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algorithm to generate a route table based on that database.
Autonomous system (AS) is a self-managed interconnected network. In large networks,
such as the Internet, a giant interconnected network is broken down to autonomous
systems. Big enterprise networks connecting to the Internet are independent AS, since the
other host on the Internet are not managed by those AS and they don’t share interior
routing information with the layer3 switches on the Internet.
Each link-state layer3 switches can provide information about the topology with its
neighboring layer3 switches.
• The segment (link) connecting to the layer3 switches
• State of the connecting link
Link-state information is flooded throughout the network so that all layer3 switches can
get firsthand information. Link-state layer3 switches will not broadcast all information
contained in their route tables; instead, they only send changed link-state information.
Link-state layer3 switches establish neighborhood by sending “HELLO” to their
neighbors, then link-state advertisements (LSA) will be sent among neighboring layer3
switches. Neighboring layer3 switch copy the LSA to their routing table and transfer the
information to the rest part of the network. This process is referred to as “flooding”. In this
way, firsthand information is sent throughout the network to provide accurate map for
creating and updating routes in the network. Link-state routing protocols use cost instead
of hops to decide the route. Cost is assigned automatically or manually. According to the
algorithm in link-state protocol, cost can be used to calculate the hop number for packets
to pass, link bandwidth, current load of the link, and can even add metric by the
administrator for better assessment of the link-state.
1) When a link-state layer3 switch enters a link-state interconnected network, it sends
a HELLO packet to get to know its neighbors and establish neighborhood.
2) The neighbors respond with information about the link they are connecting and the
related costs.
3) The originate layer3 switch uses this information to build its own routing table.
4) Then, as part of the regular update, layer3 switch send link-state advertisement
(LSA) packets to its neighboring layer3 switches. The LSA include links and related costs
of that layer3 switch.
5) Each neighboring layer3 switch copies the LSA packet and passes it to the next
neighbor (flooding).
6) Since routing database is not recalculated before layer3 switch forwards LSA
flooding, the converging time is greatly reduced.
One major advantage of link-state routing protocols is the fact that infinite counting is
impossible, this is because of the way link-state routing protocols build up their routing
table. The second advantage is, converging in a link-state interconnected network is very
fast, once the routing topology changes, updates will be flooded throughout the network
very soon. Those advantages released some layer3 switch resources, as the process
ability and bandwidth used by bad route information are minor.
The features of OSPF protocol include the following. OSPF supports networks of various
scales; several hundreds of layer3 switches can be supported in a OSPF network.
Routing topology change can be quickly found and converged. Link-state information is
used in shortest path algorithm for route calculation, eliminating endless loop. OSPF
divides the autonomous system intro areas, reducing database size, bandwidth
occupation and calculation load. (According to the position of layer3 switches in the