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Open Shortest Path First (OSPF) Routing Protocol

Posted in Routing Protocols

Introduction

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the interior gateway protocol (IGP) working group of the Internet Engineering Task Force (IETF). The working group was formed in 1988 to design an IGP based on the shortest path first (SPF) algorithm for use in the Internet. Similar to the Interior Gateway Routing Protocol (IGRP), OSPF was created because in the mid-1980s, the Routing Information Protocol (RIP) was increasingly unable to serve large, heterogeneous internetworks.

OSPF is a classless routing protocol, which means that in its updates, it includes the subnet of each route it knows about, thus, enabling variable-length subnet masks. With variable-length subnet masks, an IP network can be broken into many subnets of various sizes. This provides network administrators with extra network-configuration flexibility.These updates are multicasts at specific addresses (224.0.0.5 and 224.0.0.6).

For more information on OSPF visit our OSPF Routing Protocol section

The diagram below shows us the information that each field of an OSPF packet contains:

ospf-1

Routing Information Protocol - RIP

Posted in Routing Protocols

Introduction

Routing Information Protocol (RIP) is a true Distance-Vector routing protocol. It sends the complete routing table out to all active interfaces every 30 seconds. RIP only uses hop count to determine the best way to a remote network, but it has a maximum allowable hop count of 15, meaning that 16 is deemed unreachable. RIP works well in small networks, but it is inefficient on large networks with slow WAN links or on networks with large number of routers installed.

RIP comes in two different versions. RIP version 1 uses only classful routing, which means that all devices in the network must use the same subnet mask. This is because RIP version 1 does not include the subnet mask when it sends updates. RIP v1 uses broadcasts (255.255.255.255).

Link State Routing Protocols

Posted in Routing Protocols

Introduction

Link State routing protocols do not view networks in terms of adjacent routers and hop counts, but they build a comprehensive view of the overall network which fully describes the all possible routes along with their costs. Using the SPF (Shortest Path First) algorithm, the router creates a "topological database" which is a hierarchy reflecting the network routers it knows about. It then puts it's self on the top of this hierarchy, and has a complete picture from it's own perspective.

Link State protocols, unlike Distance Vector broadcasts, use multicast.

Distance Vector Routing Protocols

Posted in Routing Protocols

Distance Vector routing protocols use frequent broadcasts (255.255.255.255 or FF:FF:FF:FF) of their entire routing table every 30 sec. on all their interfaces in order to communicate with their neighbours. The bigger the routing tables, the more broadcasts. This methodology limits significantly the size of network on which Distance Vector can be used.

Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) are two very popular Distance Vector routing protocols. You can find links to more information on these protocols at the bottom of the page. (That's if you haven't had enough by the time you get there !)

Distance Vector protocols view networks in terms of adjacent routers and hop counts, which also happens to be the metric used. The "hop" count (max of 15 for RIP, 16 is deemed unreachable and 255 for IGMP), will increase by one every time the packet transits through a router.

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