into distinct areas that help control routing update traffic. A link is an interface on a router. A link is also a network segment that connects two routers, or a stub network such as an Ethernet LAN that is connected to a single router. Information about the state of a link is known as link-state information; this information includes the network prefix, prefix length, and cost.

 To make OSPF more efficient and scalable, OSPF supports hierarchical routing using areas. An OSPF area is a group of routers that share the same link-state information in their LSDBs.

OSPF can be implemented in one of two ways:

1- Single-area OSPF:
All routers are in one area, also known as backbone area. Best practice is to use area 0, but can use any other area number too.
2-Multiarea OSPF:
OSPF is implemented using multiple areas, in a hierarchical fashion. All areas must connect to the backbone area (area 0). Routers interconnecting the areas are referred to as area border routers (ABRs).
The hierarchical topology design options with multiarea OSPF can offer the following advantages:
  • i- Smaller routing tables: Tables are smaller because there are fewer routing table entries. This is because network addresses can be summarized between areas. Route summarization is not enabled by default.
  • ii- Reduced link-state update overhead: Designing multiarea OSPF with smaller areas minimizes processing and memory requirements.
  • iii-Reduced frequency of SPF calculations: Multiarea OSPF localize the impact of a topology change within an area. For instance, it minimizes routing update impact because LSA flooding stops at the area boundary.

Components of OSPF:

All routing protocols share similar components. They all use routing protocol messages to exchange route information. The messages help build data structures, which are then processed using a routing algorithm.
The components of OSPF are as follows:
  1. Routing protocol messages
  2. Data structures
  3. Algorithm

1- Routing Protocol Messages:

Routers running OSPF exchange messages to convey routing information using five types of packets. These packets are used to discover neighboring routers and also to exchange routing information to maintain accurate information about the network.
These packets, are as follows:
  • i- Hello packet
  • ii- Database description (DBD) packet
  • iii- Link-state request (LSR) packet
  • iv- Link-state update (LSU) packet
  • v- Link-state acknowledgment (LSAck) packet

2- Data Structures:

OSPF messages are used to create and maintain three OSPF databases or tables:
Each of these tables contains a list of neighboring routers to exchange routing information. The tables are kept and maintained in RAM.
These databases or tables, are as follows:
  • i- Adjacency database: This creates the neighbor table.
  • ii- Link-state database (LSDB): This creates the topology table.
  • iii- Forwarding database: This creates the routing table.

3- Algorithm:

A router builds a topology table by using results of calculations based on Dijkstra’s algorithm. The shortest- path first (SPF) algorithm is based on the cumulative cost to reach a destination.
The SPF algorithm creates an SPF tree by placing each router at the root of the tree and calculating the shortest path to each node. The SPF tree is then used to calculate the best routes. OSPF places the best routes into the forwarding database, which is used to make the routing table.
Source: CCNAv7 Enterprise networking, Security and Automation companion guide
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