Open Shortest Path First (OSPF)

April 24, 2025

OSPF is a link state protocol, it make use of the neighbor table, topology table, and routing table.
works on network layer
It is suited for large organization with multiple redundant paths between networks
OSPF is hierarchical, Networks and their connected hosts and routers within an autonomous system are grouped into OSPF areas.
Routers within a given area share the same topological database of the networks they serve.
Routers that can connect to multiple areas are called area border routers
A backbone (always called Area 0) is created by the collection of border routers.
Backbone router is only visible to the border routers
OSPF routers exchange hello messages, both as a form of a keep-alive packet and in order to acquire neighbors with which to exchange routing information.
Neighbors share Link State Advertisement (LSA) updates to build a consistent link state database (LSDB) that represents the network topology of the area
The router applies an algorithm called shortest path first (SPF) to analyze the LSDB and add least-cost, loop free routes to its routing table.
OSPF does adjacent layer interaction, meaning each neighbor routers share all the routing information they have.

Border Gateway Protocol(BGP)

BGP is used to route between domains in a mesh internetwork
BGP is primarily used for routing between autonomous systems.
An ASN or autonomous system number are allocated by IANA via the various regional registries.
An AS hides the private networks from the public Internet.
BGP works with classless network prefixes called Network Layer Reachability Information (NLRI)
Path selection is based on multiple metrics, including hop count, weight, local preference, origin, and community.
BGP works over TCP on port 179

Routers use the longest prefix match or the more specific prefix to perform route