Link State Protocol Concepts

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Link State Protocol concepts
• The DR and BDR
• Hello Packets
• Troubleshooting adjacency issues
• Hub and Spoke NBMA OSPF Network Configuration

A drawback to distance vector protocols such as RIP, is that they send FULL routing table updates at scheduled intervals, which can take up valuable bandwidth and drain the receiving routers CPU.

Link State protocols do not send routes and metrics, they exchange the state of their links, and the cost associated with those links.

LSA - Link State Advertisements, are advertisements that are sent from OSPF neighbours to a router, so that it can perform a series of computations on the LSA's and get a complete picture of the network. This series of computations is known as the Shortest Path First (SPF) algorithm, otherwise known as the Dijkstra algorithm.

LSU - Link State Update, contains multiple LSA's which is updated into the OSPF database of the router. (R1#show ip ospf database)

*Once the LSA's are learnt by the link state routers, they won't send any any routing updates (any additional LSAs) unless there is a change in the network topology.

*RIP Broadcasts its updates to all routers running RIP (255.255.255.255) *RIP2 Multicasts its changes to the address 224.0.0.9
*OSPF floods its changes to 224.0.0.5

OSPF Roles: If all routers in an OSPF network had to form adjacencies with every other router, and had to exchange LSA's with every other router, a large amount of bandwidth would be used anytime a router flooded a network topology change. Instead most OSPF networks will elect a DR and a BDR

The Designated Router - Will receive the LSAs from the other routers in the area when theres a change, it then floods the LSA with the change it to all the non DR and BDR routers.

Routers that are neither the DR nor the BDR for a given network segment, are indicated as DROTHERS in a show ip ospf neighbor command.

The Backup Designated Router - Should the DR fail, the BDR takes its place.

The value...
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