Accton Technology ES4710BD User Manual page 406

• State of the connecting link
Link-state information is flooded throughout the network so that all layer 3 switches can get
firsthand information. Link-state layer 3 switches will not broadcast all information contained in
their route tables; instead, they only send changed link-state information. Link-state layer 3 switches
establish neighborhoods by sending "HELLO" to their neighbors, then link-state advertisements
(LSA) will be sent among neighboring layer 3 switches. Neighboring layer 3 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 routes. Cost can be 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 metrics by the administrator for
better assessment of the link-state.
1) When a link-state layer 3 switch enters a link-state interconnected network, it sends a
HELLO packet to get to know its neighbors and establish a neighborhood.
2) The neighbors respond with information about the link they are connecting and the related
costs.
3) The originate layer 3 switch uses this information to build its own routing table.
4) Then, as part of the regular update, layer 3 switch send link-state advertisement (LSA)
packets to its neighboring layer 3 switches. The LSA include links and related costs of that layer 3
switch.
5) Each neighboring layer 3 switch copies the LSA packet and passes it to the next neighbor
(flooding).
6) Since a routing database is not recalculated before the layer 3 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. These advantages
release some layer 3 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 layer 3 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
layer 3 switches in the autonomous system, they can be grouped as internal switches, edge switches,
AS edge switches and backbone switches). OSPF supports load balance and multiple routes to the
same destination of equal costs. OSPF supports 4 level routing mechanisms (process routing
according to the order of route inside an area, route between areas, first category exterior route and
second category exterior route). OSPF support IP subnet and redistribution of routes from the other
routing protocols, and interface-based packet verification. OSPF supports sending packets in
multicast.
Each OSPF layer 3 switch maintains a database describing the topology of the whole autonomous
system. Each layer 3 switch gathers the local status information, such as available interface,
reachable neighbors, and sends link-state advertisement (sending out link-state information) to
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