SMC Networks ES4704BD Manual page 452

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.
5) Each neighboring layer3 switch copies the LSA package 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 advantages release 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 an OSPF
network. Routing topology changes can be quickly found and updating LSAs can be sent
immediately, so that routes converge quickly. Link-state information is used in shortest
path algorithm for route calculation, eliminating loop route. OSPF divides the autonomous
system into areas, reducing database size, bandwidth occupation and calculation load.
(According to the position of layer3 switches in the autonomous system, they can be
grouped as internal area switches, area 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 external route and
second category external 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 layer3 switch maintains a database describing the topology of the whole
autonomous system. Each layer3 switch gathers the local status information, such as
452