Extreme Networks Summit1 - Manual (Page 153)

S UMMIT S WITCH I NSTALLATION AND U SER G UIDE 10-5
O VERVIEW OF OSPF
O
VERVIEW OF
OSPF
OSPF is a link state protocol that distributes routing information between routers
belonging to a single IP domain, also known as an autonomous system. In a link state
routing protocol, each router maintains a database describing the topology of the
autonomous system. Each participating router has an identical database maintained
from the perspective of that router.
From the link state database (LSDB), each router constructions a tree of shortest paths,
using itself as the root. The shortest path tree provides the route to each destination in
the Autonomous System. When several equal-cost routes to a destination exist, traffic
can distributed among them. The cost of a route is described by a single metric.
L INK S TATE D ATABASE
Upon initialization, each router transmits a link state advertisement (LSA) on each of its
interfaces. The LSA contains the following information for each link:
• IP network number of the link
• Subnet mask of the link
• Metric for the link
• Operation status (up or down) of the link
LSAs are collected by each router and entered into the LSDB of each router. OSPF uses
flooding to distribute LSAs between routers. Any change in routing information is sent
to all of the routers in the network. All routers within an area have the exact same
LSDB.
A REAS
OSPF allows parts of a networks to be grouped together into areas. The topology within
an area is hidden from the rest of the autonomous system. Hiding this information
enables a significant reduction in LSA traffic, and reduces the computations needed to
maintain the LSDB. Routing within the area is determined only by the topology of the
area.
Summit.bk : 10PROT.FM Page 5 Thursday, June 18, 1998 9:27 AM