Concepts of Routing Protocols

OSPF (Open Shortest Path First):

  • OSPF is an open standard routing protocol that operates on the concept of link-state routing. It uses the Dijkstra algorithm to calculate the shortest path and builds a topology map of the network.
  • It supports multiple metrics, such as bandwidth, delay, reliability, and load, for determining the cost of a route. The cost is assigned to each link, and the lowest-cost path is chosen.
  • OSPF uses areas to divide a network into smaller, more manageable parts, which allows for better scalability. Each area has its own link-state database, and routing information is summarized between areas, reducing the impact of changes on the entire network.
  • It supports both IPv4 and IPv6 and is capable of utilizing variable-length subnet masks (VLSM), allowing for efficient IP address space utilization.
  • OSPF uses multicast addressing ( and for neighbor discovery and routing updates, minimizing network overhead.
  • The benefits of OSPF include scalability, fast convergence, load balancing, route summarization, and support for multiple network types.
  • However, OSPF can be more complex to configure and troubleshoot compared to other protocols. It requires careful planning, especially when designing areas and setting up authentication.

EIGRP (Enhanced Interior Gateway Routing Protocol):

  • EIGRP is a Cisco proprietary routing protocol that combines elements of both distance-vector and link-state routing protocols. It uses the Diffusing Update Algorithm (DUAL) to calculate the best path.
  • It uses a composite metric that takes into account bandwidth, delay, reliability, and load. The default metric formula is based on the minimum bandwidth of the slowest link in the path.
  • EIGRP supports VLSM and both IPv4 and IPv6 addressing.
  • Similar to OSPF, EIGRP supports fast convergence due to the DUAL algorithm. It maintains a topology table and uses partial updates to minimize network overhead during route changes.
  • EIGRP uses multicast addressing ( for neighbor discovery and unicast updates, reducing unnecessary traffic.
  • Key advantages of EIGRP include fast convergence, efficient use of bandwidth, low network overhead, support for multiple protocols (including IP, IPX, and AppleTalk), and seamless integration with other Cisco features like route tagging and filtering.
  • The main limitation of EIGRP is its vendor specificity, as it is only available on Cisco devices. This limits its interoperability with devices from other vendors and makes it less suitable for multi-vendor networks.

Scenarios for OSPF usage:

  • Large networks with complex topologies and scalability requirements benefit from OSPF’s hierarchical structure and area division.
  • Networks with multiple vendors’ devices that need interoperability and standardization.
  • Networks that require fast convergence and accurate calculation of the shortest path.
  • Networks with extensive VLSM implementation for efficient IP address allocation.

Scenarios for EIGRP usage:

  • Medium-sized networks that are predominantly composed of Cisco devices, as EIGRP offers seamless integration and optimized performance in such environments.
  • Networks that require fast convergence and benefit from the hybrid routing approach of EIGRP.
  • Environments where Cisco-specific features like route tagging and filtering are desired.
  • Networks that do not have strict multi-vendor requirements and can leverage the benefits of EIGRP’s simplicity and efficiency within a Cisco-centric infrastructure.

BGP (Border Gateway Protocol):

  • BGP is an exterior gateway protocol (EGP) designed for routing between different ASs or autonomous systems.
  • It is the protocol that enables the interconnection of different networks on the internet and handles the exchange of routing information between ASs.
  • BGP is a path-vector protocol that uses policies and attributes to determine the best path between ASs.
  • Unlike OSPF and EIGRP, BGP is concerned with routing on a global scale and manages routing across multiple administrative domains.
  • BGP allows for routing policies to be applied, enabling control over how routes are advertised and received between ASs.
  • BGP provides features such as route filtering, route summarization, and support for policy-based routing decisions.

Key differences between OSPF, EIGRP, and BGP:

  • Scope: OSPF and EIGRP are interior gateway protocols (IGPs) used within a single AS or organization’s network, while BGP is an exterior gateway protocol (EGP) used for routing between different ASs.
  • Routing Information: OSPF and EIGRP focus on efficient routing within a network, exchanging internal routing information, while BGP handles the exchange of routing information between different ASs on a global scale.
  • Metrics: OSPF uses a cost metric based on link-state information, EIGRP uses a composite metric, and BGP uses path attributes and policies to determine the best path.
  • Protocol Type: OSPF is an open standard protocol, EIGRP is a Cisco proprietary protocol, and BGP is an open standard protocol widely implemented across different vendors.
  • Scale: OSPF and EIGRP are typically used in medium to large networks, while BGP is used at the internet edge and in service provider networks where routing between ASs is required.

In summary, OSPF and EIGRP are interior gateway protocols used for efficient routing within a network, while BGP is an exterior gateway protocol used for routing between different autonomous systems, such as ISPs or interconnecting networks on the internet.

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