Transcript OSPF Open Shortest Path First (OSPF) is a link-state routing protocol for Internet Protocol (IP) networks.

OSPF
Open 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).
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.
OSPF advertises interfaces, not networks. Uses the wildcard mask to determine which
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0
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
Resets entire OSPF process, forcing OSPF to re-create neighbors, database, and routing
Router#clear ip ospf process
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? nw it makes more sense.
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Further Reading
1. http://computernetworkingnotes.com/routing-static-dynamics-ripospf-igrp-eigrp/ospf-routing-configurations.html
2. http://ciscoiseasy.blogspot.com/2011/02/lesson-40-ospffundamentals-part3.html
3. http://en.wikipedia.org/wiki/Open_Shortest_Path_First
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Questions?
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