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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
first hand 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, 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 packet 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)
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
(i.e. 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 that 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