Category Archives: OSPF

OSPF Summary

The characteristics of OSPF follow

  • Link-state routing protocol.
  • Uses IP protocol 89.
  • Classless protocol (supports VLSMs and CIDR).
  • Metric is cost, based on interface bandwidth by default (10^8 / BW in bps).
  • Sends partial route updates only when there are changes.
  • Send hello packets every 10 sec with dead timer of 40 sec over P-P & BC networks.
  • Send hello packets every 30 sec with dead timer of 120 sec over NBMA networks.
  • If the network is stable and there have been no updates within 30 min, a compressed update is sent.
  • Routes labeled as intra-area, interarea, external Type 1, or external Type 2.
  • Support for authentication.
  • Uses Dijkstra algorithm to calculate SPF tree.
  • Default administrative distance is 110.
  • Uses multicast address 224.0.0.5 (ALLSPFRouters).
  • Uses multicast address 224.0.0.6 (ALLDRouters).
  • Recommended for large networks.
  • For 2 routers to be adjacent :
    • 1st. Hello packets must be sent & received.
    • 2nd They must have the same hello & dead timers Also & same Net ID with subnet mask.
    • 3rd They must be in the same area.

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OSPF Basics

Introduction

The Open Short Path First (OSPF) is defined in RFC 2328.It is an interior Gateway Protocol used to distribute routing information within a single Autonomous system. It is a most common routing protocol using by Service Providers, because unlike EIGRP it is an Open Standard protocol.

Why we are calling OSPF is a link state routing protocol. Unlike RIP, OSPF is not only sending the Routing updates to its neighbors. It is sending all the information about the link like IP address of the Interface and subnet mask, the type of network it is connected to (P2P or P2Multi-point or FR) and the routers which is connected to it. The collection of these link states will form a Link state Database.

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