OSPF Design: 2 – Introduction

A dynamic routing protocol supports a routed protocol and maintains routing tables.

The most common use of static routes is in stub networks.

In Link-State routing protocols, each router sends only that portion of the routing table that describes the state of its own links.

Link-state protocols are based on the distributed map concept, which means that every router has a copy of the network map that is regularly updated.

The principle of link-state routing is that all the routers within an area maintain an identical copy of the network topology.

Link-state protocols such as OSPF flood all the routing information when they first become active in link-state packets. After the network converges, they send only small updates via link-state packets.

In OSPF, because each router knows the complete topology of the network, the use of the SPF algorithm creates an extremely fast convergence.

  • Sends updates to tables only, instead of entire tables, to routers.
  • Is a more economical routing protocol than RIP over time because it involves less network traffic.

During an external convergence event, OSPF could flood more traffic than RIP. Consider that RIP carries 25 routes per update; on the other hand, OSPF floods a single LSA per external route that is affected by the convergence event.

Distance vector means that information sent from router to router is based on an entry in a routing table that consists of the distance and vector to destination—distance being what it “costs” to get there and vector being the “direction” to get to the destination.

Call for each router to send its entire routing table, but only to its neighbors. The neighbor then forwards its entire routing table to its neighbors, and so.

Distance Vector Operations

In RIP, the router only exchanges routing information from the connected neighbors.

  • RIP broadcasts every 30 seconds to maintain network integrity.
  • RIP maintains routing tables, showing the number of hops between routers, and is limited to a 15-hop count.
  • A router using RIP passes its entire routing table to each directly connected neighbor router that it knows of.

Link-state algorithms send small updates everywhere; distance vector algorithms send large updates only to neighboring routers.

Consider the following issues before selecting a routing protocol:

  • Operational considerations—Determine how easy it is to manage a network over time. These considerations include how to determine a protocol’s capability to adapt to changes, how to minimize disruptions to the network, and how to troubleshoot problems.
  • Technical considerations—Assist in determining whether a given protocol is capable of supporting a particular set of network requirements.
  • Business considerations—Defined as business priorities and policies that influence network design decisions. These types of considerations can originate from any area within a company and can be the keystones to the success of the network.

OSPF supports only the TCP/IP protocol suite.

OSPF routers carry full topology information about the backbone and connectivity information about all of the areas. Within each area, OSPF routers exchange full topology information about that area because the boundaries of areas fall within a router.

OSPF router run separate SPF calculations for each area.

OSPF, on the other hand, injects all the connectivity information about the other areas into each area. This enables every router in an OSPF area to choose the optimal area border router (ABR) for traffic that needs to go out of its area.

If the topology change were within the area, all the existing routes affected by the change would be discarded and a new routing table would be generated.

RIP floods the full topology table every 30 seconds. OSPF floods the full topology table every 30 minutes.

Within an area, OSPF and integrated IS-IS exchange changed link-state information. Between areas, OSPF and integrated IS-IS exchange changed routes.

Changed information in a RIP network is broadcast to all of its neighbors after the network has finished updating its topology. Changed information in OSPF and integrated IS-IS propagates throughout the area in which the change occurred. If route summarization is not done, change information might also propagate to the backbone and into other areas.

The default for OSPF is to use four equal-cost paths.

OSPF computes the cost of a path by summing the metrics for each hop on that path.

The principle of link-state routing is that all the routers within a network maintain an identical copy of the network topology.

OSPF converges in 0(M.log M) iterations, where M is the number of links.

Bellman Ford algorithm, which converges in 0(N.M) iterations, where N is the number of nodes.

Directly connected networks are reached via a metric (cost) of 0.

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Author: Mo Moghaddas

Traveler. Casual investor. Hobbyist landscape photographer loving Networks. Blockchain/Cryptocurrency enthusiast. Internet citizen!

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