Open Shortest Path First protocol (Open Shortest Path First, OSPF) proposed by the IETF in 1988, is a dynamic routing protocol based on link status. OSPF is a class Interior Gateway Protocol Interior Gateway Protocol IGP. 
Introduction OSPF
link-state routing protocol, to overcome the two deadly weaknesses of RIP
convergence speed (more than 240 seconds)
size limit, only 15 hops
subregions concept:
the backbone area Area 0, Area N non-backbone areas
backbone continuity region, non-backbone area must be connected to the backbone
RFC 2328 defines:
each router to which all the routers in the same administrative domain, the broadcast information transmitting link state, link state router first collect information related to each node is calculated according to a certain algorithm the shortest path.
Features OSPF routing protocol
can be adapted to large-scale network
routing changes fast convergence
without routing ring
supports variable length subnet mask VLSM
support zoning
support protocol packets to multicast addresses.
Hello packets
OSPF process, each router will send Hello packets at intervals of time;
Hello packets sent by the multicast address 224.0.0.5;
OSPF uses Hello packets to initiate, establish adjacencies, monitoring of the presence and disappearance of such relations
in the broadcast network or mesh point, Hello transmission interval is 10 seconds;
on the NBMA network, Hello transmission interval is 30 seconds
OSPF works ---- Router ID! []
Running OSPF router must have a Router ID (RID).
RID is a 32-bit unsigned integer, in an autonomous system that uniquely identifies a router.
RID can be configured manually or automatically generated.
If no command RID, generating the following order a RID:
current device configuration Loopback interface, select all the maximum values Loopback port IP address as RID;
current device is not configured Loopback interface, and select all IP addresses configured on the active interfaces on the largest IP as RID.
