OSPF [Open Shortest Path First]

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Routing protocols tell routers how to route packets. BGP [Border Gateway Protocol], RIP [Routing Information Protocol], OSPF [Open Shortest Path First], RTMP [Real Time Message Protocol], IGRP [Interior Gateway Routing Protocol] or other routing protocols select the route that is available based on routing table/list. OSPF is a link-state routing protocol which broadcasts or advertises routing information only when a change occurs by sending “hello” messages.

Key points of Link-State protocols are:

• Maintain knowledge of all routers

• Route information is flooded to all routers called advertising
• Each router builds its own routing tables
• Example: OSPF [Open Shortest Path First] and its derivatives

Here is a brief overview of OSPF:

OSPF distributes routing information to other routers in same or internal (interior) to an AS [Autonomous System] (same carrier or service provider). OSPF is an interior gateway protocol, not an external or exterior gateway protocol like BGP.

An AS is a common group of routers sharing or exchanging information (routing) via common (same) routing protocol which can be a serial connections or ethernet where networks just “broadcast” (yell, flood, advertise) at one another.

OSPF is a link-state protocol, which means that each router distributes (or floods) changes to all routers in the same AS (area). Because of the volume of changes that could occur fewer than 30 routers per AS are recommended. Also, each router may have access (routes) to only three other AS (areas).

OSPF v3 now provides support for IPv6 packet forwarding, multicast support (one-to-many), and as many as 24 optional capabilities (up from 8). OSPFv3 adds more security and supports IPv6’s packet security subheaders.

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