Ospf; Introduction To Ospf - Edge-Core ES4624-SFP Manual

17.6 OSPF

17.6.1 Introduction to OSPF

OSPF is abbreviation for Open Shortest Path First. It is an interior dynamic routing
protocol for autonomous system based on link-state. The protocol creates a link-state
database by exchanging link-states among layer3 switches, and then uses the Shortest
Path First algorithm to generate a route table basing 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 switch can provide information about the topology with its
neighboring Layer3 switches.
• The network segment (link) connecting to the layer3 switch
• 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
packages to pass, link bandwidth, and current load of the link.. The administrator can
even add weight for better assessment of the link-state.
1) When a link-state layer3 switch enters a link-state interconnected network, it
sends a HELLO package to get to know its neighbors and establish neighborhood.
2) The neighbors respond with information about the links 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) packages to its neighboring layer3 switches. The LSA include links and related
costs of that layer3 switch.
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