OSPFOpen Shortest Path First (OSPF) is a link-state routing protocol for

Internet Protocol (IP) networks. It uses a link state routing algorithm and falls into the group of interior routing protocols (also referred to as

IGP), operating within a single autonomous system (AS).

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Areas

• Backbone area (area 0)• Stub area• Not-so-stubby area (NSSA)

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Router ID

Router ID is going to be chosen automatically unless configured manually, using the 'router-id A.B.C.D' command in OSPF configuration context.

Initially, a router tries to choose its ID based on the following order of operation:

1. Router chooses numerically the highest IP address off of the loopback interface.

2. In case there is no loopback interface configured, the router is going to choose numerically the highest IP address off of the physical interface.

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Link State Advertisement (LSA)

OSPF routers exchange LSAs by flooding them to all OSPF neighbors. This flooding results in creating the same Link-State Database (LSDB) on all routers in the area. LSDB is a topology database, (kind of a "road map"), shared by all routers in the area. Then, each router runs Dijkstra's SPF algorithm to choose the best path to each destination, placing itself as the root (starting point). The product of SPF algorithm operation is the routing table.

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Designated Router (DR)

A designated router (DR) is the router interface elected among all routers on a particular multi-access network segment, generally assumed to be a broadcast multi-access network.

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Backup Designated Router (BDR)

A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election.

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Useful OSPF Commands (Cisco IOS)

Commands Descriptions

Router(config)#router ospf 1 Starts OSPF process 1. The process ID is any positive integer value between 1 and 65,535.

Router(config-router)#network 192.168.1.0 0.0.0.255 area 0OSPF advertises interfaces, not networks. Uses the wildcard mask to determine which

interfaces to advertise.

Router(config-if)#ip ospf hellointerval timer 20 Changes the Hello Interval timer to 20 seconds.

Router(config-if)#ip ospf deadinterval 80 Changes the Dead Interval timer to 80 seconds.

NOTE: Hello and Dead Interval timers must match for routers to become neighbors

Router#show ip protocol Displays parameters for all protocols running on the router

Router#show ip route Displays a complete IP routing table

Router#show ip ospf Displays basic information about OSPF routing processes

Router#show ip ospf interface Displays OSPF info as it relates to all interfaces

Router#show ip ospf interface fastethernet 0/0 Displays OSPF information for interface fastethernet 0/0

Router#show ip ospf border-routers Displays border and boundary router information

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Useful OSPF Commands (Cisco IOS) CONTD.Commands Descriptions

Router#show ip ospf neighbor Lists all OSPF neighbors and their states

Router#show ip ospf neighbor detail Displays a detailed list of neighbors

Router#clear ip route * Clears entire routing table, forcing it to rebuild

Router#clear ip route a.b.c.d Clears specific route to network a.b.c.d

Router#clear ip opsf counters Resets OSPF counters

Router#clear ip ospf processResets entire OSPF process, forcing OSPF to re-create neighbors, database, and routing

table

Router#debug ip ospf events Displays all OSPF events

Router#debug ip ospf adjacency Displays various OSPF states and DR/ BDR election between adjacent routers

Router#debug ip ospf packets Displays OPSF packets

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Enough with the Boring Stuff, Show us a DEMO!!

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Hands On Lab – Setup OSPF on All R1,2 and 3

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That’s it? nw it makes more sense.

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Further Reading

1. http://computernetworkingnotes.com/routing-static-dynamics-rip-ospf-igrp-eigrp/ospf-routing-configurations.html

2. http://ciscoiseasy.blogspot.com/2011/02/lesson-40-ospf-fundamentals-part3.html

3. http://en.wikipedia.org/wiki/Open_Shortest_Path_First

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Questions?