OSPF [Open Shortest Path First] - More

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Here is a brief overview of OSPF [Open Shortest Path First]:

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

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” (yells, floods, advertises) 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.

OSPF, a link-state protocol is an expandable (scalable) routing protocol with key advantages over distance-vector protocols such as RIP. Some of the advantages include:

• Unlimited hops - allows limitless reachability

• VLSM [Variable-Length Subnet Masks] - “wildcard subnet masks” create multiple route options based on sub-dividing internet protocol addresses - see other TECHtionary terms for explanation - Route Summarization - sub-divided route tables/lists to improve performance.
• Maintains three databases - adjacency, topology and route
  • Adjacency database/table - maintains route information on neighbors

  • Topology database/table - maintains all route information
  • Route database/table - maintains “best route” information

OSPF supports redistribution of many other protocols. This process is called Filtering is implemented on ASBR [Autonomous System Boundary Routers] (routers with external interfaces). Cisco suggests filtering to prevent unwanted network protocols from entering.

Route Redistribution - multi-protocol support for:

• BGP [Border Gateway Protocol]

• EGP [Exterior Gateway Protocol]
• EIGRP [Enhance Interior Gateway Routing Protocol]
• IGRP [Interior Gateway Routing Protocol]
• ISIS [Intermediate System-to-Intermediate System]
• ISO-IGRP [IGRP for mobile systems]
• ODR [On Demand stub Routers]
• RIP [Routing Information Protocol]
• Static routes

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[tags]eigrp,border gateway protocol,ospf,open shortest path first,bgp,enhance interior gateway routing protocol[/tags]