Saturday, September 5, 2015

Understanding OSPF Routing

OSPF is a fast-converging, link-state IGP used by millions.

OSPF forms adjacencies with neighbors and shares information via the DR and BDR using Link State Advertisements.

Areas in OSPF are used to limit LSAs and summarize routes. Everyone connects to area zero, the backbone.

Open Shortest Path First is a robust link-state interior gateway protocol (IGP). People use OSPF when they discover that RIP just isn’t going to work for their larger network, or when they need very fast convergence

OSPF is the most widely used IGP. When we discuss IGPs, we’re talking about one routing domain, or Autonomous System (AS). Imagine a medium-sized company with multiple buildings and departments, all connected together and sharing two redundant Internet links. All of the buildings on-site are part of the same AS. But with OSPF we also have the concept of an Area, which allows further segmentation, perhaps by department in each building.

To understand the design needs for areas in OSPF, let’s start by discussing how OSPF works. There’s some terminology you may not have encountered before, including:

Router ID: In OSPF this is a unique 32-bit number assigned to each router. This is chosen as the highest IP address on a router, and can be set large by configuring an address on a loopback interface of the chosen router.

Neighbor Routers: two routers with a common link that can talk to each other.
Adjacency: a two-way relationship between two neighbor routers. Neighbors don’t always form adjacencies.

LSA: Link State Advertisements are flooded; they describe routes within a given link.

Hello Protocol: this is how routers on a network determine their neighbors and form LSAs.

Area: a hierarchy. A set of routers that exchange LSAs, with others in the same area. Areas limit LSAs and encourage aggregate routes.

ABR:- An Area Border Router is a router that is in area zero, and one or more other areas.

DR, BDR:- A Designated Router, as we said, is the router that keeps the database for the subnet. It sends and receives updates (via multicast) from the other routers in the same network.

ASBR:- The Autonomous System Boundary Router is very special, but confusing. The ASBR connects one or more AS, and exchanges routes between them. The ASBR’s purpose is to redistribute routes from another AS into its own AS.

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